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{{Short description|Device for receiving radio broadcasts}}
[[File:Sony-walkman-srfs84s 0001.JPG|thumb|A portable battery-powered AM/FM broadcast receiver, used to listen to audio broadcast by local [[radio station]]s.]]
[[File:Icom IC-R7000 radio receiver.jpg|thumb|A modern [[communications receiver]], used in [[two-way radio]] communication stations to talk with remote locations by [[shortwave radio]].]]
[[Image:Girl listening to radio.gif|thumb|Girl listening to [[vacuum tube]] console radio in the 1940s. During the [[golden age of radio]], 1925–1955, families gathered to listen to the home radio receiver in the evening]]

In [[radio|radio communications]], a '''radio receiver''', also known as a '''receiver''', a '''wireless''', or simply a '''radio''', is an electronic device that receives [[radio wave]]s and converts the information carried by them to a usable form. It is used with an [[antenna (radio)|antenna]]. The antenna intercepts radio waves ([[electromagnetic wave]]s of [[radio frequency]]) and converts them to tiny [[alternating current]]s which are applied to the receiver, and the receiver extracts the desired information. The receiver uses [[electronic filter]]s to separate the desired [[radio frequency]] signal from all the other signals picked up by the antenna, an [[electronic amplifier]] to increase the power of the signal for further processing, and finally recovers the desired information through [[demodulation]].

Radio receivers are essential components of all systems that use [[radio]]. The information produced by the receiver may be in the form of sound, video ([[television]]), or [[digital signal|digital data]].<ref>[http://www.radio-electronics.com/info/rf-technology-design/index.php Radio-Electronics, ''Radio Receiver Technology'']</ref> A radio receiver may be a separate piece of electronic equipment, or an [[electronic circuit]] within another device. The most familiar type of radio receiver for most people is a broadcast radio receiver, which reproduces sound transmitted by [[radio broadcasting]] stations, historically the first mass-market radio application. A broadcast receiver is commonly called a "radio". However radio receivers are very widely used in other areas of modern technology, in [[television]]s, [[cell phone]]s, [[wireless modem]]s, [[radio clock]]s and other components of communications, remote control, and wireless networking systems.

==Broadcast radio receivers==
{{redirect-distinguish|AM/FM|AM/FM/GIS|AM/FM (song)}}

The most familiar form of radio receiver is a broadcast receiver, often just called a ''radio'', which receives [[audio signal|audio]] programs intended for public reception transmitted by local [[radio station]]s. The sound is reproduced either by a [[loudspeaker]] in the radio or an [[earphone]] which plugs into a jack on the radio. The radio requires [[electric power]], provided either by [[battery (electricity)|batteries]] inside the radio or a power cord which plugs into an [[electric outlet]]. All radios have a [[Loudness|volume control]] to adjust the loudness of the audio, and some type of "tuning" control to select the radio station to be received.

===Modulation types===
[[Modulation]] is the process of adding information to a radio [[carrier wave]].

====AM and FM====
Two types of modulation are used in analog radio broadcasting systems; AM and FM.

In [[amplitude modulation]] (<span class="ltr">AM</span>) the strength of the radio signal is varied by the audio signal. [[AM broadcasting]] is allowed in the [[AM broadcast band]]s which are between 148 and 283&nbsp;kHz in the [[longwave]] range, and between 526 and 1706&nbsp;kHz in the [[medium frequency]] (<span class="ltr">MF</span>) range of the [[radio spectrum]]. AM broadcasting is also permitted in [[shortwave]] bands, between about 2.3 and 26&nbsp;MHz, which are used for long distance international broadcasting.

In [[frequency modulation]] (<span class="ltr">FM</span>), the [[frequency]] of the radio signal is varied slightly by the audio signal. [[FM broadcasting]] is permitted in the [[FM broadcast band]]s between about 65 and 108&nbsp;MHz in the [[very high frequency]] (VHF) range. The exact frequency ranges vary somewhat in different countries.

[[FM stereo]] radio stations broadcast in [[stereophonic sound]] (stereo), transmitting two sound channels representing left and right [[microphone]]s. A stereo receiver contains the additional circuits and parallel signal paths to reproduce the two separate channels. A [[monaural]] receiver, in contrast, only receives a single audio channel that is a combination (sum) of the left and right channels.<ref name="Pizzi">{{cite book
 | last1  = Pizzi
 | first1 = Skip 
 | last2  = Jones
 | first2 = Graham
 | title  = A Broadcast Engineering Tutorial for Non-Engineers
 | publisher = CRC Press
 | date   = 2014
 | pages  = 208
 | url    = https://books.google.com/books?id=sQ1xAwAAQBAJ&q=monaural+mono+receiver+left+right+combined&pg=PA208
 | isbn   = 978-1317906834
 }}</ref><ref name="Libbey">{{cite book
 | last1  = Libbey
 | first1 = Robert 
 | title  = Signal And Image Processing Sourcebook
 | publisher = Springer Science and Business Media
 | date   = 1994
 | pages  = 63
 | url    = https://books.google.com/books?id=DoU2CLLZ2tsC&q=mono+receiver+L+R+combined&pg=PA63
 | isbn   = 978-0442308612
 }}</ref><ref name="Olsen">{{cite book
 | last1  = Olsen
 | first1 = George H. 
 | title  = Electronics Made Simple
 | publisher = Elsevier
 | date   = 2013
 | pages  = 258
 | url    = https://books.google.com/books?id=KeZbCQAAQBAJ&q=%22mono+receiver%22+left+right+combines&pg=PA258
 | isbn   = 978-1483140780
 }}</ref>  While [[AM stereo]] transmitters and receivers exist, they have not achieved the popularity of FM stereo.

Most modern radios are able to receive both AM and FM radio stations, and have a switch to select which band to receive; these are called ''AM/FM radios''.

====Digital audio broadcasting (DAB)====
{{main|Digital audio broadcasting}}

[[Digital audio broadcasting]] (DAB) is an advanced radio technology which debuted in some countries in 1998 that transmits audio from terrestrial radio stations as a [[digital signal]] rather than an [[analog signal]] as AM and FM do. Its advantages are that DAB has the potential to provide higher quality sound than FM (although many stations do not choose to transmit at such high quality), has greater immunity to [[radio noise]] and interference, makes better use of scarce [[radio spectrum]] bandwidth, and provides advanced user features such as [[electronic program guide]], sports commentaries, and image slideshows. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased. As of 2017, 38 countries offer DAB, with 2,100 stations serving listening areas containing 420 million people. The United States and Canada have chosen not to implement DAB.

DAB radio stations work differently from AM or FM stations: a single DAB station transmits a wide 1,500&nbsp;kHz bandwidth signal that carries from 9 to 12 channels from which the listener can choose. Broadcasters can transmit a channel at a range of different [[bit rate]]s, so different channels can have different audio quality. In different countries DAB stations broadcast in either [[Band III]] (174–240&nbsp;MHz) or [[L band]] (1.452–1.492&nbsp;GHz).

===Reception===
The [[signal strength]] of radio waves decreases the farther they travel from the transmitter, so a radio station can only be received within a limited range of its transmitter. The range depends on the power of the transmitter, the sensitivity of the receiver, atmospheric and internal [[noise (electronics)|noise]], as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as [[ground wave]]s which follow the contour of the Earth, so AM radio stations can be reliably received at hundreds of miles distance. Due to their higher frequency, FM band radio signals cannot travel far beyond the visual horizon; limiting reception distance to about 40 miles (64&nbsp;km), and can be blocked by hills between the transmitter and receiver. However FM radio is less susceptible to interference from [[radio noise]] ([[Radio frequency interference|RFI]], [[sferics]], static) and has higher [[fidelity]]; better [[frequency response]] and less [[audio distortion]], than AM. So in countries that still broadcast AM radio, serious music is typically only broadcast by FM stations, and AM stations specialize in [[radio news]], [[talk radio]], and [[sports radio]]. Like FM, DAB signals travel by [[line-of-sight propagation|line of sight]] so reception distances are limited by the visual horizon to about 30–40&nbsp;miles (48–64&nbsp;km).

===Types of broadcast receivers===
[[File:Radio-with-Alarm clock.jpg|thumb|A bedside [[clock radio]] that combines a radio receiver with an [[alarm clock]]]]
Radios are manufactured in a range of styles and functions:
*''Console radio'' - A self-contained radio with speaker designed to stand on the floor.
*''[[Table radio]]'' also called a "''Mantel radio''" - A self-contained radio with speaker designed to sit on a table, cabinet, or [[fireplace mantel]].<ref>Marianne Fedunkiw, Inventing the Radio, Crabtree Publishing Company, 2007, page 17</ref><ref>[https://radioattic.com/gallery.htm?style=Table/Mantle_Radios Radio Attic Gallery of Table/Mantle Radios]</ref>  Table radios typically plug into a wall outlet, although some "cordless" battery powered table radios exist.
*''[[Clock radio]]'' - A bedside [[table radio]] that also includes an [[alarm clock]]. The alarm clock can be set to turn on the radio in the morning instead of an alarm, to wake the owner. 
*''[[Tuner (radio)|Tuner]]'' - A [[high fidelity]] AM/FM radio receiver in a component [[home audio|home audio system]]. It has no speakers but outputs an [[audio signal]] which is fed into the system and played through the system's speakers. 
*''[[Portable radio]]'' - a radio powered by [[battery (electricity)|batteries]] that can be carried with a person. Radios are now often integrated with other audio sources in [[CD player]]s and [[portable media player]]s. Portable radios typically are small enough to be hand held, or, for larger radios, have a handle or carrying strap. Portable radios may have an arrangement for powering from an outlet, conserving the batteries when an outlet is available. Portable "emergency" radios may be solar and/or hand crank powered. <ref>[https://radioattic.com/item.htm?radio=9590006 Solar/Hand Crank Powered Radio]</ref>
**''[[Boom box]]'' - a portable battery-powered [[high fidelity]] stereo sound system in the form of a box with a handle, which became popular during the mid 1970s. 
**''[[Transistor radio]]'' - an older term for a portable pocket-sized broadcast radio receiver. Made possible by the invention of the [[transistor]] and developed in the 1950s, transistor radios were hugely popular during the 1960s and early 1970s, and changed the public's listening habits. 
*''[[Car radio]]'' - A radio integrated into the dashboard of a vehicle, used for entertainment while driving. Virtually all modern cars and trucks are equipped with radios, which usually also includes a [[CD player]].
*''[[Satellite radio]]'' receiver - subscription radio receiver that receives audio programming from a [[direct broadcast satellite]]. The subscriber must pay a monthly fee. They are mostly designed as car radios.
*''[[Shortwave receiver]]'' - This is a broadcast radio that also receives the shortwave bands. It is used for [[shortwave listening]].
* An ''[[AV receiver|AV or Stereo receiver]]'' (in context often just called a ''receiver'') is a component in a [[high fidelity|hi-fi]] or [[home cinema|home theatre]] system combining a radio and audio amplifier in one unit that connects to the speakers and often to other input and output components (eg turntable, television, tape deck, and CD and DVD players)

==Other applications==
Radio receivers are essential components of all systems that use [[radio]]. Besides the broadcast receivers described above, radio receivers are used in a huge variety of electronic systems in modern technology. They can be a separate piece of equipment (a ''radio''), or a subsystem incorporated into other electronic devices. A [[transceiver]] is a [[transmitter]] and receiver combined in one unit. Below is a list of a few of the most common types, organized by function.

*'''Broadcast television reception''' - Televisions receive a [[video signal]] representing a moving image, composed of a sequence of still images, and a synchronized [[audio signal]] representing the associated sound. The [[television channel]] received by a TV occupies a wider [[bandwidth (signal processing)|bandwidth]] than an audio signal, from 600&nbsp;kHz to 6&nbsp;MHz.
**''[[Television receiver|Terrestrial television receiver]]'', ''broadcast television'' or just ''television'' (<span class="ltr">TV</span>) - Televisions contains an integral receiver ([[TV tuner]]) which receives free [[broadcast television]] from local [[television station]]s on [[TV channel]]s in the [[very high frequency|VHF]] and [[ultrahigh frequency|UHF]] bands.
**''[[Satellite TV]]'' receiver - a [[set-top box]] which receives subscription [[direct-broadcast satellite television]], and displays it on an ordinary [[television]]. A rooftop [[satellite dish]] receives many channels all modulated on a [[Ku band|K<sub>u</sub> band]] [[microwave]] [[downlink]] signal from a [[geostationary]] [[direct broadcast satellite]] {{convert|22,000|mi}} above the Earth, and the signal is converted to a lower [[intermediate frequency]] and transported to the box through a coaxial cable. The subscriber pays a monthly fee.
*'''Two-way voice communications''' - A [[two-way radio]] is an audio [[transceiver]], a receiver and [[transmitter]] in the same device, used for bidirectional person-to-person voice communication. The radio link may be [[half-duplex]], using a single radio channel in which only one radio can transmit at a time. so different users take turns talking, pressing a [[push to talk]] button on their radio which switches on the transmitter. Or the radio link may be [[full duplex]], a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone.
**''[[Cellphone]]'' - a portable [[telephone]] that is connected to the [[telephone network]] by radio signals exchanged with a local antenna called a [[cell tower]]. Cellphones have highly automated digital receivers working in the UHF and microwave band that receive the incoming side of the [[duplex (telecommunications)|duplex]] voice channel, as well as a control channel that handles dialing calls and switching the phone between cell towers. They usually also have several other receivers that connect them with other networks: a [[wireless modem|WiFi modem]], a [[bluetooth]] modem, and a [[GPS receiver]]. The cell tower has sophisticated multichannel receivers that receive the signals from many cell phones simultaneously.
**''[[Cordless phone]]'' - a [[landline telephone]] in which the [[handset]] is portable and communicates with the rest of the phone by a short range [[duplex (telecommunications)|duplex]] radio link, instead of being attached by a cord. Both the handset and the [[base station]] have radio receivers operating in the [[ultrahigh frequency|UHF]] band that receive the short range bidirectional [[duplex (telecommunications)|duplex]] radio link.
**''[[Citizens band radio]]'' - a two-way half-duplex radio operating in the 27&nbsp;MHz band that can be used without a license. They are often installed in vehicles and used by truckers and delivery services.
**''[[Walkie-talkie]]'' - a handheld short range half-duplex two-way radio. 
**[[Image:ABOedit2015-19.jpg|thumb|upright=0.5|Handheld scanner]]''[[Scanner (radio)|Scanner]]'' - a receiver that continuously monitors multiple frequencies or [[radio channel]]s by stepping through the channels repeatedly, listening briefly to each channel for a transmission. When a transmitter is found the receiver stops at that channel. Scanners are used to monitor emergency police, fire, and ambulance frequencies, as well as other two way radio frequencies such as [[citizens band]]. Scanning capabilities have also become a standard feature in communications receivers, walkie-talkies, and other two-way radios. 
**[[Image:ICOM RC-9500.jpg|thumb|upright=0.5|Modern communications receiver, ICOM RC-9500]]''[[Communications receiver]]'' or ''[[shortwave receiver]]'' - a general purpose audio receiver covering the [[low frequency|LF]], [[medium frequency|MF]], [[shortwave]] ([[high frequency|HF]]), and [[very high frequency|VHF]] bands. Used mostly with a separate shortwave transmitter for two-way voice communication in communication stations, [[amateur radio]] stations, and for [[shortwave listening]].
*'''One-way (simplex) voice communications'''
**''[[Wireless microphone]]'' receiver - these receive the short range signal from [[wireless microphone]]s used onstage by musical artists, public speakers, and television personalities.
**[[Image:Babymonitor.JPG|thumb|upright=0.5|Baby monitor. The receiver is on the left]]''[[Baby monitor]]'' - this is a cribside appliance for parents of infants that transmits the baby's sounds to a receiver carried by the parents, so they can monitor the baby while they are in other parts of the house. Many baby monitors now have video cameras to show a picture of the baby.
*'''Data communications'''
**''[[Wireless modem|Wireless (WiFi) modem]]'' - an automated short range digital data transmitter and receiver on a portable wireless device that communicates by microwaves with a nearby [[wireless access point|access point]], a [[router (computing)|router]] or gateway, connecting the portable device with a local computer network ([[wireless local area network|WLAN]]) to exchange data with other devices. 
**''[[Bluetooth]]'' modem - a very short range (up to 10 m) 2.4-2.83&nbsp;GHz data transceiver on a portable wireless device used as a substitute for a wire or cable connection, mainly to exchange files between portable devices and connect cellphones and music players with wireless earphones.
**''[[Microwave relay]]'' - a long-distance high bandwidth point-to-point data transmission link consisting of a dish antenna and transmitter that transmits a beam of microwaves to another dish antenna and receiver. Since the antennas must be in [[line-of-sight propagation|line-of-sight]], distances are limited by the visual horizon to 30–40 miles. Microwave links are used for private business data, [[Wide area network|wide area computer networks]] (WANs), and by [[telephone company|telephone companies]] to transmit distance phone calls and television signals between cities.
*'''[[Satellite communication]]s''' - [[Communication satellite]]s are used for data transmission between widely separated points on Earth. Other satellites are used for search and rescue, [[remote sensing]], weather reporting and scientific research. Radio communication with [[satellite]]s and [[spacecraft]] can involve very long path lengths, from 35,786&nbsp;km (22,236&nbsp;mi) for [[geosynchronous]] satellites to billions of kilometers for [[interplanetary spaceflight|interplanetary]] spacecraft. This and the limited power available to a spacecraft transmitter mean very sensitive receivers must be used.
**''[[transponder (satellite communications)|Satellite transponder]]'' - A receiver and transmitter in a [[communications satellite]] that receives multiple data channels carrying long-distance telephone calls, television signals. or internet traffic on a microwave [[uplink]] signal from a [[satellite ground station]] and retransmits the data to another ground station on a different [[downlink]] frequency. In a [[direct broadcast satellite]] the transponder broadcasts a stronger signal directly to [[satellite radio]] or [[satellite television]] receivers in consumer's homes. 
**''[[Satellite ground station]] receiver'' - [[communication satellite]] ground stations receive data from communications satellites orbiting the Earth. Deep space ground stations such as those of the [[NASA Deep Space Network]] receive the weak signals from distant scientific spacecraft on [[interplanetary spaceflight|interplanetary]] exploration missions. These have large [[parabolic antenna|dish antennas]] around 85&nbsp;ft (25 m) in diameter, and extremely sensitive radio receivers similar to [[radio telescope]]s. The [[RF front end]] of the receiver is often [[cryogenic]]ally cooled to −195.79&nbsp;°C (−320&nbsp;°F) by [[liquid nitrogen]] to reduce [[radio noise]] in the circuit.
*'''[[Remote control]]''' - [[Remote control]] receivers receive digital commands that control a device, which may be as complex as a space vehicle or [[unmanned aerial vehicle]], or as simple as a [[garage door opener]]. Remote control systems often also incorporate a [[telemetry]] channel to transmit data on the state of the controlled device back to the controller. [[Radio controlled model]] and other models include multichannel receivers in model cars, boats, airplanes, and helicopters. A short-range radio system is used in [[keyless entry]] systems.
*'''[[Radiolocation]]''' - This is the use of radio waves to determine the location or direction of an object.
**''[[Radar]]'' - a device that transmits a narrow beam of microwaves which reflect from a target back to a receiver, used to locate objects such as aircraft, spacecraft, missiles, ships or land vehicles. The reflected waves from the target are received by a receiver usually connected to the same antenna, indicating the direction to the target. Widely used in aviation, shipping, navigation, weather forecasting, space flight, vehicle [[collision avoidance system]]s, and the military. 
**''[[Global navigation satellite system]]'' (GNSS) receiver, such as a [[GPS receiver]] used with the US [[Global Positioning System]] - the most widely used electronic navigation device. An automated digital receiver that receives simultaneous data signals from several satellites in low Earth orbit. Using extremely precise time signals it calculates the distance to the satellites, and from this the receiver's location on Earth. GNSS receivers are sold as portable devices, and are also incorporated in cell phones, vehicles and weapons, even [[artillery shell]]s. 
**''[[VHF omnidirectional range|VOR]]'' receiver - navigational instrument on an aircraft that uses the VHF signal from [[VHF omnidirectional range|VOR]] navigational beacons between 108 and 117.95&nbsp;MHz to determine the direction to the beacon very accurately, for air navigation.
**''[[Animal migration tracking#Radio tracking|Wild animal tracking]]'' receiver - a receiver with a directional antenna used to track wild animals which have been tagged with a small VHF transmitter, for [[wildlife management]] purposes.
*'''Other'''
**''[[Telemetry]]'' receiver - this receives data signals to monitor conditions of a process. Telemetry is used to monitor missile and spacecraft in flight, [[well logging]] during [[oil and gas drilling]], and unmanned scientific instruments in remote locations.
**''[[Measuring receiver]]'' - a calibrated, laboratory grade radio receiver used to measure the characteristics of radio signals. Often incorporates a [[spectrum analyzer]].
**''[[Radio telescope]]'' - specialized antenna and radio receiver used as a scientific instrument to study weak radio waves from [[astronomical radio source]]s in space like stars, nebulas and galaxies in [[radio astronomy]]. They are the most sensitive radio receivers that exist, having large [[parabolic antenna|parabolic (dish) antennas]] up to 500 meters in diameter, and extremely sensitive radio circuits. The [[RF front end]] of the receiver is often [[cryogenic]]ally cooled by [[liquid nitrogen]] to reduce [[radio noise]].

==How receivers work==
{{see also|Radio receiver design}}
[[File:Antenna schematic symbol.svg|thumb|upright=0.25|Symbol for an antenna]]
A radio receiver is connected to an [[antenna (radio)|antenna]] which converts some of the energy from the incoming radio wave into a tiny [[radio frequency]] AC [[voltage]] which is applied to the receiver's input. An antenna typically consists of an arrangement of metal conductors. The oscillating [[electric field|electric]] and [[magnetic field]]s of the radio wave push the [[electron]]s in the antenna back and forth, creating an oscillating voltage.

The [[Antenna (radio)|antenna]] may be enclosed inside the receiver's case, as with the [[loopstick antenna|ferrite loop antennas]] of [[AM radio]]s and the flat [[inverted F antenna]] of cell phones; attached to the outside of the receiver, as with [[whip antenna]]s used on [[FM radio]]s, or mounted separately and connected to the receiver by a cable, as with rooftop [[television antenna]]s and [[satellite dish]]es.

===Main functions of a receiver===
Practical radio receivers perform three basic functions on the signal from the antenna: [[bandpass filter|filtering]], [[amplifier|amplification]], and [[demodulation]]:<ref name="Ganguly">{{cite book
 | last1  = Ganguly
 | first1 = Partha Kumar
 | title  = Principles of Electronics
 | publisher = PHI Learning Pvt. Ltd.
 | date   = 2015
 | pages  = 286–289
 | url    = https://books.google.com/books?id=8fmUCgAAQBAJ&q=%22radio+receiver%22+filter+amplification+demodulation&pg=PA287
 | isbn   = 978-8120351240
 }}</ref>

==== Bandpass filtering ====
{{main|Bandpass filter}}
[[Image:Bandpass filter symbol.svg|thumb|upright=0.5|Symbol for a bandpass filter used in [[block diagram]]s of radio receivers]]

Radio waves from many transmitters pass through the air simultaneously without interfering with each other and are received by the antenna. These can be separated in the receiver because they have different [[frequency|frequencies]]; that is, the radio wave from each transmitter oscillates at a different rate. To separate out the desired radio signal, the [[bandpass filter]] allows the frequency of the desired radio transmission to pass through, and blocks signals at all other frequencies.

The bandpass filter consists of one or more [[resonant circuit]]s (tuned circuits). The resonant circuit is connected between the antenna input and ground. When the incoming radio signal is at the resonant frequency, the resonant circuit has high impedance and the radio signal from the desired station is passed on to the following stages of the receiver. At all other frequencies the resonant circuit has low impedance, so signals at these frequencies are conducted to ground.
*'''''Bandwidth and selectivity''''': See graphs. The information ([[modulation]]) in a radio transmission is contained in two narrow bands of frequencies called [[sideband]]s ''(<span class="ltr">SB</span>)'' on either side of the [[carrier wave|carrier]] frequency ''(<span class="ltr">C</span>)'', so the filter has to pass a band of frequencies, not just a single frequency. The band of frequencies received by the receiver is called its ''[[passband]]'' ''(<span class="ltr">PB</span>)'', and the width of the passband in [[kilohertz]] is called the [[bandwidth (signal processing)|bandwidth]] ''(<span class="ltr">BW</span>)''. The bandwidth of the filter must be wide enough to allow the sidebands through without distortion, but narrow enough to block any interfering transmissions on adjacent frequencies (such as ''S2'' in the diagram). The ability of the receiver to reject unwanted radio stations near in frequency to the desired station is an important parameter called ''[[Selectivity (radio)|selectivity]]'' determined by the filter. In modern receivers [[crystal filter|quartz crystal]], [[ceramic resonator]], or [[surface acoustic wave]] (SAW) filters are often used which have sharper selectivity compared to networks of capacitor-inductor tuned circuits. 
*'''''Tuning''''': To select a particular station the radio is "''tuned''" to the frequency of the desired transmitter. The radio has a dial or digital display showing the frequency it is tuned to. ''Tuning'' is adjusting the frequency of the receiver's passband to the frequency of the desired radio transmitter. Turning the tuning knob changes the [[resonant frequency]] of the [[tuned circuit]]. When the resonant frequency is equal to the radio transmitter's frequency the tuned circuit oscillates in sympathy, passing the signal on to the rest of the receiver.
{{multiple image
| align = center
| direction = horizontal
| header   =
| image1   = Modulated radio signal frequency spectrum.svg
| caption1 = The [[frequency spectrum]] of a typical radio signal from an AM or FM radio transmitter. It consists of a component (<span class="ltr">C</span>) at the [[carrier wave]] frequency ''f''<sub>C</sub>, with the modulation contained in narrow frequency bands called [[sideband]]s (<span class="ltr">SB</span>) just above and below the carrier.
| width1   = 350
| image2   = Bandpass filtering in a radio receiver.svg
| caption2 = How the bandpass filter selects a single radio signal ''S1'' from all the radio signals ''S2, S3 ...'' received by the antenna. From top, the graphs show the voltage from the antenna applied to the filter ''V''<sub>in</sub>, the [[transfer function]] of the filter ''T'', and the voltage at the output of the filter ''V''<sub>out</sub> as a function of frequency ''f''. The transfer function ''T'' is the amount of signal that gets through the filter at each frequency:<br /><math>V_\text{out}(<span class="ltr">f</span>) = \text{T}(<span class="ltr">f</span>) V_\text{in}(<span class="ltr">f</span>)</math>
| width2   = 250
| footer   = 
}}
{{breakafterimages}}

==== Amplification ====
{{main|Amplifier}}
[[Image:Amplifier symbol.svg|thumb|upright=0.5|Symbol for an [[amplifier]]]]
The power of the radio waves picked up by a receiving antenna decreases with the square of its distance from the transmitting antenna. Even with the powerful transmitters used in radio broadcasting stations, if the receiver is more than a few miles from the transmitter the power intercepted by the receiver's antenna is very small, perhaps as low as [[picowatt]]s or [[femtowatt]]s. To increase the power of the recovered signal, an [[amplifier]] circuit uses electric power from batteries or the wall plug to increase the [[amplitude]] (voltage or current) of the signal. In most modern receivers, the electronic components which do the actual amplifying are [[transistor]]s.

Receivers usually have several stages of amplification: the radio signal from the bandpass filter is amplified to make it powerful enough to drive the demodulator, then the audio signal from the demodulator is amplified to make it powerful enough to operate the speaker. The degree of amplification of a radio receiver is measured by a parameter called its ''[[sensitivity (radio receiver)|sensitivity]]'', which is the minimum signal strength of a station at the antenna, measured in [[microvolt]]s, necessary to receive the signal clearly, with a certain [[signal-to-noise ratio]]. Since it is easy to amplify a signal to any desired degree, the limit to the sensitivity of many modern receivers is not the degree of amplification but random [[electronic noise]] present in the circuit, which can drown out a weak radio signal.

==== Demodulation ====
{{main|Demodulation}}
[[Image:Demodulator symbol.svg|thumb|upright=0.5|Symbol for a demodulator]] 
After the radio signal is filtered and amplified, the receiver must extract the information-bearing [[modulation]] signal from the modulated radio frequency [[carrier wave]]. This is done by a circuit called a [[demodulator]] ([[detector (radio)|detector]]). Each type of modulation requires a different type of demodulator
*an AM receiver that receives an ([[amplitude modulation|amplitude modulated]]) radio signal uses an AM demodulator
*an FM receiver that receives a [[frequency modulation|frequency modulated]] signal uses an FM demodulator
*an FSK receiver which receives [[frequency-shift keying]] (used to transmit digital data in wireless devices) uses an FSK demodulator 
Many other types of modulation are also used for specialized purposes.

The modulation signal output by the demodulator is usually amplified to increase its strength, then the information is converted back to a human-usable form by some type of [[transducer]]. An [[audio signal]], representing sound, as in a broadcast radio, is converted to [[sound wave]]s by an [[earphone]] or [[loudspeaker]]. A [[video signal]], representing moving images, as in a [[television receiver]], is converted to light by a [[display device|display]]. [[Digital data]], as in a [[wireless modem]], is applied as input to a [[computer]] or [[microprocessor]], which interacts with human users.

:'''AM demodulation'''
{{main|Envelope detector}}
[[Image:Envelope detector circuit.svg|thumb|Envelope detector circuit]]
[[Image:Amplitude modulation detection.png|thumb|How an envelope detector works]]
:The easiest type of demodulation to understand is AM demodulation, used in [[AM radio]]s to recover the [[audio signal|audio]] modulation signal, which represents sound and is converted to [[sound wave]]s by the radio's [[loudspeaker|speaker]]. It is accomplished by a circuit called an [[envelope detector]] ''(see circuit)'', consisting of a [[diode]] ''(<span class="ltr">D</span>)'' with a bypass [[capacitor]] ''(<span class="ltr">C</span>)'' across its output.

:See graphs. The [[Amplitude modulation|amplitude modulated]] radio signal from the tuned circuit is shown at ''<span style="color:red;">(<span class="ltr">A</span>)</span>''. The rapid oscillations are the [[radio frequency]] [[carrier wave]]. The [[audio signal]] (the sound) is contained in the slow variations ([[modulation]]) of the [[amplitude]] (size) of the waves. If it was applied directly to the speaker, this signal cannot be converted to sound, because the audio excursions are the same on both sides of the axis, averaging out to zero, which would result in no net motion of the speaker's diaphragm. ''<span style="color:red;">(<span class="ltr">B</span>)</span>'' When this signal is applied as input ''V''<sub>I</sub> to the detector, the diode ''(<span class="ltr">D</span>)'' conducts current in one direction but not in the opposite direction, thus allowing through pulses of current on only one side of the signal. In other words, it [[rectifier|rectifies]] the AC current to a pulsing DC current. The resulting voltage ''V''<sub>O</sub> applied to the load ''R''<sub>L</sub> no longer averages zero; its peak value is proportional to the audio signal. ''<span style="color:red;">(<span class="ltr">C</span>)</span>'' The bypass capacitor ''(<span class="ltr">C</span>)'' is charged up by the current pulses from the diode, and its voltage follows the peaks of the pulses, the envelope of the audio wave. It performs a smoothing ([[low pass filter]]ing) function, removing the radio frequency carrier pulses, leaving the low frequency audio signal to pass through the load ''R''<sub>L</sub>. The audio signal is amplified and applied to earphones or a speaker.
{{breakafterimages}}

=== Tuned radio frequency (TRF) receiver ===
{{Main|Tuned radio frequency receiver}}

[[File:Tuned radio frequency (TRF) receiver block diagram 2.svg|thumb|upright=2.0|Block diagram of a tuned radio frequency receiver. To achieve enough [[Selectivity (radio)|selectivity]] to reject stations on adjacent frequencies, multiple cascaded bandpass filter stages had to be used. The dotted line indicates that the bandpass filters must be tuned together.]]

In the simplest type of radio receiver, called a [[tuned radio frequency receiver|tuned radio frequency (TRF) receiver]], the three functions above are performed consecutively:<ref name="Rudersdorfer1">{{cite book
 | last1  = Rudersdorfer
 | first1 = Ralf 
 | title  = Radio Receiver Technology: Principles, Architectures and Applications
 | publisher = John Wiley and Sons
 | date   = 2013
 | url    = https://books.google.com/books?id=6aRMAgAAQBAJ&pg=PT14
 | isbn   = 978-1118647844
 }} Chapter 1</ref> (<span class="ltr">1</span>) the mix of radio signals from the antenna is filtered to extract the signal of the desired transmitter; (<span class="ltr">2</span>) this oscillating voltage is sent through a [[radio frequency]] (<span class="ltr">RF</span>) [[amplifier]] to increase its strength to a level sufficient to drive the demodulator; (<span class="ltr">3</span>) the demodulator recovers the [[modulation]] signal (which in broadcast receivers is an [[audio signal]], a voltage oscillating at an [[audio frequency]] rate representing the sound waves) from the modulated radio [[carrier wave]]; (<span class="ltr">4</span>) the modulation signal is amplified further in an [[audio amplifier]], then is applied to a [[loudspeaker]] or [[earphone]] to convert it to sound waves.

Although the TRF receiver is used in a few applications, it has practical disadvantages which make it inferior to the superheterodyne receiver below, which is used in most applications.<ref name="Rudersdorfer1" /> The drawbacks stem from the fact that in the TRF the filtering, amplification, and demodulation are done at the high frequency of the incoming radio signal. The bandwidth of a filter increases with its center frequency, so as the TRF receiver is tuned to different frequencies its bandwidth varies. Most important, the increasing congestion of the [[radio spectrum]] requires that radio channels be spaced very close together in frequency. It is extremely difficult to build filters operating at radio frequencies that have a narrow enough bandwidth to separate closely spaced radio stations. TRF receivers typically must have many cascaded tuning stages to achieve adequate selectivity. The [[#Advantages|Advantages]] section below describes how the superheterodyne receiver overcomes these problems.

=== The superheterodyne design ===
{{Main|Superheterodyne receiver}}
[[File:Superheterodyne receiver block diagram 2.svg|thumb|upright=1.8|Block diagram of a superheterodyne receiver. The dotted line indicates that the RF filter and local oscillator must be tuned in tandem.]]

The [[superheterodyne]] receiver, invented in 1918 by [[Edwin Armstrong]]<ref name="Armstrong1">{{cite journal
  | last1  = Armstrong
  | first1 = Edwin H.
  | title = A new system of radio frequency amplification
  | journal = Proceedings of the Institute of Radio Engineers
  | volume = 9
  | issue = 1
  | pages = 3–11
  | date = February 1921
  | url = https://books.google.com/books?id=rk1JAQAAIAAJ&pg=RA1-PA5
  | access-date = December 23, 2015}}</ref> is the design used in almost all modern receivers<ref name="Lee2" /><ref name="Rudersdorfer1" /><ref name="Dixon" /><ref name="Williams1">[https://books.google.com/books?id=XiKgKdeBi6cC&dq=superheterodyne&pg=PA28 Williams, Lyle Russell (2006) ''The New Radio Receiver Building Handbook'', p. 28-30]</ref> except a few specialized applications.

In the superheterodyne, the radio frequency signal from the antenna is shifted down to a lower "[[intermediate frequency]]" (<span class="ltr">IF</span>), before it is processed.<ref name="ArmyManual5" /><ref name="McNicol15" /><ref name="Terman8">[http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Radio-Engineer%27s-Handbook-1.pdf Terman, Frederick E. (1943) ''Radio Engineers' Handbook'', p. 636-638]</ref><ref name="CarrHandbook1">{{cite book
 | last1  = Carr
 | first1 = Joseph J. 
 | title  = The Technician's Radio Receiver Handbook: Wireless and Telecommunication Technology
 | publisher = Newnes
 | date   = 2001
 | pages  = 8–11
 | url    = https://books.google.com/books?id=PNnJnWMQCNkC&pg=PA8
 | isbn   = 978-0750673198
 }}</ref> The incoming radio frequency signal from the antenna is mixed with an unmodulated signal generated by a ''[[local oscillator]]'' (<span class="ltr">LO</span>) in the receiver. The mixing is done in a nonlinear circuit called the "''[[frequency mixer|mixer]]''". The result at the output of the mixer is a [[heterodyne]] or beat frequency at the difference between these two frequencies. The process is similar to the way two musical notes at different frequencies played together produce a [[beat (acoustics)|beat note]]. This lower frequency is called the ''[[intermediate frequency]]'' (<span class="ltr">IF</span>). The IF signal also has the [[modulation]] [[sideband]]s that carry the information that was present in the original RF signal. The IF signal passes through filter and amplifier stages,<ref name="Dixon" /> then is [[demodulate]]d in a detector, recovering the original modulation.

The receiver is easy to tune; to receive a different frequency it is only necessary to change the local oscillator frequency. The stages of the receiver after the mixer operates at the fixed intermediate frequency (<span class="ltr">IF</span>) so the IF bandpass filter does not have to be adjusted to different frequencies. The fixed frequency allows modern receivers to use sophisticated [[quartz crystal]], [[ceramic resonator]], or [[surface acoustic wave]] (SAW) IF filters that have very high [[Q factor]]s, to improve selectivity.

The RF filter on the front end of the receiver is needed to prevent interference from any radio signals at the [[image frequency]]. Without an input filter the receiver can receive incoming RF signals at two different frequencies,.<ref name="Rembovsky">{{cite book
 | last1  = Rembovsky
 | first1 = Anatoly
 | last2  = Ashikhmin
 | first2 = Alexander
 | last3  = Kozmin
 | first3 = Vladimir
 | title  = Radio Monitoring: Problems, Methods and Equipment
 | publisher = Springer Science and Business Media
 | date   = 2009
 | pages  = 26
 | url    = https://books.google.com/books?id=2ra1lg9MCLgC&q=selectivity+sensitivity+image&pg=PA26
 | isbn   = 978-0387981000
 |display-authors=etal}}</ref><ref name="Williams1" /><ref name="CarrHandbook1" /><ref name="Terman9">[http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Radio-Engineer%27s-Handbook-1.pdf Terman, Frederick E. (1943) ''Radio Engineers' Handbook'', p. 645]</ref> The receiver can be designed to receive on either of these two frequencies; if the receiver is designed to receive on one, any other radio station or radio noise on the other frequency may pass through and interfere with the desired signal. A single tunable RF filter stage rejects the image frequency; since these are relatively far from the desired frequency, a simple filter provides adequate rejection. Rejection of interfering signals much closer in frequency to the desired signal is handled by the multiple sharply-tuned stages of the intermediate frequency amplifiers, which do not need to change their tuning.<ref name="Williams1" /> This filter does not need great selectivity, but as the receiver is tuned to different frequencies it must "track" in tandem with the local oscillator. The RF filter also serves to limit the bandwidth applied to the RF amplifier, preventing it from being overloaded by strong out-of-band signals.

[[File:Double-conversion superheterodyne receiver block diagram.svg|thumb|upright=2.1|Block diagram of a dual-conversion superheterodyne receiver]]

To achieve both good image rejection and selectivity, many modern superhet receivers use two intermediate frequencies; this is called a ''[[dual-conversion (superhet)|dual-conversion]]'' or ''double-conversion'' superheterodyne.<ref name="Rudersdorfer1" /> The incoming RF signal is first mixed with one local oscillator signal in the first mixer to convert it to a high IF frequency, to allow efficient filtering out of the image frequency, then this first IF is mixed with a second local oscillator signal in a second mixer to convert it to a low IF frequency for good bandpass filtering. Some receivers even use [[triple-conversion (superhet)|triple-conversion]].

At the cost of the extra stages, the superheterodyne receiver provides the advantage of greater selectivity than can be achieved with a TRF design. Where very high frequencies are in use, only the initial stage of the receiver needs to operate at the highest frequencies; the remaining stages can provide much of the receiver gain at lower frequencies which may be easier to manage. Tuning is simplified compared to a multi-stage TRF design, and only two stages need to track over the tuning range. The total amplification of the receiver is divided between three amplifiers at different frequencies; the RF, IF, and audio amplifier. This reduces problems with feedback and [[parasitic oscillation]]s that are encountered in receivers where most of the amplifier stages operate at the same frequency, as in the TRF receiver.<ref name="ArmyManual5" />

The most important advantage is that better [[Selectivity (radio)|selectivity]] can be achieved by doing the filtering at the lower intermediate frequency.<ref name="Rudersdorfer1" /><ref name="Dixon" /><ref name="ArmyManual5" /> One of the most important parameters of a receiver is its [[bandwidth (signal processing)|bandwidth]], the band of frequencies it accepts. In order to reject nearby interfering stations or noise, a narrow bandwidth is required. In all known filtering techniques, the bandwidth of the filter increases in proportion with the frequency, so by performing the filtering at the lower <math>f_\text{IF}</math>, rather than the frequency of the original radio signal <math>f_\text{RF}</math>, a narrower bandwidth can be achieved. Modern FM and television broadcasting, cellphones and other communications services, with their narrow channel widths, would be impossible without the superheterodyne.<ref name="Dixon" />

===Automatic gain control (AGC)===
{{main|Automatic gain control}}
The [[signal strength]] ([[amplitude]]) of the radio signal from a receiver's antenna varies drastically, by orders of magnitude, depending on how far away the radio transmitter is, how powerful it is, and [[radio propagation|propagation]] conditions along the path of the radio waves.<ref name="Drentea">{{cite book
 | last1  = Drentea
 | first1 = Cornell 
 | title  = Modern Communications Receiver Design and Technology
 | publisher = Artech House 
 | date   = 2010
 | pages  = 325–330
 | url    = https://books.google.com/books?id=9juUwbKP-58C&q=%22automatic+gain+control%22&pg=PA339
 | isbn   = 978-1596933101
 }}</ref>  The strength of the signal received from a given transmitter varies with time due to changing propagation conditions of the path through which the radio wave passes, such as [[multipath interference]]; this is called ''[[fading]]''.<ref name="Drentea" /><ref name="Rudersdorfer1" /> In an AM receiver, the amplitude of the audio signal from the detector, and the sound volume, is proportional to the amplitude of the radio signal, so fading causes variations in the volume. In addition as the receiver is tuned between strong and weak stations, the volume of the sound from the speaker would vary drastically. Without an automatic system to handle it, in an AM receiver, constant adjustment of the volume control would be required.

With other types of modulation like FM or FSK the amplitude of the modulation does not vary with the radio signal strength, but in all types the demodulator requires a certain range of signal amplitude to operate properly.<ref name="Rudersdorfer1" /><ref name="JonHagen">{{cite book
 | last1  = Hagen
 | first1 = Jon B. 
 | title  = Radio-Frequency Electronics: Circuits and Applications
 | publisher = Cambridge Univ. Press
 | date   = 1996
 | pages  = 60
 | url    = https://books.google.com/books?id=QtJ5tNdlyYAC&q=%22automatic+gain+control%22&pg=PA60
 | isbn   = 978-0521553568
 }}</ref>  Insufficient signal amplitude will cause an increase of noise in the demodulator, while excessive signal amplitude will cause amplifier stages to overload (saturate), causing distortion (clipping) of the signal.

Therefore, almost all modern receivers include a [[feedback]] [[control system]] which monitors the ''average'' level of the radio signal at the detector, and adjusts the [[amplifier gain|gain]] of the amplifiers to give the optimum signal level for demodulation.<ref name="Rudersdorfer1" /><ref name="JonHagen" /><ref name="Drentea" /> This is called [[automatic gain control]] (AGC). AGC can be compared to the [[dark adaptation]] mechanism in the [[human eye]]; on entering a dark room the gain of the eye is increased by the iris opening.<ref name="Drentea" /> In its simplest form, an AGC system consists of a [[rectifier]] which converts the RF signal to a varying DC level, a [[lowpass filter]] to smooth the variations and produce an average level.<ref name="JonHagen" /> This is applied as a control signal to an earlier amplifier stage, to control its gain. In a superheterodyne receiver, AGC is usually applied to the [[intermediate frequency|IF amplifier]], and there may be a second AGC loop to control the gain of the RF amplifier to prevent it from overloading, too.

In certain receiver designs such as modern digital receivers, a related problem is [[DC offset]] of the signal. This is corrected by a similar feedback system.

==History==
{{Main|History of radio}}

Radio waves were first identified in German physicist [[Heinrich Hertz]]'s 1887 series of experiments to prove James Clerk Maxwell's [[electromagnetic theory]]. Hertz used spark-excited dipole antennas to generate the waves and micrometer [[spark gap]]s attached to [[dipole antenna|dipole]] and [[loop antenna]]s to detect them.<ref name="Lee">{{cite book
  | last = Lee
  | first = Thomas H.
  | title = The Design of CMOS Radio-Frequency Integrated Circuits, 2nd Ed.
  | publisher = Cambridge University Press
  | date = 2004
  | location = UK
  | pages = 1–8
  | url = https://books.google.com/books?id=io1hL48OqBsC&pg=PA2 
  | isbn = 978-0521835398}}</ref><ref name="Appleyard">{{cite journal
  | last1  = Appleyard
  | first1 = Rollo
  | title = Pioneers of Electrical Communication part 5 - Heinrich Rudolph Hertz
  | journal = Electrical Communication
  | volume = 6
  | issue = 2
  | pages = 67
  | date = October 1927
  | url = http://www.americanradiohistory.com/Archive-ITT/20s/ITT-Vol-06-1927-02.pdf
  | access-date = December 19, 2015}}</ref><ref name="Phillips1">{{cite book
 | last1  = Phillips
 | first1  = Vivian J.
 | title  = Early Radio Wave Detectors
 | publisher  = Inst. of Electrical Engineers
 | date  = 1980
 | location  = London
 | pages  = [https://archive.org/details/EarlyRadioWaveDetectors/page/4 4–12]
 | url  = https://archive.org/details/EarlyRadioWaveDetectors/page/4
 | isbn  = 978-0906048245
 }}</ref>  These primitive devices are more accurately described as radio wave sensors, not "receivers", as they could only detect radio waves within about 100 feet of the transmitter, and were not used for communication but instead as laboratory instruments in scientific experiments.

===Spark era===
{{Main|Wireless telegraphy}}
[[File:Guglielmo Marconi 1901 wireless signal.jpg|thumb|upright=1.5|[[Guglielmo Marconi]], who built the first radio receivers, with his early spark transmitter ''(right)'' and coherer receiver ''(left)'' from the 1890s. The receiver records the Morse code on paper tape]]
[[File:Crystal radio receiver block diagram.svg|thumb|Generic block diagram of an unamplified radio receiver from the wireless telegraphy era<ref name="Rudersdorfer2">{{cite book
 | last1  = Rudersdorfer
 | first1 = Ralf 
 | title  = Radio Receiver Technology: Principles, Architectures and Applications
 | publisher = John Wiley and Sons
 | date   = 2013
 | pages  = 1–2
 | url    = https://books.google.com/books?id=6aRMAgAAQBAJ&pg=PT14
 | isbn   = 978-1118647844
 }}</ref>]]
[[File:Radiotelegraph syphon recorder tape 1922.png|thumb|upright=1.6|Example of transatlantic radiotelegraph message recorded on paper tape by a [[siphon recorder]] at RCA's New York receiving center in 1920. The translation of the Morse code is given below the tape.]]

The first [[radio transmitter]]s, used during the initial three decades of radio from 1887 to 1917, a period called the ''spark era'', were [[spark gap transmitter]]s which generated radio waves by discharging a [[capacitance]] through an [[electric spark]].<ref name="Nahin">{{cite book
 | last1  = Nahin
 | first1 = Paul J.
 | title  = The Science of Radio: With Matlab and Electronics Workbench Demonstration, 2nd Ed.
 | publisher = Springer Science & Business Media 
 | date   = 2001
 | pages  = 45–48
 | url    = https://books.google.com/books?id=V1GBW6UD4CcC&pg=PA45
 | isbn   = 978-0387951508
 }}</ref><ref name="Coe2">{{cite book
 | last1  = Coe
 | first1 = Lewis 
 | title  = Wireless Radio: A History
 | publisher = McFarland
 | date   = 2006
 | pages  = 3–8
 | url    = https://books.google.com/books?id=W1JAeg1PiWIC&pg=PA3
 | isbn   = 978-0786426621
 }}</ref><ref name="McNicol14">{{cite book
 | last1  = McNicol
 | first1 = Donald
 | title  = Radio's Conquest of Space
 | publisher = Murray Hill Books
 | date   = 1946
 | pages  = 57–68
 | isbn = 9780405060526
 | url    = https://archive.org/stream/radiosconquestof00mcnirich#page/57/mode/2up
 }}</ref>  Each spark produced a transient pulse of radio waves which decreased rapidly to zero.<ref name="Lee" /><ref name="Phillips1" /> These [[damped wave]]s could not be modulated to carry sound, as in modern [[amplitude modulation|AM]] and [[frequency modulation|FM]] transmission. So spark transmitters could not transmit sound, and instead transmitted information by [[radiotelegraphy]]. The transmitter was switched on and off rapidly by the operator using a [[telegraph key]], creating different length pulses of damped radio waves ("dots" and "dashes") to spell out text messages in [[Morse code]].<ref name="Phillips1" /><ref name="Coe2" />

Therefore, the first radio receivers did not have to extract an audio signal from the radio wave like modern receivers, but just detected the presence of the radio signal, and produced a sound during the "dots" and "dashes".<ref name="Phillips1" /> The device which did this was called a "''[[detector (radio)|detector]]''". Since there were no [[amplifier|amplifying]] devices at this time, the sensitivity of the receiver mostly depended on the detector. Many different detector devices were tried. Radio receivers during the spark era consisted of these parts:<ref name="Rudersdorfer1" />

*An ''[[antenna (radio)|antenna]]'', to intercept the radio waves and convert them to tiny radio frequency [[electric current]]s.
*A ''[[resonant circuit|tuned circuit]]'', consisting of a capacitor connected to a coil of wire, which acted as a [[bandpass filter]] to select the desired signal out of all the signals picked up by the antenna.   Either the capacitor or coil was adjustable to tune the receiver to the frequency of different transmitters. The earliest receivers, before 1897, did not have tuned circuits, they responded to all radio signals picked up by their antennas, so they had little frequency-discriminating ability and received any transmitter in their vicinity.<ref name="Carr" /> Most receivers used a pair of tuned circuits with their coils [[inductive coupling|magnetically coupled]], called a [[resonant transformer]] (oscillation transformer) or "loose coupler".
*A ''[[detector (radio)|detector]]'', which produced a pulse of DC current for each damped wave received.
*An indicating device such as an ''[[earphone]]'', which converted the pulses of current into sound waves. The first receivers used an [[electric bell]] instead. Later receivers in commercial wireless systems used a Morse [[siphon recorder]],<ref name="Lee" /> which consisted of an ink pen mounted on a needle swung by an [[electromagnet]] (a [[galvanometer]]) which drew a line on a moving [[paper tape]]. Each string of damped waves constituting a Morse "dot" or "dash" caused the needle to swing over, creating a displacement of the line, which could be read off the tape. With such an automated receiver a radio operator did not have to continuously monitor the receiver.
The signal from the spark gap transmitter consisted of damped waves repeated at an audio frequency rate, from 120 to perhaps 4000 per second, so in the earphone the signal sounded like a musical tone or buzz, and the Morse code "dots" and "dashes" sounded like beeps.

The first person to use radio waves for ''communication'' was [[Guglielmo Marconi]].<ref name="Coe2" /><ref name="Beauchamp1">{{cite book
 | last1  = Beauchamp
 | first1 = Ken 
 | title  = History of Telegraphy
 | publisher = IET 
 | date   = 2001
 | pages  = 184–186 
 | url    = https://books.google.com/books?id=k3XCkncd83AC&pg=PA185
 | isbn   = 978-0852967928
 }}</ref>  Marconi invented little himself, but he was first to believe that radio could be a practical communication medium, and singlehandedly developed the first [[wireless telegraphy]] systems, transmitters and receivers, beginning in 1894–5,<ref name="Beauchamp1" /> mainly by improving technology invented by others.<ref name="Coe2" /><ref name="Nahin1" /><ref name="Klooster">{{cite book
 | last1  = Klooster
 | first1 = John W.
 | title  = Icons of Invention
 | publisher = ABC-CLIO
 | date   = 2007
 | pages  = 159–161
 | url    = https://books.google.com/books?id=WKuG-VIwID8C&q=Tesla&pg=PA161
 | isbn   = 978-0313347436
 }}</ref><ref name="McNicol">{{cite book
 | last1  = McNicol
 | first1 = Donald
 | title  = Radio's Conquest of Space
 | publisher = Murray Hill Books
 | date   = 1946
 | pages  = 37–45
 | isbn = 9780405060526
 | url    = https://archive.org/stream/radiosconquestof00mcnirich#page/38/mode/2up
 }}</ref>
<ref name="Hong1">{{cite book
 | last1  = Hong
 | first1 = Sungook 
 | title  = Wireless: From Marconi's Black-box to the Audion
 | publisher = MIT Press
 | date   = 2001
 | pages  = 1–2
 | url    = https://books.google.com/books?id=UjXGQSPXvIcC&pg=PA2
 | isbn   = 978-0262082983
 }}</ref><ref name="Sarkar3">[https://books.google.com/books?id=NBLEAA6QKYkC&pg=PA353 Sarkar et al. (2006) ''History of Wireless'', p. 349-358], [http://www.ee-techs.com/techbook/wireless-3.pdf archive] {{webarchive|url=http://arquivo.pt/wayback/20160517072919/http://www.ee-techs.com/techbook/wireless-3.pdf |date=2016-05-17 }}</ref> [[Oliver Lodge]] and [[Alexander Stepanovich Popov|Alexander Popov]] were also experimenting with similar radio wave receiving apparatus at the same time in 1894–5,<ref name="Klooster" /><ref name="Fleming1">{{cite book
 | last1  = Fleming
 | first1 = John Ambrose
 | title  = The Principles of Electric Wave Telegraphy and Telephony, 2nd Ed.
 | publisher = Longmans, Green and Co.
 | date   = 1910
 | location = London
 | pages  = 420–428
 | url    = https://books.google.com/books?id=qQFVAAAAMAAJ&pg=PA420
 }}</ref> but they are not known to have transmitted Morse code during this period,<ref name="Coe2" /><ref name="Beauchamp1" /> just strings of random pulses. Therefore, Marconi is usually given credit for building the first radio receivers.

====Coherer receiver====
{{Main|Coherer}}

{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Marconi's Coherer Receiver at Oxford Museum History of Science (cropped).jpg
| caption1 = One of Marconi's first coherer receivers, used in his "black box" demonstration at Toynbee Hall, London, 1896. The coherer is at right, with the "tapper" just behind it, The relay is at left, batteries are in background
| width1   = 160
| image2   = Radiotelegraph receiver from 1914.jpg
| caption2 = A typical commercial radiotelegraphy receiver from the first decade of the 20th century. The [[coherer]] ''(right)'' detects the pulses of radio waves, and the "dots" and "dashes" of [[Morse code]] were recorded in ink on paper tape by a [[siphon recorder]] ''(left)'' and transcribed later.
| width2   = 375
}}
<gallery mode=nolines widths="180px" >
Rx marconi.jpg|Circuit of Marconi's first coherer radio receiver from 1896
Cohere(Ag-Ni).jpg|Coherer from 1904 as developed by Marconi.
</gallery>
The first radio receivers invented by Marconi, [[Oliver Lodge]] and [[Alexander Stepanovich Popov|Alexander Popov]] in 1894-5 used a primitive radio wave [[detector (radio)|detector]] called a [[coherer]], invented in 1890 by [[Edouard Branly]] and improved by Lodge and Marconi.<ref name="Lee" /><ref name="Coe2" /><ref name="Carr">{{cite book
 | last1  = Carr
 | first1 = Joseph 
 | title  = Old Time Radios! Restoration and Repair
 | publisher = McGraw-Hill Professional
 | date   = 1990
 | pages  = 5–13
 | url    = https://books.google.com/books?id=4dxDsYuFdTkC&pg=PA5
 | isbn   = 978-0071507660
 }}</ref><ref name="Klooster" /><ref name="Fleming1"/><ref name="Stone1">{{cite book
 | last1  = Stone
 | first1 = Ellery W. 
 | title  = Elements of Radiotelegraphy
 | publisher = D. Van Nostrand Co.
 | date   = 1919
 | pages  = [https://archive.org/details/elementsradiote01stongoog/page/n253 203]–208
 | url    = https://archive.org/details/elementsradiote01stongoog
 }}</ref><ref name="Phillips3">[https://archive.org/stream/EarlyRadioWaveDetectors/Phillips-EarlyRadioWaveDetectors#page/n16/mode/1up Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 18-21]</ref> The coherer was a glass tube with metal electrodes at each end, with loose metal powder between the electrodes.<ref name="Lee" /><ref name="Coe2" /><ref name="McNicol2">[https://archive.org/stream/radiosconquestof00mcnirich#page/114/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 107-113]</ref> It initially had a high [[electrical resistance|resistance]]. When a radio frequency voltage was applied to the electrodes, its resistance dropped and it conducted electricity. In the receiver the coherer was connected directly between the antenna and ground. In addition to the antenna, the coherer was connected in a [[direct current|DC]] circuit with a [[Battery (electricity)|battery]] and [[relay]]. When the incoming radio wave reduced the resistance of the coherer, the current from the battery flowed through it, turning on the relay to ring a bell or make a mark on a paper tape in a [[siphon recorder]]. In order to restore the coherer to its previous nonconducting state to receive the next pulse of radio waves, it had to be tapped mechanically to disturb the metal particles.<ref name="Lee" /><ref name="Coe2" /><ref name="Fleming1" /><ref name="Phillips4">[https://archive.org/stream/EarlyRadioWaveDetectors/Phillips-EarlyRadioWaveDetectors#page/n26/mode/1up Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 38-42]</ref> This was done by a "decoherer", a clapper which struck the tube, operated by an [[electromagnet]] powered by the relay.

The coherer is an obscure antique device, and even today there is some uncertainty about the exact physical mechanism by which the various types worked.<ref name="Lee" /><ref name="Nahin1">[https://books.google.com/books?id=V1GBW6UD4CcC&pg=PA53 Nahin, Paul J. (2001) ''The Science of Radio'', p. 53-56]</ref><ref name="Phillips5">[https://archive.org/stream/EarlyRadioWaveDetectors/Phillips-EarlyRadioWaveDetectors#page/n35/mode/1up Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 57-60]</ref> However it can be seen that it was essentially a [[Bistability|bistable]] device, a radio-wave-operated switch, and so it did not have the ability to [[rectifier|rectify]] the radio wave to [[demodulate]] the later [[amplitude modulated]] (<span class="ltr">AM</span>) radio transmissions that carried sound.<ref name="Lee" /><ref name="Nahin1" />

In a long series of experiments Marconi found that by using an elevated wire [[monopole antenna]] instead of Hertz's [[dipole antenna]]s he could transmit longer distances, beyond the curve of the Earth, demonstrating that radio was not just a laboratory curiosity but a commercially viable communication method. This culminated in his historic transatlantic wireless transmission on December 12, 1901 from [[Poldhu|Poldhu, Cornwall]] to [[St. John's, Newfoundland and Labrador|St. John's, Newfoundland]], a distance of 3500&nbsp;km (2200 miles), which was received by a coherer.<ref name="Nahin1" /><ref name="Sarkar3" /> However the usual range of coherer receivers even with the powerful transmitters of this era was limited to a few hundred miles.

The coherer remained the dominant detector used in early radio receivers for about 10 years,<ref name="McNicol2" /> until replaced by the [[crystal detector]] and [[electrolytic detector]] around 1907. In spite of much development work, it was a very crude unsatisfactory device.<ref name="Lee" /><ref name="Coe2" /> It was not very sensitive, and also responded to impulsive [[radio noise]] ([[radio frequency interference|RFI]]), such as nearby lights being switched on or off, as well as to the intended signal.<ref name="Coe2" /><ref name="McNicol2" /> Due to the cumbersome mechanical "tapping back" mechanism it was limited to a data rate of about 12-15 words per minute of [[Morse code]], while a spark-gap transmitter could transmit Morse at up to 100 WPM with a paper tape machine.<ref name="Maver">{{cite journal
  | last1  = Maver
  | first1 = William Jr.
  | title = Wireless Telegraphy To-Day
  | journal = American Monthly Review of Reviews
  | volume = 30
  | issue = 2
  | pages = 192
  | date = August 1904
  | url = https://books.google.com/books?id=PbsGAQAAIAAJ&pg=PA191
  | access-date = January 2, 2016}}</ref><ref name="Aitken3">{{cite book
 | last1  = Aitken
 | first1 = Hugh G.J.
 | title  = The Continuous Wave: Technology and American Radio, 1900-1932
 | publisher = Princeton Univ. Press
 | date   = 2014
 | pages  = 190
 | url    = https://books.google.com/books?id=ebr_AwAAQBAJ&q=coherer+%22words+per+minute%22&pg=PA190
 | isbn   = 978-1400854608
 }}</ref>
{{breakafterimages}}

====Other early detectors====
[[Image:Attempt to use human brain to receive radio waves.jpg|thumb|upright=0.8|Experiment to use human brain as a radio wave detector, 1902]]

The coherer's poor performance motivated a great deal of research to find better radio wave detectors, and many were invented. Some strange devices were tried; researchers experimented with using [[frog leg]]s<ref name="Lefeuvre">{{cite journal
  | title = Frog's leg method of detecting wireless waves
  | journal = Electrical Review and Western Electrician
  | volume = 62
  | issue = 3
  | pages = 164
  | date = January 18, 1913
  | url = https://books.google.com/books?id=8Y0fAQAAMAAJ&q=frog+leg&pg=PA164
  | access-date = January 30, 2018| last1 = Worthington
 | first1 = George
 }}</ref> and even a [[human brain]]<ref name="Brain">{{cite journal
  | last1  = Collins
  | first1 = Archie Frederick
  | title = The effect of electric waves on the human brain
  | journal = Electrical World and Engineer
  | volume = 39
  | issue = 8
  | pages = 335–338
  | date = February 22, 1902
  | url = https://books.google.com/books?id=DLdQAAAAYAAJ&q=detector+brain&pg=PA335
  | access-date = January 26, 2018}}</ref> from a cadaver as detectors.<ref name="Lee" /><ref name="Phillips2">[https://archive.org/stream/EarlyRadioWaveDetectors/Phillips-EarlyRadioWaveDetectors#page/n106/mode/1up Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 198-203]</ref>

By the first years of the 20th century, experiments in using [[amplitude modulation]] (<span class="ltr">AM</span>) to transmit sound by radio ([[radiotelephony]]) were being made. So a second goal of detector research was to find detectors that could [[demodulate]] an AM signal, extracting the [[audio signal|audio]] (sound) signal from the radio [[carrier wave]]. It was found by trial and error that this could be done by a detector that exhibited "asymmetrical conduction"; a device that conducted current in one direction but not in the other.<ref name="Phillips8">[https://archive.org/stream/EarlyRadioWaveDetectors/Phillips-EarlyRadioWaveDetectors#page/n109/mode/1up Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 205-209]</ref> This [[rectifier|rectified]] the alternating current radio signal, removing one side of the carrier cycles, leaving a pulsing [[direct current|DC]] current whose amplitude varied with the audio modulation signal. When applied to an earphone this would reproduce the transmitted sound.

Below are the detectors that saw wide use before vacuum tubes took over around 1920.<ref name="Marriott" /><ref name="Secor">{{cite journal
  | last1  = Secor
  | first1 = H. Winfield
  | title = Radio Detector Development
  | journal = Electrical Experimenter
  | volume = 4
  | issue = 9
  | pages = 652–656
  | date = January 1917
  | url = http://www.nuenergy.org/radio-detector-development/
  | access-date = January 3, 2016}}</ref> All except the magnetic detector could rectify and therefore receive AM signals:
{{breakafterimages}}

[[Image:DÉTECTEUR MAGNÉTIQUE (radio).jpg|thumb|Magnetic detector]]
*'''[[Magnetic detector]]''' - Developed by [[Guglielmo Marconi]] in 1902 from a method invented by [[Ernest Rutherford]] and used by the Marconi Co. until it adopted the Audion vacuum tube around 1912, this was a mechanical device consisting of an endless band of iron wires which passed between two pulleys turned by a windup mechanism.<ref name="McNicol3">{{cite book
 | last1  = McNicol
 | first1 = Donald
 | title  = Radio's Conquest of Space
 | publisher = Murray Hill Books 
 | date   = 1946
 | pages  = 121–123
 | isbn = 9780405060526
 | url    = https://archive.org/stream/radiosconquestof00mcnirich#page/121/mode/2up
 }}</ref><ref name="Stone2">[https://archive.org/details/elementsradiote01stongoog/page/n259  Stone, Ellery (1919) ''Elements of Radiotelegraphy'', p. 209-221]</ref><ref name="Fleming2">[https://books.google.com/books?id=qQFVAAAAMAAJ&pg=PA446 Fleming, John Ambrose (1910) ''The Principles of Electric Wave Telegraphy and Telephony'', p. 446-455]</ref><ref name="Phillips7">[https://archive.org/stream/EarlyRadioWaveDetectors/Phillips-EarlyRadioWaveDetectors#page/n49/mode/1up Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 85-108]</ref> The iron wires passed through a coil of fine wire attached to the antenna, in a [[magnetic field]] created by two [[magnet]]s. The [[hysteresis]] of the iron induced a pulse of current in a sensor coil each time a radio signal passed through the exciting coil. The magnetic detector was used on shipboard receivers due to its insensitivity to vibration. One was part of the wireless station of the [[RMS Titanic|RMS ''Titanic'']] which was used to summon help during its famous 15 April 1912 sinking.<ref name="Stephenson">{{cite journal
  | last1  = Stephenson
  | first1 = Parks 
  | title = The Marconi Wireless Installation in R.M.S. Titanic
  | journal = Old Timer's Bulletin
  | volume = 42
  | issue = 4
  | date = November 2001
  | url = http://marconigraph.com/titanic/wireless/mgy_wireless.html
  | access-date = May 22, 2016}} copied on Stephenson's marconigraph.com personal website</ref>

{{breakafterimages}}
[[File:Electrolytic detector.jpg|thumb|upright=0.7|Electrolytic detector]]
*'''[[Electrolytic detector]]''' ("'''liquid barretter'''") - Invented in 1903 by [[Reginald Fessenden]], this consisted of a thin silver-plated platinum wire enclosed in a glass rod, with the tip making contact with the surface of a cup of [[nitric acid]].<ref name="Lee" /><ref name="Stone2" /><ref name="McNicol4">[https://archive.org/stream/radiosconquestof00mcnirich#page/115/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 115-119]</ref><ref name="Fleming3">[https://books.google.com/books?id=qQFVAAAAMAAJ&pg=PA460 Fleming, John Ambrose (1910) ''The Principles of Electric Wave Telegraphy and Telephony'', p. 460-464]</ref><ref name="Phillips6">[https://archive.org/stream/EarlyRadioWaveDetectors/Phillips-EarlyRadioWaveDetectors#page/n39/mode/1up Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 65-81]</ref> The electrolytic action caused current to be conducted in only one direction. The detector was used until about 1910.<ref name="Stone2" /> Electrolytic detectors that Fessenden had installed on US Navy ships received the first [[AM broadcasting|AM radio broadcast]] on Christmas Eve, 1906, an evening of Christmas music transmitted by Fessenden using his new alternator transmitter.<ref name="Lee" />

{{breakafterimages}}
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Fleming valve.jpg
| caption1 = Early Fleming valve.
| width1   = 190
| image2   = Marconi Valve Tuner.jpg
| caption2 = Marconi valve receiver for use on ships had two Fleming valves ''(top)'' in case one burned out. It was used on the RMS ''Titanic''.
| width2   = 240
| footer   =
}}
*'''[[Thermionic diode]]''' ('''[[Fleming valve]]''') - The first [[vacuum tube]], invented in 1904 by [[John Ambrose Fleming]], consisted of an evacuated glass bulb containing two electrodes: a [[cathode]] consisting of a hot wire [[electrical filament|filament]] similar to that in an incandescent light bulb, and a metal plate [[plate electrode|anode]].<ref name="Carr" /><ref name="Lee1">[https://books.google.com/books?id=WqdzSq56lSQC&pg=PA9 Lee, Thomas H. (2004) ''The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed.'', p. 9-11]</ref><ref name="McNicol5">[https://archive.org/stream/radiosconquestof00mcnirich#page/157/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 157-162]</ref><ref name="Fleming4">[https://books.google.com/books?id=qQFVAAAAMAAJ&pg=PA476 Fleming, John Ambrose (1910) ''The Principles of Electric Wave Telegraphy and Telephony'', p. 476-483]</ref> Fleming, a consultant to Marconi, invented the valve as a more sensitive detector for transatlantic wireless reception. The filament was heated by a separate current through it and emitted electrons into the tube by [[thermionic emission]], an effect which had been discovered by [[Thomas Edison]]. The radio signal was applied between the cathode and anode. When the anode was positive, a current of electrons flowed from the cathode to the anode, but when the anode was negative the electrons were repelled and no current flowed. The Fleming valve was used to a limited extent but was not popular because it was expensive, had limited filament life, and was not as sensitive as electrolytic or crystal detectors.<ref name="Lee1" />

{{breakafterimages}}
[[File:Kristallradio (<span class="ltr">3</span>).jpg|thumb|A galena cat's whisker detector from a 1920s [[crystal radio]] ]]
*'''[[Crystal detector]]''' ('''cat's whisker detector''') - invented around 1904–1906 by Henry H. C. Dunwoody and [[Greenleaf Whittier Pickard]], based on [[Karl Ferdinand Braun]]'s 1874 discovery of "asymmetrical conduction" in crystals, these were the most successful and widely used detectors before the vacuum tube era<ref name="Phillips8" /><ref name="Marriott">{{Cite journal | last = Marriott | first = Robert H. | title = United States Radio Development | journal = Proc. of the Inst. Of Radio Engineers | volume = 5 | issue = 3 | page = 184 | date = September 17, 1915 | url = https://books.google.com/books?id=bh0B93CuXnkC&pg=PA184 | doi = 10.1109/jrproc.1917.217311| s2cid = 51644366 | access-date = 2010-01-19 }}</ref> and gave their name to the ''[[crystal radio]]'' receiver ''(below)''.<ref name="Stone2" /><ref name="McNicol6">[https://archive.org/stream/radiosconquestof00mcnirich#page/123/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 123-131]</ref><ref name="Fleming5">[https://books.google.com/books?id=qQFVAAAAMAAJ&pg=PA471 Fleming, John Ambrose (1910) ''The Principles of Electric Wave Telegraphy and Telephony'', p. 471-475]</ref> One of the first [[semiconductor device|semiconductor electronic devices]], a crystal detector consisted of a pea-sized pebble of a crystalline semiconductor mineral such as [[galena]] ([[lead sulfide]]) whose surface was touched by a fine springy metal wire mounted on an adjustable arm.<ref name="Carr" /> This functioned as a primitive [[diode]] which conducted electric current in only one direction. In addition to their use in crystal radios, [[carborundum]] crystal detectors were also used in some early vacuum tube radios because they were more sensitive than the vacuum tube [[grid-leak detector]].

{{breakafterimages}}
During the vacuum tube era, the term "detector" changed from meaning a radio wave detector to mean a [[demodulator]], a device that could extract the audio [[modulation]] signal from a radio signal. That is its meaning today.

====Tuning====
"Tuning" means adjusting the frequency of the receiver to the frequency of the desired radio transmission. The first receivers had no tuned circuit, the detector was connected directly between the antenna and ground. Due to the lack of any frequency selective components besides the antenna, the [[bandwidth (signal processing)|bandwidth]] of the receiver was equal to the broad bandwidth of the antenna.<ref name="McNicol14" /><ref name="Carr" /><ref name="Stone1" /><ref name="Hong2">{{cite book
 | last1  = Hong
 | first1 = Sungook 
 | title  = Wireless: From Marconi's Black-box to the Audion
 | publisher = MIT Press
 | date   = 2001
 | pages  = 89–100
 | url    = https://books.google.com/books?id=UjXGQSPXvIcC&pg=PA89
 | isbn   = 978-0262082983
 }}</ref>  This was acceptable and even necessary because the first Hertzian spark transmitters also lacked a tuned circuit. Due to the impulsive nature of the spark, the energy of the radio waves was spread over a very wide band of frequencies.<ref name="Aitken2" /><ref name="Beauchamp2">[https://books.google.com/books?id=k3XCkncd83AC&pg=PA185 Beauchamp, Ken (2001) ''History of Telegraphy'', p. 189-190]</ref> To receive enough energy from this wideband signal the receiver had to have a wide bandwidth also.

When more than one spark transmitter was radiating in a given area, their frequencies overlapped, so their signals interfered with each other, resulting in garbled reception.<ref name="McNicol14" /><ref name="Hong2" /><ref name="Kennelly">{{cite book
 | last1  = Kennelly
 | first1 = Arthur E.
 | title  = Wireless Telegraphy: An Elementary Treatise
 | publisher = Moffatt, Yard and Co.
 | date   = 1906
 | location = New York
 | pages  = [https://archive.org/details/wirelesstelegra01kenngoog/page/n184 173]–183
 | url    = https://archive.org/details/wirelesstelegra01kenngoog
 | quote  = selective signaling.
 }}</ref> Some method was needed to allow the receiver to select which transmitter's signal to receive.<ref name="Kennelly" /><ref name="Aitken4">[https://books.google.com/books?id=Mez_AwAAQBAJ&dq=tesla&pg=PA31  Aitken, Hugh 2014 ''Syntony and Spark: The origins of radio'', p. 31-48]</ref> Multiple wavelengths produced by a poorly tuned transmitter caused the signal to "dampen", or die down, greatly reducing the power and range of transmission.<ref>Jed Z. Buchwald, Scientific Credibility and Technical Standards in 19th and early 20th century Germany and Britain, Springer Science & Business Media - 1996, page 158</ref> In 1892, [[William Crookes]] gave a lecture<ref name="Crookes">{{cite journal
  | last1 = Crookes
  | first1 = William
  | title = Some Possibilities of Electricity
  | journal = The Fortnightly Review
  | volume = 51
  | pages = 174–176
  | date = February 1, 1892
  | url = http://www.tfo.upm.es/ImperialismoWeb/ArtCrookes.htm
  | access-date = August 19, 2015
  | archive-date = September 29, 2018
  | archive-url = https://web.archive.org/web/20180929160315/http://www.tfo.upm.es/ImperialismoWeb/ArtCrookes.htm
  | url-status = dead
  }}</ref> on radio in which he suggested using [[resonance]] to reduce the bandwidth of transmitters and receivers. Different transmitters could then be "tuned" to transmit on different frequencies so they did not interfere.<ref name="Sarkar3" /><ref name="Aitken2">[https://books.google.com/books?id=Mez_AwAAQBAJ&dq=tesla&pg=PA71 Aitken, Hugh 2014 ''Syntony and Spark: The origins of radio'', p. 70-73]</ref><ref name="Rockman">{{cite book
 | last1  = Rockman
 | first1 = Howard B. 
 | title  = Intellectual Property Law for Engineers and Scientists
 | publisher = John Wiley and Sons
 | date   = 2004
 | pages  = 196–199
 | url    = https://books.google.com/books?id=6B88P-32IIQC&pg=PA196
 | isbn   = 978-0471697398
 }}</ref>   The receiver would also have a [[resonant circuit]] (tuned circuit), and could receive a particular transmission by "tuning" its resonant circuit to the same frequency as the transmitter, analogously to tuning a musical instrument to resonance with another. This is the system used in all modern radio.

Tuning was used in Hertz's original experiments<ref>Cecil Lewis Fortescue, Wireless Telegraphy, Read Books Ltd - 2013, chapter XIII</ref> and practical application of tuning showed up in the early to mid 1890s in wireless systems not specifically designed for radio communication. [[Nikola Tesla]]'s March 1893 lecture demonstrating the [[Wireless power|wireless transmission of power]] for lighting (mainly by what he thought was ground conduction<ref name="Hong3">{{cite book
 | last1  = Hong
 | first1 = Sungook 
 | title  = Wireless: From Marconi's Black-box to the Audion
 | publisher = MIT Press
 | date   = 2001
 | pages  = 199
 | url    = https://books.google.com/books?id=UjXGQSPXvIcC&pg=PA89
 | isbn   = 978-0262082983
 }}</ref>) included elements of tuning. The wireless lighting system consisted of a spark-excited grounded [[resonant transformer]] with a wire antenna which transmitted power across the room to another resonant transformer tuned to the frequency of the transmitter, which lighted a [[Geissler tube]].<ref name="Klooster" /><ref name="Rockman" /> Use of tuning in free space "Hertzian waves" (radio) was explained and demonstrated in Oliver Lodge's 1894 lectures on Hertz's work.<ref>Peter Rowlands, Oliver Lodge and the Liverpool Physical Society, Liverpool University Press - 1990, page 117</ref> At the time Lodge was demonstrating the physics and optical qualities of radio waves instead of attempting to build a communication system but he would go on to develop methods (patented in 1897) of tuning radio (what he called "syntony"), including using variable inductance to tune antennas.<ref>Jed Z. Buchwald, Scientific Credibility and Technical Standards in 19th and early 20th century Germany and Britain, Springer Science & Business Media - 1996, pages 158-159</ref><ref name="Aitken1">{{cite book
 | last1  = Aitken
 | first1 = Hugh G.J. 
 | title  = Syntony and Spark: The Origins of Radio
 | publisher = Princeton Univ. Press
 | date   = 2014
 | pages  = 255
 | url    = https://books.google.com/books?id=Mez_AwAAQBAJ&pg=PA254
 | isbn   = 978-1400857883
 }}</ref><ref>Thomas H. Lee, The Design of CMOS Radio-Frequency Integrated Circuits, Cambridge University Press - 2004, page 35</ref>

By 1897 the advantages of tuned systems had become clear, and Marconi and the other wireless researchers had incorporated [[tuned circuit]]s, consisting of [[capacitor]]s and [[inductor]]s connected together, into their transmitters and receivers.<ref name="McNicol14" /><ref name="Klooster" /><ref name="Sarkar3" /><ref name="Stone1" /><ref name="Hong2" /><ref name="Aitken1" /> The tuned circuit acted like an electrical analog of a [[tuning fork]]. It had a high [[impedance (electrical)|impedance]] at its [[resonant frequency]], but a low impedance at all other frequencies. Connected between the antenna and the detector it served as a [[bandpass filter]], passing the signal of the desired station to the detector, but routing all other signals to ground.<ref name="Carr" /> The frequency of the station received '''''f''''' was determined by the [[capacitance]] '''''C''''' and [[inductance]] '''''L''''' in the tuned circuit:
<math display="block">f = \frac{1}{ 2 \pi \sqrt{LC} }</math>

{{breakafterimages}}

=====Inductive coupling=====
[[File:Rx marconi.jpg|thumb|upright=0.8|Marconi's inductively coupled coherer receiver from his controversial April 1900 "four circuit" patent no. 7,777.]]
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Braun wireless receiving transformer 1905.jpg
| caption1 = Braun receiving transformer from 1904
| width1   = 122
| image2   = Crystal radio receiver from wireless era.png
| caption2 = Crystal receiver from 1914 with "loose coupler" tuning transformer. The secondary coil ''(<span class="ltr">1</span>)'' can be slid in or out of the primary ''(in box)'' to adjust the coupling. Other components: ''(<span class="ltr">2</span>)'' primary tuning capacitor, ''(<span class="ltr">3</span>)'' secondary tuning capacitor, ''(<span class="ltr">4</span>)'' loading coil, ''(<span class="ltr">5</span>)'' crystal detector, ''(<span class="ltr">8</span>)'' headphones
| width2   = 292
| footer   =
}}

In order to reject [[radio noise]] and interference from other transmitters near in frequency to the desired station, the bandpass filter (tuned circuit) in the receiver has to have a narrow [[bandwidth (signal processing)|bandwidth]], allowing only a narrow band of frequencies through.<ref name="McNicol14" /><ref name="Carr" /> The form of bandpass filter that was used in the first receivers, which has continued to be used in receivers until recently, was the double-tuned [[inductive coupling|inductively-coupled]] circuit, or [[resonant transformer]] ([[oscillation transformer]] or RF transformer).<ref name="McNicol14" /><ref name="Klooster" /><ref name="Sarkar3" /><ref name="Stone1" /><ref name="Aitken1" /><ref name="McNicol7">[https://archive.org/stream/radiosconquestof00mcnirich#page/243/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 242-253]</ref> The antenna and ground were connected to a coil of wire, which was magnetically coupled to a second coil with a capacitor across it, which was connected to the detector.<ref name="Carr" /> The RF alternating current from the antenna through the primary coil created a [[magnetic field]] which induced a current in the secondary coil which fed the detector. Both primary and secondary were tuned circuits;<ref name="Hong2" /> the primary coil resonated with the capacitance of the antenna, while the secondary coil resonated with the capacitor across it. Both were adjusted to the same [[resonant frequency]].

This circuit had two advantages.<ref name="Carr" /> One was that by using the correct turns ratio, the [[Electrical impedance|impedance]] of the antenna could be matched to the impedance of the receiver, to transfer maximum RF power to the receiver. [[Impedance matching]] was important to achieve maximum receiving range in the unamplified receivers of this era.<ref name="Rudersdorfer2" /><ref name="Carr" /> The coils usually had taps which could be selected by a multiposition switch. The second advantage was that due to "loose coupling" it had a much narrower bandwidth than a simple [[tuned circuit]], and the bandwidth could be adjusted.<ref name="McNicol14" /><ref name="McNicol7" /> Unlike in an ordinary transformer, the two coils were "loosely coupled"; separated physically so not all the magnetic field from the primary passed through the secondary, reducing the [[mutual inductance]]. This gave the coupled tuned circuits much "sharper" tuning, a narrower bandwidth than a single tuned circuit. In the "Navy type" loose coupler ''(see picture)'', widely used with [[crystal radio|crystal receivers]], the smaller secondary coil was mounted on a rack which could be slid in or out of the primary coil, to vary the [[mutual inductance]] between the coils.<ref name="McNicol14" /><ref name="Marx">{{cite book
 | last1  = Marx
 | first1 = Harry J.
 | last2  = Van Muffling
 | first2 = Adrian 
 | title  = Radio Reception
 | publisher = G. Putnam's Sons
 | date   = 1922
 | location = New York
 | pages  = [https://archive.org/details/RadioReception/page/n115 95]–103
 | url    = https://archive.org/details/RadioReception
 | quote  = loose coupler variometer variocoupler.
 }}</ref>  When the operator encountered an interfering signal at a nearby frequency, the secondary could be slid further out of the primary, reducing the coupling, which narrowed the bandwidth, rejecting the interfering signal.  A disadvantage was that all three adjustments in the loose coupler - primary tuning, secondary tuning, and coupling - were interactive; changing one changed the others. So tuning in a new station was a process of successive adjustments.

Selectivity became more important as spark transmitters were replaced by [[continuous wave]] transmitters which transmitted on a narrow band of frequencies, and broadcasting led to a proliferation of closely spaced radio stations crowding the radio spectrum.<ref name="Carr" /> Resonant transformers continued to be used as the bandpass filter in vacuum tube radios, and new forms such as the ''[[inductor|variometer]]'' were invented.<ref name="Marx" /><ref name="McNicol8" /> Another advantage of the double-tuned transformer for AM reception was that when properly adjusted it had a "flat top" frequency response curve as opposed to the "peaked" response of a single tuned circuit.<ref name="Terman1">{{cite book
 | last1  = Terman
 | first1 = Frederick E.
 | title  = Radio Engineers' Handbook
 | publisher = McGraw-Hill Book Co. 
 | date   = 1943
 | location = New York
 | pages  = 170
 | url    = http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Radio-Engineer%27s-Handbook-1.pdf
 }}</ref> This allowed it to pass the [[sideband]]s of AM modulation on either side of the [[carrier wave|carrier]] with little distortion, unlike a single tuned circuit which attenuated the higher audio frequencies. Until recently the bandpass filters in the superheterodyne circuit used in all modern receivers were made with resonant transformers, called [[Resonant transformer|IF transformers]].

=====Patent disputes=====
Marconi's initial radio system had relatively poor tuning limiting its range and adding to interference.<ref name="Hong, Sungook 2001 pp. 91-99">Hong, Sungook (2001). Wireless: From Marconi's Black-box to the Audion. MIT Press. pp. 91-99</ref> To overcome this drawback he developed a four circuit system with tuned coils in "''syntony''" at both the transmitters and receivers.<ref name="Hong, Sungook 2001 pp. 91-99"/> His 1900 British #7,777 (four sevens) patent for tuning filed in April 1900 and granted a year later opened the door to patents disputes since it infringed on the Syntonic patents of Oliver Lodge, first filed in May 1897, as well as patents filed by [[Ferdinand Braun]].<ref name="Hong, Sungook 2001 pp. 91-99"/> Marconi was able to obtain patents in the UK and France but the US version of his tuned four circuit patent, filed in November 1900, was initially rejected based on it being anticipated by Lodge's tuning system, and refiled versions were rejected because of the prior patents by Braun, and Lodge.<ref name="Howard B. Rockman 2004, page 198">Howard B. Rockman, Intellectual Property Law for Engineers and Scientists, John Wiley & Sons - 2004, page 198</ref> A further clarification and re-submission was rejected because it infringed on parts of two prior patents Tesla had obtained for his wireless power transmission system.<ref>U.S. [http://www.teslauniverse.com/nikola-tesla/patents/us-patent-649621-apparatus-transmission-electrical-energy Patent No. 649,621], 3/15/1900, and part of [http://www.teslauniverse.com/nikola-tesla/patents/us-patent-645576-system-transmission-electrical-energy 645,576], 3/20/1900 (filed Sept. 2, 1897) [https://www.law.cornell.edu/supremecourt/text/320/1#fn3 Marconi Wireless Telegraph Co. of America v. United States. United States v. Marconi Wireless Telegraph Co. of America. 320 U.S. 1 (63 S.Ct. 1393, 87 L.Ed. 1731) ]</ref> Marconi's lawyers managed to get a resubmitted patent reconsidered by another examiner who initially rejected it due to a pre-existing [[John Stone Stone]] tuning patent, but it was finally approved it in June 1904 based on it having a unique system of variable inductance tuning that was different from Stone<ref name="Patent714756">US Patent no. 714,756, John Stone Stone [https://patents.google.com/patent/US714756 Method of electric signaling], filed: February 8, 1900, granted: December 2, 1902</ref><ref>[https://www.law.cornell.edu/supremecourt/text/320/1#fn3 Marconi Wireless Telegraph Co. of America v. United States. United States v. Marconi Wireless Telegraph Co. of America. 320 U.S. 1 (63 S.Ct. 1393, 87 L.Ed. 1731) ]</ref> who tuned by varying the length of the antenna.<ref name="Howard B. Rockman 2004, page 198"/> When Lodge's Syntonic patent was extended in 1911 for another 7 years the Marconi Company agreed to settle that patent dispute, purchasing Lodge's radio company with its patent in 1912, giving them the priority patent they needed.<ref>Hong, Sungook (2001). Wireless: From Marconi's Black-box to the Audion. MIT Press. p. 48</ref><ref>Susan J. Douglas, Listening in: Radio and the American Imagination, U of Minnesota Press, page 50</ref> Other patent disputes would crop up over the years including a 1943 [[US Supreme Court]] ruling on the Marconi Company's ability to sue the US government over patent infringement during World War I. The Court rejected the Marconi Company's suit saying they could not sue for patent infringement when their own patents did not seem to have priority over the patents of Lodge, Stone, and Tesla.<ref name="Klooster" /><ref name="Rockman" />
{{breakafterimages}}

====Crystal radio receiver====
{{Main|Crystal radio}}
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Marconi Type 106 crystal radio receiver.jpg
| caption1 = Prior to 1920 the crystal receiver was the main type used in wireless telegraphy stations, and sophisticated models were made, like this Marconi Type 106 from 1915.
| width1   = 200
| image2   = Crystal radio advertisement.png
| caption2 = Family listening to the first broadcasts around 1920 with a crystal receiver. The mother and father have to share an earphone
| width2   = 233
| image3   = Crystal radio.agr.jpg
| caption3 = After vacuum-tube receivers appeared around 1920, the crystal set became a simple cheap alternative radio used by youth and the poor.
| width3   = 173
| footer   = 
}}
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = CrystalRadio.jpg
| caption1 = Simple crystal radio. The capacitance of the wire antenna connected to the coil serves as the capacitor in the tuned circuit.
| width1   = 227
| image2   = Inductively_coupled_crystal_radio_circuit.svg
| caption2 = Typical "loose coupler" crystal radio circuit
| width2   = 200
}}

Although it was invented in 1904 in the wireless telegraphy era, the crystal radio receiver could also rectify AM transmissions and served as a bridge to the broadcast era. In addition to being the main type used in commercial stations during the wireless telegraphy era, it was the first receiver to be used widely by the public.<ref name="Basalla">{{Cite book | last = Basalla | first = George | title = The Evolution of Technology | publisher = Cambridge University Press | year = 1988 | location = UK | page = 44 | url = https://books.google.com/books?id=EBtnG36-1WIC&pg=PA44 | isbn = 978-0-521-29681-6 }}</ref> During the first two decades of the 20th century, as radio stations began to transmit in AM voice ([[radiotelephony]]) instead of radiotelegraphy, radio listening became a popular hobby, and the crystal was the simplest, cheapest detector. The millions of people who purchased or homemade these inexpensive reliable receivers created the mass listening audience for the first [[radio broadcasting|radio broadcasts]], which began around 1920.<ref name="Corbin">{{Cite book | last = Corbin | first = Alfred | title = The Third Element: A Brief History of Electronics | publisher = AuthorHouse | year = 2006 | pages = 44–45 | url = https://books.google.com/books?id=-9lt4HL-AlwC&pg=PA45 | isbn = 978-1-4208-9084-6 }}</ref> By the late 1920s the crystal receiver was superseded by vacuum tube receivers and became commercially obsolete. However it continued to be used by youth and the poor until [[World War II]].<ref name="Basalla" /> Today these simple radio receivers are constructed by students as educational science projects.

The crystal radio used a [[cat's whisker detector]], invented by Harrison H. C. Dunwoody and [[Greenleaf Whittier Pickard]] in 1904, to extract the audio from the radio frequency signal.<ref name="Carr" /><ref name="Stone2" /><ref name="ArmyManual1">{{cite book
 | title  = Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers
 | publisher = US Dept. of the Army
 | date   = 1952
 | pages  = 167–169
 | url    = https://books.google.com/books?id=f9QXAAAAYAAJ&pg=PA168
 }}</ref>   It consisted of a mineral crystal, usually [[galena]], which was lightly touched by a fine springy wire (the "cat whisker") on an adjustable arm.<ref name="Stone2" /><ref name="Williams3">{{cite book
 | last1  = Williams
 | first1 = Lyle Russell 
 | title  = The New Radio Receiver Building Handbook
 | publisher = Lulu
 | date   = 2006
 | pages  = 20–24
 | url    = https://books.google.com/books?id=XiKgKdeBi6cC&pg=PA20
 | isbn   = 978-1847285263
 }}</ref>  The resulting crude [[semiconductor junction]] functioned as a [[Schottky barrier diode]], conducting in only one direction. Only particular sites on the crystal surface worked as detector junctions, and the junction could be disrupted by the slightest vibration. So a usable site was found by trial and error before each use; the operator would drag the cat's whisker across the crystal until the radio began functioning. Frederick Seitz, a later semiconductor researcher, wrote:
<blockquote>Such variability, bordering on what seemed the mystical, plagued the early history of crystal detectors and caused many of the vacuum tube experts of a later generation to regard the art of crystal rectification as being close to disreputable.<ref name="Riordan">{{cite book  | last =  Riordan  | first = Michael |author1-link=Michael Riordan (physicist) |author2-link=Lillian Hoddeson |author2=Lillian Hoddeson  | title = Crystal fire: the invention of the transistor and the birth of the information age  | publisher = W. W. Norton & Company  | year = 1988  | location = USA  | pages = 19–21  | url = https://books.google.com/books?id=SZ6wm5ZSUmsC&pg=PA92  | isbn = 978-0-393-31851-7}}</ref></blockquote>
The crystal radio was unamplified and ran off the power of the radio waves received from the radio station, so it had to be listened to with [[earphone]]s; it could not drive a [[loudspeaker]].<ref name="Carr" /><ref name="Williams3" />  It required a long wire antenna, and its sensitivity depended on how large the antenna was.  During the wireless era it was used in commercial and military longwave stations with huge antennas to receive long distance radiotelegraphy traffic, even including transatlantic traffic.<ref name="Beauchamp">{{cite book     | last = Beauchamp  | first = Ken   | title = History of Telegraphy  | publisher = Institution of Electrical Engineers | year = 2001 | pages = 191 | url = https://books.google.com/books?id=k3XCkncd83AC&pg=PA191 | isbn = 978-0852967928}}</ref><ref name="Bucher">{{cite book
 | last1  = Bucher
 | first1 = Elmer Eustice 
 | title  = Practical Wireless Telegraphy
 | publisher = Wireless Press
 | date   = 1917
 | location = New York
 | pages  = [https://archive.org/details/practicalwirele05buchgoog/page/n318 306]
 | url    = https://archive.org/details/practicalwirele05buchgoog
 }}</ref>  However, when used to receive broadcast stations a typical home crystal set had a more limited range of about 25 miles.<ref name="Lescarboura" >{{Cite book | last = Lescarboura | first = Austin C. | title = Radio for Everybody | publisher = Scientific American Publishing Co. | year = 1922 | location = New York | pages = [https://archive.org/details/RadioForEverybody/page/n113 93]–94 | url = https://archive.org/details/RadioForEverybody }}</ref> In sophisticated crystal radios the "loose coupler" inductively coupled tuned circuit was used to increase the [[Q factor|Q]]. However it still had poor [[Selectivity (radio)|selectivity]] compared to modern receivers.<ref name="Williams3" />

{{breakafterimages}}

====Heterodyne receiver and BFO====
{{Main|Heterodyne}}
[[Image:Poulsen tikker radio receiver circuit.png|thumb|Radio receiver with Poulsen "tikker" consisting of a [[commutator (electric)|commutator]] disk turned by a motor to interrupt the carrier.]]

Beginning around 1905 [[continuous wave]] (<span class="ltr">CW</span>) transmitters began to replace spark transmitters for radiotelegraphy because they had much greater range. The first continuous wave transmitters were the [[Poulsen arc]] invented in 1904 and the [[Alexanderson alternator]] developed 1906–1910, which were replaced by vacuum tube transmitters beginning around 1920.<ref name="Phillips1" />

The continuous wave radiotelegraphy signals produced by these transmitters required a different method of reception.<ref name="Lauer">{{cite book
 | last1  = Lauer
 | first1 = Henri
 | last2  = Brown
 | first2 = Harry L. 
 | title  = Radio Engineering Principles
 | publisher = McGraw-Hill
 | date   = 1920
 | pages  = [https://archive.org/details/radioengineerin01browgoog/page/n161 135]–142
 | url    = https://archive.org/details/radioengineerin01browgoog
 | quote  = tikker heterodyne.
 }}</ref><ref name="Phillips9">[https://archive.org/stream/EarlyRadioWaveDetectors/Phillips-EarlyRadioWaveDetectors#page/n94/mode/1up Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 172-185]</ref> The radiotelegraphy signals produced by spark gap transmitters consisted of strings of [[damped wave]]s repeating at an audio rate, so the "dots" and "dashes" of Morse code were audible as a tone or buzz in the receivers' earphones. However the new continuous wave radiotelegraph signals simply consisted of pulses of unmodulated [[carrier signal|carrier]] ([[sine wave]]s). These were inaudible in the receiver headphones.  To receive this new modulation type, the receiver had to produce some kind of tone during the pulses of carrier.

The first crude device that did this was the [[tikker]], invented in 1908 by [[Valdemar Poulsen]].<ref name="Marriott" /><ref name="Lauer" />
<ref name="McNicol1">{{cite book
 | last1  = McNicol
 | first1 = Donald
 | title  = Radio's Conquest of Space
 | publisher = Murray Hill Books
 | date   = 1946
 | location = New York
 | pages  = 133–136
 | isbn = 9780405060526
 | url    = https://archive.org/stream/radiosconquestof00mcnirich#page/133/mode/2up
 }}</ref> This was a vibrating interrupter with a [[capacitor]] at the tuner output which served as a rudimentary [[modulator]], interrupting the carrier at an audio rate, thus producing a buzz in the earphone when the carrier was present.<ref name="Lee2">[https://books.google.com/books?id=WqdzSq56lSQC&pg=PA14 Lee, Thomas H. (2004) ''The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed.'', p. 14-15]</ref>  A similar device was the "tone wheel" invented by [[Rudolph Goldschmidt]], a wheel spun by a motor with contacts spaced around its circumference, which made contact with a stationary brush.

[[File:Heterodyne radio receiver circuit 1920.png|thumb|Fessenden's heterodyne radio receiver circuit]]

In 1901 [[Reginald Fessenden]] had invented a better means of accomplishing this.<ref name="Lauer" /><ref name="McNicol1" /><ref name="Patent1050441">US patent no. 1050441, Reginald A. Fessenden, ''[https://patents.google.com/patent/US1050441?oq=1050441 Electrical signaling apparatus]'', filed July 27, 1905; granted January 14, 1913</ref><ref name="Hogan">{{cite journal
  | last1  = Hogan
  | first1 = John V. L.
  | title = The Heterodyne Receiver
  | journal = The Electric Journal
  | volume = 18
  | issue = 4
  | pages = 116–119
  | date = April 1921
  | url = https://archive.org/stream/electricjournal18elecuoft#page/116/mode/2up
  | access-date = January 28, 2016}}</ref> In his ''heterodyne receiver'' an unmodulated sine wave radio signal at a frequency '''''f''<sub>O</sub>''' offset from the incoming radio wave carrier '''''f''<sub>C</sub>''' was applied to a rectifying detector such as a [[crystal detector]] or [[electrolytic detector]], along with the radio signal from the antenna. In the detector the two signals mixed, creating two new ''[[heterodyne]]'' ([[beat (acoustics)|beat]]) frequencies at the sum '''''f''<sub>C</sub>'''&nbsp;+&nbsp;'''''f''<sub>O</sub>''' and the difference '''''f''<sub>C</sub>'''&nbsp;−&nbsp;'''''f''<sub>O</sub>''' between these frequencies. By choosing '''''f''<sub>O</sub>''' correctly the lower heterodyne '''''f''<sub>C</sub>'''&nbsp;−&nbsp;'''''f''<sub>O</sub>''' was in the [[audio frequency]] range, so it was audible as a tone in the earphone whenever the carrier was present. Thus the "dots" and "dashes" of Morse code were audible as musical "beeps". A major attraction of this method during this pre-amplification period was that the heterodyne receiver actually amplified the signal somewhat, the detector had "mixer gain".<ref name="McNicol1" />

The receiver was ahead of its time, because when it was invented there was no oscillator capable of producing the radio frequency sine wave '''''f''<sub>O</sub>''' with the required stability.<ref name="Nahin2">[https://books.google.com/books?id=V1GBW6UD4CcC&pg=PA91 Nahin, Paul J. (2001) ''The Science of Radio'', p. 91]</ref> Fessenden first used his large radio frequency [[alternator]],<ref name="Lee2" /> but this was not practical for ordinary receivers. The heterodyne receiver remained a laboratory curiosity until a cheap compact source of continuous waves appeared, the vacuum tube [[electronic oscillator]]<ref name="McNicol1" /> invented by [[Edwin Armstrong]] and [[Alexander Meissner]] in 1913.<ref name="Marriott" /><ref name="McNicol16">[https://archive.org/stream/radiosconquestof00mcnirich#page/267/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 267-270]</ref> After this it became the standard method of receiving CW radiotelegraphy. The heterodyne oscillator is the ancestor of the ''[[beat frequency oscillator]]'' (BFO) which is used to receive radiotelegraphy in [[communications receiver]]s today. The heterodyne oscillator had to be retuned each time the receiver was tuned to a new station, but in modern [[superheterodyne]] receivers the BFO signal beats with the fixed [[intermediate frequency]], so the beat frequency oscillator can be a fixed frequency.

Armstrong later used Fessenden's heterodyne principle in his superheterodyne receiver ''(below)''.<ref name="McNicol1" /><ref name="Lee2" />
{{breakafterimages}}

===Vacuum tube era===
[[File:Radio receivers used in 1920s.png|thumb|Unlike today, when almost all radios use a variation of the superheterodyne design, during the 1920s vacuum tube radios used a variety of competing circuits.]]
[[File:Vintage Zenith Console Radio, Model 12S-568, With the Zenith Robot (or Shutter) Dial, Circa 1941 (8655513293).jpg|thumb|During the "[[Golden Age of Radio]]" (1920 to 1950), families gathered to listen to the home radio in the evening, such as this Zenith console model 12-S-568 from 1938, a 12-tube superheterodyne with pushbutton tuning and 12-inch cone speaker.]]

The [[Audion]] ([[triode]]) [[vacuum tube]] invented by [[Lee De Forest]] in 1906 was the first practical [[amplifier|amplifying]] device and revolutionized radio.<ref name="Lee1" /> Vacuum tube transmitters replaced spark transmitters and made possible four new types of [[modulation]]: [[continuous wave]] (<span class="ltr">CW</span>) radiotelegraphy, [[amplitude modulation]] (<span class="ltr">AM</span>) around 1915 which could carry audio (sound), [[frequency modulation]] (<span class="ltr">FM</span>) around 1938 which had much improved audio quality, and [[Single-sideband modulation|single sideband]] (SSB).

The amplifying vacuum tube used energy from a battery or electrical outlet to increase the power of the radio signal, so vacuum tube receivers could be more sensitive and have a greater reception range than the previous unamplified receivers. The increased audio output power also allowed them to drive [[loudspeaker]]s instead of [[earphone]]s, permitting more than one person to listen. The first loudspeakers were produced around 1915. These changes caused radio listening to evolve explosively from a solitary hobby to a popular social and family pastime. The development of [[amplitude modulation]] (<span class="ltr">AM</span>) and vacuum-tube transmitters during World War I, and the availability of cheap receiving tubes after the war, set the stage for the start of [[AM broadcasting]], which sprang up spontaneously around 1920.

The advent of [[radio broadcasting]] increased the market for radio receivers greatly, and transformed them into a consumer product.<ref name="McNicol12">[https://archive.org/stream/radiosconquestof00mcnirich#page/341/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 341-344]</ref><ref name="Wurtzler">{{cite book
 | last1  = Wurtzler
 | first1 = Steve J. 
 | title  = Electric Sounds: Technological Change and the Rise of Corporate Mass Media
 | publisher = Columbia Univ. Press
 | date   = 2007
 | pages  = 147–148
 | url    = https://books.google.com/books?id=E14aAwAAQBAJ&pg=PA147
 | isbn   = 978-0231510080
 }}</ref><ref name="Nebeker">{{cite book
 | last1  = Nebeker
 | first1 = Frederik 
 | title  = Dawn of the Electronic Age: Electrical Technologies in the Shaping of the Modern World, 1914 to 1945
 | publisher = John Wiley and Sons
 | date   = 2009
 | pages  = 159–160
 | url    = https://books.google.com/books?id=xwmH6-q5O5AC&pg=PA160
 | isbn   = 978-0470409749
 }}</ref> At the beginning of the 1920s the radio receiver was a forbidding high-tech device, with many cryptic knobs and controls requiring technical skill to operate, housed in an unattractive black metal box, with a tinny-sounding [[horn loudspeaker]].<ref name="Wurtzler" /> By the 1930s, the broadcast receiver had become a piece of furniture, housed in an attractive wooden case, with standardized controls anyone could use, which occupied a respected place in the home living room. In the early radios the multiple tuned circuits required multiple knobs to be adjusted to tune in a new station. One of the most important ease-of-use innovations was "single knob tuning", achieved by linking the tuning capacitors together mechanically.<ref name="Wurtzler" /><ref name="Nebeker" /> The [[Loudspeaker|dynamic cone loudspeaker]] invented in 1924 greatly improved audio [[frequency response]] over the previous horn speakers, allowing music to be reproduced with good fidelity.<ref name="Wurtzler" /><ref name="McNicol13">[https://archive.org/stream/radiosconquestof00mcnirich#page/336/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 336-340]</ref> Convenience features like large lighted dials, [[Tone control circuit|tone controls]], pushbutton tuning, [[tuning indicator]]s and [[automatic gain control]] (AGC) were added.<ref name="McNicol12" /><ref name="Nebeker" /> The receiver market was divided into the above ''broadcast receivers'' and ''[[communications receiver]]s'', which were used for [[two-way radio]] communications such as [[shortwave radio]].<ref name="Terman4">[http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Radio-Engineer%27s-Handbook-1.pdf Terman, Frederick E. (1943) ''Radio Engineers' Handbook'', p. 656]</ref>

A vacuum-tube receiver required several power supplies at different voltages, which in early radios were supplied by separate batteries. By 1930 adequate [[rectifier]] tubes were developed, and the expensive batteries were replaced by a transformer power supply that worked off the house current.<ref name="McNicol12" /><ref name="Wurtzler" />

Vacuum tubes were bulky, expensive, had a limited lifetime, consumed a large amount of power and produced a lot of waste heat, so the number of tubes a receiver could economically have was a limiting factor. Therefore, a goal of tube receiver design was to get the most performance out of a limited number of tubes. The major radio receiver designs, listed below, were invented during the vacuum tube era.

A defect in many early vacuum-tube receivers was that the amplifying stages could oscillate, act as an [[electronic oscillator|oscillator]], producing unwanted radio frequency alternating currents.<ref name="Carr" /><ref name="Williams" /><ref name="Lee3">[https://books.google.com/books?id=WqdzSq56lSQC&pg=PA15 Lee, Thomas H. (2004) ''The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed.'', p. 15-18]</ref> These [[parasitic oscillation]]s mixed with the [[carrier wave|carrier]] of the radio signal in the detector tube, producing audible [[beat (acoustics)|beat]] notes ([[heterodyne]]s); annoying whistles, moans, and howls in the speaker. The oscillations were caused by [[feedback]] in the amplifiers; one major feedback path was the [[capacitance]] between the plate and grid in early [[triode]]s.<ref name="Williams" /><ref name="Lee3" /> This was solved by the [[Neutrodyne]] circuit, and later the development of the [[tetrode]] and [[pentode]] around 1930.

[[Edwin Armstrong]] is one of the most important figures in radio receiver history, and during this period invented technology which continues to dominate radio communication.<ref name="Lee2" /> He was the first to give a correct explanation of how De Forest's triode tube worked. He invented the [[electronic oscillator|feedback oscillator]], [[regenerative receiver]], the [[superregenerative receiver]], the [[superheterodyne receiver]], and modern [[frequency modulation]] (<span class="ltr">FM</span>).
{{breakafterimages}}

====The first vacuum-tube receivers====
{{disputed section|date=October 2020}}
[[File:De Forest RJ6 Audion radio receiver.jpg|thumb|upright=1.4|De Forest's first commercial Audion receiver, the RJ6 which came out in 1914. The Audion tube was always mounted upside down, with its delicate filament loop hanging down, so it did not sag and touch the other electrodes in the tube.]]

{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Single triode receiver 1924.jpg
| caption1 = 
| width1   = 200
| image2   = Single tube triode receiver circuit.png
| caption2 = 
| width2   = 360
| footer   = Example of single tube triode grid-leak receiver from 1920, the first type of amplifying radio receiver. In the grid leak circuit, electrons attracted to the grid during the positive half cycles of the radio signal charge the grid capacitor with a negative voltage of a few volts, [[biasing]] the grid near its [[cutoff voltage]], so the tube conducts only during the positive half-cycles, [[rectification (electricity)|rectifying]] the radio carrier. 
}}

The first amplifying vacuum tube, the [[Audion]], a crude [[triode]], was invented in 1906 by [[Lee De Forest]] as a more sensitive [[detector (radio)|detector]] for radio receivers, by adding a third electrode to the thermionic diode detector, the [[Fleming valve]].<ref name="Lee1" /><ref name="McNicol8">[https://archive.org/stream/radiosconquestof00mcnirich#page/254/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 254-259]</ref><ref name="Okamura">{{cite book
  | last = Okamura
  | first = Sōgo 
  | title = History of Electron Tubes
  | publisher = IOS Press
  | year = 1994
  | pages = 17–22
  | url = https://books.google.com/books?id=VHFyngmO95YC&q=Audion+triode&pg=PA21
  | isbn = 978-9051991451}}</ref><ref name="De Forest">{{cite journal
  | last = De Forest
  | first = Lee
  | title = The Audion; A New Receiver for Wireless Telegraphy
  | journal = Trans. AIEE
  | volume = 25
  | pages = 735–763
  | date = January 1906
  | url = https://babel.hathitrust.org/cgi/pt?id=uiug.30112089533605&view=1up&seq=356
  | doi = 10.1109/t-aiee.1906.4764762
  | access-date = March 30, 2021}} The link is to a reprint of the paper in the ''Scientific American Supplement'', Nos. 1665 and 1666, November 30, 1907 and December 7, 1907, p.348-350 and 354-356.</ref> It was not widely used until its [[amplifier|amplifying]] ability was recognized around 1912.<ref name="Lee1" /> The first tube receivers, invented by De Forest and built by hobbyists until the mid 1920s, used a single Audion which functioned as a [[grid-leak detector]] which both [[rectifier|rectified]] and amplified the radio signal.<ref name="McNicol8" /><ref name="Williams" /><ref name="Terman7">{{cite book
 | last1  = Terman
 | first1 = Frederick E.
 | title  = Radio Engineers' Handbook
 | publisher = McGraw-Hill Book Co. 
 | date   = 1943
 | location = New York
 | pages  = 564–565
 | url    = http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Radio-Engineer%27s-Handbook-1.pdf
 }}</ref> There was uncertainty about the operating principle of the Audion until [[Edwin Armstrong]] explained both its amplifying and demodulating functions in a 1914 paper.<ref name="Armstrong4">{{cite journal
  | last1  = Armstrong
  | first1 = Edwin
  | title = Operating features of the Audion
  | journal = Electrical World
  | volume = 64
  | issue = 24
  | pages = 1149–1152
  | date = December 12, 1914
  | url = https://books.google.com/books?id=rdpQAAAAYAAJ&pg=PA1149
  | doi = 10.1111/j.1749-6632.1916.tb55188.x
  | access-date = May 14, 2017| bibcode = 1916NYASA..27..215A| s2cid = 85101768
 }}</ref><ref name="McNicol118">[https://archive.org/stream/radiosconquestof00mcnirich#page/180/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 180]</ref><ref name="Lee4">[https://books.google.com/books?id=WqdzSq56lSQC&pg=PA13 Lee, Thomas H. (2004) ''The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed.'', p. 13]</ref> The grid-leak detector circuit was also used in [[Regenerative circuit|regenerative]], [[Tuned radio frequency receiver|TRF]], and early [[superheterodyne receiver]]s ''(below)'' until the 1930s.

To give enough output power to drive a loudspeaker, 2 or 3 additional vacuum tube stages were needed for audio amplification.<ref name="McNicol8" /> Many early hobbyists could only afford a single tube receiver, and listened to the radio with earphones, so early tube amplifiers and speakers were sold as add-ons.

In addition to very low [[gain (electronics)|gain]] of about 5 and a short lifetime of about 30 – 100 hours, the primitive Audion had erratic characteristics because it was incompletely evacuated. De Forest believed that [[ionization]] of residual air was key to Audion operation.<ref name="Langmuir">{{cite journal
  | last1  = Langmuir
  | first1 = Irving 
  | title = The Pure Electron Discharge and its Applications in Radio Telegraphy and Telephony
  | journal = Proceedings of the IRE
  | volume = 3
  | issue = 3
  | pages = 261–293
  | date = September 1915
  | url = http://www.ieee.org/publications_standards/publications/proceedings/langmuir.pdf
  | doi = 10.1109/jrproc.1915.216680
  | access-date = January 12, 2016}}</ref><ref name="Tyne9">{{cite journal
  | last1  = Tyne
  | first1 = Gerald F. J.
  | title = The Saga of the Vacuum Tube, Part 9
  | journal = Radio News
  | volume = 30
  | issue = 6
  | pages = 30–31, 56, 58
  | date = December 1943
  | url = http://www.americanradiohistory.com/Archive-Radio-News/40s/Radio-News-1943-12-R.pdf#page=30
  | access-date = June 17, 2016}}</ref> This made it a more sensitive detector<ref name="Langmuir" /> but also caused its electrical characteristics to vary during use.<ref name="McNicol8" /><ref name="Okamura" /> As the tube heated up, gas released from the metal elements would change the pressure in the tube, changing the plate current and other characteristics, so it required periodic [[biasing (electronics)|bias]] adjustments to keep it at the correct operating point. Each Audion stage usually had a [[rheostat]] to adjust the filament current, and often a [[potentiometer]] or multiposition switch to control the plate voltage. The filament rheostat was also used as a volume control. The many controls made multitube Audion receivers complicated to operate.

By 1914, Harold Arnold at [[Western Electric]] and [[Irving Langmuir]] at [[General Electric|GE]] realized that the residual gas was not necessary; the Audion could operate on electron conduction alone.<ref name="Okamura" /><ref name="Langmuir" /><ref name="Tyne9" /> They evacuated tubes to a lower pressure of 10<sup>−9</sup> atm, producing the first "hard vacuum" triodes. These more stable tubes did not require bias adjustments, so radios had fewer controls and were easier to operate.<ref name="Okamura" /> During [[World War I]] civilian radio use was prohibited, but by 1920 large-scale production of vacuum tube radios began. The "soft" incompletely evacuated tubes were used as detectors through the 1920s then became obsolete.

{{breakafterimages}}

====Regenerative (autodyne) receiver====
{{Main|Regenerative receiver}}
[[File:Regenerative receiver block diagram 2.svg|thumb|Block diagram of regenerative receiver]]
[[File:Armstrong regenerative receiver circuit.svg|thumb|Circuit of single tube Armstrong regenerative receiver]]
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Man listening to regenerative receiver 1922.jpg
| caption1 = Homemade Armstrong regenerative receiver, 1922. The "tickler" coil ''(L3)'' is visible on the front panel, coupled to the input tuning coils.
| width1   = 150
| image2   = Paragon RA-10 Radio Receiver, 1920 - National Electronics Museum - DSC00230.JPG
| caption2 = Commercial regenerative receiver from the early 1920s, the Paragon RA-10 ''(center)'' with separate 10R single tube RF amplifier ''(left)'' and three tube DA-2 detector and 2-stage audio amplifier unit ''(right)''.  The 4 cylindrical dry cell "A" batteries ''(right rear)'' powered the tube filaments, while the 2 rectangular "B" batteries provided plate voltage. 
| width2   = 480
| image3   = Regenerartive Receiver-S7300056.JPG
| caption3 = Homemade one-tube Armstrong regenerative receiver from the 1940s. The tickler coil is a variometer winding mounted on a shaft inside the tuning coil ''(upper right)'' which can be rotated by a knob on the front panel.  
| width3   = 210
| footer   =
}}

The [[regenerative receiver]], invented by [[Edwin Armstrong]]<ref name="Armstrong2">{{cite journal 
|last=Armstrong 
|first=Edwin H. 
|title=Some recent developments in the Audion receiver 
|journal=Proc. IRE 
|volume=3 
|issue=9 
|pages=215&ndash;247 
|date=September 1915 
|url=http://www.ieee.org/documents/00573757.pdf 
|access-date=August 29, 2012 
|doi= 10.1109/JRPROC.1915.216677|s2cid=2116636 
}}</ref> in 1913 when he was a 23-year-old college student,<ref name="Armstrong3">{{cite journal
  | last1  = Armstrong
  | first1 = Edwin H.
  | title = The Regenerative Circuit
  | journal = The Electrical Journal
  | volume = 18
  | issue = 4
  | pages = 153–154
  | date = April 1921
  | url = https://books.google.com/books?id=sHs2AQAAMAAJ&pg=PA153
  | access-date = January 11, 2016}}</ref> was used very widely until the late 1920s particularly by hobbyists who could only afford a single-tube radio. Today transistor versions of the circuit are still used in a few inexpensive applications like [[walkie-talkie]]s. In the regenerative receiver the [[gain (electronics)|gain]] (amplification) of a [[vacuum tube]] or transistor is increased by using ''regeneration'' ([[positive feedback]]); some of the energy from the tube's output circuit is fed back into the input circuit with a [[feedback loop]].<ref name="Carr" /><ref name="Williams">{{cite book
 | last1  = Williams
 | first1 = Lyle Russell 
 | title  = The New Radio Receiver Building Handbook
 | publisher = Lulu
 | date   = 2006
 | pages  = 24–27
 | url    = https://books.google.com/books?id=XiKgKdeBi6cC&q=regenerative+%22tuned+radio+frequency%22&pg=PA24
 | isbn   = 978-1847285263
 }}</ref><ref name="ArmyManual2">[https://books.google.com/books?id=f9QXAAAAYAAJ&pg=PA187 Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 187-190]</ref><ref name="Terman6">[http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Radio-Engineer%27s-Handbook-1.pdf Terman, Frederick E. (1943) ''Radio Engineers' Handbook'', p. 574-575]</ref><ref name="McNicol9">[https://archive.org/stream/radiosconquestof00mcnirich#page/260/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 260-262]</ref> The early vacuum tubes had very low gain (around 5). Regeneration could not only increase the gain of the tube enormously, by a factor of 15,000 or more, it also increased the [[Q factor]] of the tuned circuit, decreasing (sharpening) the [[bandwidth (signal processing)|bandwidth]] of the receiver by the same factor, improving [[Selectivity (radio)|selectivity]] greatly.<ref name="Williams" /><ref name="ArmyManual2" /><ref name="Terman6" /> The receiver had a control to adjust the feedback. The tube also acted as a [[grid-leak detector]] to rectify the AM signal.<ref name="Williams" />

Another advantage of the circuit was that the tube could be made to oscillate, and thus a single tube could serve as both a beat frequency oscillator and a detector, functioning as a heterodyne receiver to make [[continuous wave|CW]] [[radiotelegraphy]] transmissions audible.<ref name="Williams" /><ref name="ArmyManual2" /><ref name="Terman6" /> This mode was called an [[autodyne]] receiver. To receive radiotelegraphy, the feedback was increased until the tube oscillated, then the oscillation frequency was tuned to one side of the transmitted signal. The incoming radio [[carrier signal]] and local oscillation signal mixed in the tube and produced an audible [[heterodyne]] (beat) tone at the difference between the frequencies.

A widely used design was the [[Armstrong oscillator|Armstrong circuit]], in which a "tickler" coil in the plate circuit was coupled to the tuning coil in the grid circuit, to provide the feedback.<ref name="Carr" /><ref name="Williams" /><ref name="McNicol9" /> The feedback was controlled by a variable resistor, or alternately by moving the two windings physically closer together to increase loop gain, or apart to reduce it.<ref name="ArmyManual2" /> This was done by an adjustable air core transformer called a [[Inductor#Variable inductors|variometer]]{{Broken anchor|date=2024-06-25|bot=User:Cewbot/log/20201008/configuration|target_link=Inductor#Variable inductors|reason= }} (variocoupler). Regenerative detectors were sometimes also used in TRF and superheterodyne receivers.

One problem with the regenerative circuit was that when used with large amounts of regeneration the selectivity (<span class="ltr">Q</span>) of the tuned circuit could be ''too'' sharp, attenuating the AM sidebands, thus distorting the audio modulation.<ref name="Langford-Smith2">{{cite book
 | last1  = Langford-Smith
 | first1 = F.
 | title  = Radiotron Designer's Handbook, 4th Ed.
 | publisher = Wireless Press for RCA
 | date   = 1953
 | pages  = 1223–1224
 | url    = https://docs.google.com/viewer?docex=1&url=http://www.tubebooks.org/Books/intro_RDH4.pdf
 }}</ref>  This was usually the limiting factor on the amount of feedback that could be employed.

A more serious drawback was that it could act as an inadvertent [[radio transmitter]], producing interference ([[radio frequency interference|RFI]]) in nearby receivers.<ref name="Carr" /><ref name="Williams" /><ref name="ArmyManual2" /><ref name="Terman6" /><ref name="McNicol9" /><ref name="Wing">In the early 1920s Armstrong, David Sarnoff head of RCA, and other radio pioneers testified before the US Congress on the need for legislation against radiating regenerative receivers. {{cite journal
  | last1  = Wing
  | first1 = Willis K.
  | title = The Case Against the Radiating Receiver
  | journal = Broadcast Radio
  | volume = 5
  | issue = 6
  | pages = 478–482
  | date = October 1924
  | url = http://www.americanradiohistory.com/Archive-Radio-Broadcast/Radio-Broadcast-1924-10.pdf
  | access-date = January 16, 2016}}</ref> In AM reception, to get the most sensitivity the tube was operated very close to instability and could easily break into oscillation (and in CW reception ''did'' oscillate), and the resulting radio signal was radiated by its wire antenna. In nearby receivers, the regenerative's signal would beat with the signal of the station being received in the detector, creating annoying [[heterodyne]]s, ([[beat (acoustics)|beat]]s), howls and whistles.<ref name="Carr" /> Early regeneratives which oscillated easily were called "bloopers". One preventive measure was to use a stage of RF amplification before the regenerative detector, to isolate it from the antenna.<ref name="Williams" /><ref name="ArmyManual2" /> But by the mid 1920s "regens" were no longer sold by the major radio manufacturers.<ref name="Carr" />
{{breakafterimages}}

====Superregenerative receiver====
{{Main|Regenerative receiver#Superregenerative receiver}}
[[Image:Edwin Armstrong and superregenerative receiver.jpg|thumb|upright=0.5|Armstrong presenting his superregenerative receiver, June 28, 1922, Columbia University]]

This was a receiver invented by [[Edwin Armstrong]] in 1922 which used regeneration in a more sophisticated way, to give greater gain.<ref name="Lee3" /><ref name="ArmyManual3">[https://books.google.com/books?id=f9QXAAAAYAAJ&pg=PA190 Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 190-193]</ref><ref name="Terman5">{{cite book
 | last1  = Terman
 | first1 = Frederick E.
 | title  = Radio Engineers' Handbook
 | publisher = McGraw-Hill Book Co. 
 | date   = 1943
 | location = New York
 | pages  = 662–663
 | url    = http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Radio-Engineer%27s-Handbook-1.pdf
 }}</ref><ref name="Williams2">[https://books.google.com/books?id=XiKgKdeBi6cC&dq=superheterodyne&pg=PA31 Williams, Lyle Russell (2006) ''The New Radio Receiver Building Handbook'', p. 31-32]</ref><ref name="McNicol11">[https://archive.org/stream/radiosconquestof00mcnirich#page/279/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 279-282]</ref> It was used in a few shortwave receivers in the 1930s, and is used today in a few cheap high frequency applications such as [[walkie-talkie]]s and [[garage door opener]]s.

In the regenerative receiver the [[loop gain]] of the feedback loop was less than one, so the tube (or other amplifying device) did not oscillate but was close to oscillation, giving large gain.<ref name="ArmyManual3" /> In the superregenerative receiver, the loop gain was made equal to one, so the amplifying device actually began to oscillate, but the oscillations were interrupted periodically.<ref name="Lee3" /><ref name="Dixon" /> This allowed a single tube to produce gains of over 10<sup>6</sup>.

{{breakafterimages}}

====TRF receiver====
{{Main|Tuned radio frequency receiver}}
{{multiple image
| align = center
| direction = horizontal
| header   = 
| image1   = RadioBaquelita314.jpg
| caption1 = Early 6 tube TRF receiver from around 1920. The 3 large knobs adjust the 3 tuned circuits to tune in stations
| width1   = 200
| image2   = Atwater Kent Type TA detector and 2 stage AF amplifier - Bayernhof Museum - DSC06297.JPG
| caption2 = Atwater-Kent TRF receiver from the 1920s with 2 RF stages ''(left)'', detector and two audio amplifier tubes ''(right)''. The loudspeaker consists of an earphone coupled to an acoustic horn which amplifies the sound.
| width2   = 200
| image3   = Tuning a TRF radio 1925.jpg
| caption3 = Tuning all 3 stages of a TRF set in unison. This 1925 Grebe Synchrophase receiver has [[thumbwheel]]s instead of knobs which can be turned with a finger, so a third hand is not needed. 
| width3   = 193
| footer   =
}}

The [[tuned radio frequency receiver|tuned radio frequency (TRF) receiver]], invented in 1916 by [[Ernst Alexanderson]], improved both sensitivity and [[Selectivity (radio)|selectivity]] by using several stages of amplification before the detector, each with a [[tuned circuit]], all tuned to the frequency of the station.<ref name="Carr" /><ref name="Lee3" /><ref name="Dixon">{{cite book
 | last1  = Dixon
 | first1 = Robert 
 | title  = Radio Receiver Design
 | publisher = CRC Press
 | date   = 1998
 | pages  = 57–61
 | url    = https://books.google.com/books?id=hqkKAV1KsrQC&pg=PA57
 | isbn   = 978-0824701611
 }}</ref><ref name="ArmyManual6">[https://books.google.com/books?id=f9QXAAAAYAAJ&pg=PA170 Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 170-175]</ref><ref name="McNicol10">[https://archive.org/stream/radiosconquestof00mcnirich#page/263/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 263-267]</ref>

A major problem of early TRF receivers was that they were complicated to tune, because each resonant circuit had to be adjusted to the frequency of the station before the radio would work.<ref name="Carr" /><ref name="Lee3" /> In later TRF receivers the tuning capacitors were linked together mechanically ("ganged") on a common shaft so they could be adjusted with one knob, but in early receivers the frequencies of the tuned circuits could not be made to "track" well enough to allow this, and each tuned circuit had its own tuning knob.<ref name="Dixon" /><ref name="ArmyManual4">[https://books.google.com/books?id=f9QXAAAAYAAJ&pg=PA177 Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 177-179]</ref> Therefore, the knobs had to be turned simultaneously. For this reason most TRF sets had no more than three tuned RF stages.<ref name="Williams" /><ref name="Langford-Smith2" />

A second problem was that the multiple radio frequency stages, all tuned to the same frequency, were prone to oscillate,<ref name="ArmyManual4" /><ref name="Terman2">{{cite book
 | last1  = Terman
 | first1 = Frederick E.
 | title  = Radio Engineers' Handbook
 | publisher = McGraw-Hill Book Co. 
 | date   = 1943
 | location = New York
 | pages  = 438–439
 | url    = http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Radio-Engineer%27s-Handbook-1.pdf
 }}</ref> and the [[parasitic oscillation]]s mixed with the radio station's [[carrier signal|carrier]] in the detector, producing audible [[heterodyne]]s ([[beat (music)|beat]] notes), whistles and moans, in the speaker.<ref name="Carr" /><ref name="Williams" /><ref name="Lee3" /><ref name="McNicol10" /> This was solved by the invention of the Neutrodyne circuit ''(below)'' and the development of the [[tetrode]] later around 1930, and better shielding between stages.<ref name="McNicol10" />

Today the TRF design is used in a few integrated (<span class="ltr">IC</span>) receiver chips. From the standpoint of modern receivers the disadvantage of the TRF is that the gain and bandwidth of the tuned RF stages are not constant but vary as the receiver is tuned to different frequencies.<ref name="Terman2" /> Since the bandwidth of a filter with a given [[Q factor|Q]] is proportional to the frequency, as the receiver is tuned to higher frequencies its bandwidth increases.<ref name="ArmyManual5">[https://books.google.com/books?id=f9QXAAAAYAAJ&pg=PA195 Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 195-197]</ref><ref name="Rembovsky" />
{{breakafterimages}}

====Neutrodyne receiver====
{{Main|Neutrodyne }}
{{multiple image
| image1   = First neutrodyne radio receiver.jpg
| caption1 = Hazeltine's prototype Neutrodyne receiver, presented at a March 2, 1923 meeting of the Radio Society of America at Columbia University.
| width1   = 150
| image2   = Tuning Neutrodyne receiver 1924.jpg
| caption2 = Tuning a Neutrodyne TRF receiver with 3 tuned circuits ''(large knobs)'', 1924. For each station the index numbers on the dials had to be written down so that the station could be found again.
| width2   = 158
| footer   =
}}

The Neutrodyne receiver, invented in 1922 by [[Louis Alan Hazeltine|Louis Hazeltine]],<ref name="Patent1450080">US Patent No. 1450080, Louis Alan Hazeltine, [https://patents.google.com/patent/US1450080?oq=1450080 "Method and electric circuit arrangement for neutralizing capacity coupling"]; filed August 7, 1919; granted March 27, 1923</ref><ref name="Hazeltine">{{cite journal
 |last       = Hazeltine
 |first      = Louis A.
 |title      = Tuned Radio Frequency Amplification With Neutralization of Capacity Coupling
 |journal    = Proc. of the Radio Club of America
 |volume     = 2
 |issue      = 8
 |pages      = 7–12
 |date       = March 1923
 |url        = http://www.radioclubofamerica.org/images/proceedings%20index%20author%20sort.pdf
 |access-date = March 7, 2014
}}{{dead link|date=April 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> was a TRF receiver with a "neutralizing" circuit added to each radio amplification stage to cancel the feedback to prevent the oscillations which caused the annoying whistles in the TRF.<ref name="Carr" /><ref name="Lee3" /><ref name="McNicol10" /><ref name="ArmyManual4" /><ref name="Terman3">{{cite book
 | last1  = Terman
 | first1 = Frederick E.
 | title  = Radio Engineers' Handbook
 | publisher = McGraw-Hill Book Co. 
 | date   = 1943
 | location = New York
 | pages  = 468–469
 | url    = http://www.americanradiohistory.com/Archive-Handbooks-Miscellaneous/Radio-Engineer%27s-Handbook-1.pdf
 }}</ref>  In the neutralizing circuit a capacitor fed a feedback current from the plate circuit to the grid circuit which was 180° [[out of phase]] with the feedback which caused the oscillation, canceling it.<ref name="Williams" /> The Neutrodyne was popular until the advent of cheap [[tetrode]] tubes around 1930.

{{breakafterimages}}

====Reflex receiver====
{{Main|Reflex receiver}}
[[File:Reflex receiver block diagram.svg|thumb|upright=1.3|Block diagram of simple single tube reflex receiver]]

The [[reflex receiver]], invented in 1914 by Wilhelm Schloemilch and Otto von Bronk,<ref name="Grimes">{{cite journal
  | last1  = Grimes
  | first1 = David
  | title = The Story of Reflex and Radio Frequency
  | journal = Radio in the Home
  | volume = 2
  | issue = 12
  | pages = 9–10 
  | date = May 1924
  | url = https://docs.google.com/viewer?docex=1&url=http://www.americanradiohistory.com/Archive-Radio-in-the-Home/20s/RitH-1924-May.pdf
  | access-date = January 24, 2016}}</ref> and rediscovered and extended to multiple tubes in 1917 by Marius Latour<ref name="Grimes" /><ref name="Latour">US Patent no. 1405523, Marius Latour ''[https://patents.google.com/patent/US1405523?oq=1405523+latour Audion or lamp relay or amplifying apparatus]'', filed December 28, 1917; granted February 7, 1922</ref> and William H. Priess, was a design used in some inexpensive radios of the 1920s<ref name="McNicol17">[https://archive.org/stream/radiosconquestof00mcnirich#page/283/mode/2up    McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 283-284]</ref> which enjoyed a resurgence in small portable tube radios of the 1930s<ref name="ReflexingToday">{{cite journal
 |title      = Reflexing Today: Operating economy with the newer tubes
 |journal    = Radio World
 |volume     = 23
 |issue      = 17
 |pages      = 3
 |date       = July 8, 1933
 |url        = http://www.americanradiohistory.com/hd2/Archive-Radio-World-IDX/IDX/33/Radio-World-1933-Jul-01-OCR-Page-0019.pdf
 |access-date = January 16, 2016
}}{{Dead link|date=August 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> and again in a few of the first transistor radios in the 1950s.<ref name="Lee3" /><ref name="Langford-Smith1">{{cite book
 | last1  = Langford-Smith
 | first1 = F.
 | title  = Radiotron Designer's Handbook, 4th Ed.
 | publisher = Wireless Press for RCA
 | date   = 1953
 | pages  = 1140–1141
 | url    = https://docs.google.com/viewer?docex=1&url=http://www.tubebooks.org/Books/intro_RDH4.pdf
 }}</ref>  It is another example of an ingenious circuit invented to get the most out of a limited number of active devices. In the reflex receiver the RF signal from the tuned circuit is passed through one or more amplifying tubes or transistors, [[demodulation|demodulated]] in a [[detector (radio)|detector]], then the resulting audio signal is passed ''again'' though the same amplifier stages for audio amplification.<ref name="Lee3" /> The separate radio and audio signals present simultaneously in the amplifier do not interfere with each other since they are at different frequencies, allowing the amplifying tubes to do "double duty". In addition to single tube reflex receivers, some TRF and superheterodyne receivers had several stages "reflexed".<ref name="Langford-Smith1" /> Reflex radios were prone to a defect called "play-through" which meant that the volume of audio did not go to zero when the volume control was turned down.<ref name="Langford-Smith1" />

{{breakafterimages}}

====Superheterodyne receiver====
{{Main|Superheterodyne receiver}}

[[Image:Prototype Armstrong superheterodyne receiver 1920.jpg|thumb|upright=1.5|The first superheterodyne receiver built at Armstrong's Signal Corps laboratory in Paris during World War I. It is constructed in two sections, the [[frequency mixer|mixer]] and [[local oscillator]] ''(left)'' and three IF amplification stages and a detector stage ''(right)''. The intermediate frequency was 75 kHz. ]]
{{multiple image
| align = right
| direction = horizontal
| image1   = Philco radio model PT44 front.jpg
| width1   = 150
| image2   = Philco radio model PT44 chassis back.jpg
| width2   = 150
| footer   = During the 1940s the vacuum tube superheterodyne receiver was refined into a cheap-to-manufacture form called the "[[All American Five]]" because it only required 5 tubes, which was used in almost all broadcast radios until the end of the tube era in the 1970s.
}}

The [[superheterodyne]], invented in 1918 during [[World War I]] by [[Edwin Armstrong]]<ref name="Armstrong1" /> when he was in the [[Signal Corps]], is the design used in almost all modern receivers, except a few specialized applications.<ref name="Lee2" /><ref name="Dixon" /><ref name="Williams1" /> It is a more complicated design than the other receivers above, and when it was invented required 6 - 9 vacuum tubes, putting it beyond the budget of most consumers, so it was initially used mainly in commercial and military communication stations.<ref name="McNicol15">[https://archive.org/stream/radiosconquestof00mcnirich#page/272/mode/2up McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 272-278]</ref> However, by the 1930s the "superhet" had replaced all the other receiver types above.

In the superheterodyne, the "[[heterodyne]]" technique invented by [[Reginald Fessenden]] is used to shift the frequency of the radio signal down to a lower "[[intermediate frequency]]" (<span class="ltr">IF</span>), before it is processed.<ref name="ArmyManual5" /><ref name="McNicol15" /><ref name="Terman8" /> Its operation and advantages over the other radio designs in this section are described above in [[#The superheterodyne design|The superheterodyne design]]

By the 1940s the superheterodyne AM broadcast receiver was refined into a cheap-to-manufacture design called the "[[All American Five]]", because it only used five vacuum tubes: usually a converter (mixer/local oscillator), an IF amplifier, a detector/audio amplifier, audio power amplifier, and a rectifier. This design was used for virtually all commercial radio receivers until the transistor replaced the vacuum tube in the 1970s.
{{breakafterimages}}

===Semiconductor era===
The invention of the [[transistor]] in 1947 revolutionized radio technology, making truly portable receivers possible, beginning with [[transistor radio]]s in the late 1950s. Although portable vacuum tube radios were made, tubes were bulky and inefficient, consuming large amounts of power and requiring several large batteries to produce the filament and plate voltage. Transistors did not require a heated filament, reducing power consumption, and were smaller and much less fragile than vacuum tubes.

====Portable radios====
{{See also|Transistor radio}}

[[File:Vintage Zenith Royal 2000 Trans-Oceanic Transistor Radio, Chassis 11ET40Z2, Made in the USA (12125632465).jpg|thumb|A Zenith transistor based portable radio receiver]]

Companies first began manufacturing radios advertised as portables shortly after the start of commercial broadcasting in the early 1920s. The vast majority of tube radios of the era used batteries and could be set up and operated anywhere, but most did not have features designed for portability such as handles and built in speakers. Some of the earliest portable tube radios were the Winn "Portable Wireless Set No. 149" that appeared in 1920 and the Grebe Model KT-1 that followed a year later. Crystal sets such as the Westinghouse Aeriola Jr. and the [[RCA]] Radiola 1 were also advertised as portable radios.<ref name="Schiffer1991">{{cite book|author=Michael B. Schiffer|title=The Portable Radio in American Life|url=https://books.google.com/books?id=c5C87BmhRegC&pg=PA66|year=1991|publisher=University of Arizona Press|isbn=978-0-8165-1284-3|pages=66–}}</ref>

Thanks to miniaturized vacuum tubes first developed in 1940, smaller portable radios appeared on the market from manufacturers such as [[Zenith Electronics|Zenith]] and [[General Electric]]. First introduced in 1942, Zenith's [[Trans-Oceanic]] line of portable radios were designed to provide entertainment broadcasts as well as being able to tune into weather, marine and international shortwave stations. By the 1950s, a "golden age" of tube portables included [[lunchbox]]-sized tube radios like the Emerson 560, that featured molded plastic cases. So-called "pocket portable" radios like the RCA BP10 had existed since the 1940s, but their actual size was compatible with only the largest of coat pockets.<ref name="Schiffer1991" /> But some, like the Privat-ear and Dyna-mite pocket radios, were small enough to fit a pocket.<ref>[https://books.google.com/books?id=ODRrEAAAQBAJ&dq=subminiature+tubes+shirt-pocket+portables+Pocket-Mite+Micro-Electronics+Privat-ear&pg=PA166 The Portable Radio in American Life]</ref><ref>[https://books.google.com/books?id=8NsDAAAAMBAJ&dq=dyna-mite+pocket+radio&pg=PA44 Popular Mechanics aug 1953]</ref>

The development of the [[bipolar junction transistor]] in the early 1950s resulted in it being licensed to a number of electronics companies, such as [[Texas Instruments]], who produced a limited run of transistorized radios as a sales tool. The [[Regency TR-1]], made by the Regency Division of I.D.E.A. (Industrial Development Engineering Associates) of Indianapolis, Indiana, was launched in 1951. The era of true, shirt-pocket sized portable radios followed, with manufacturers such as [[Sony]], Zenith, RCA, DeWald, and [[Powel Crosley Jr.|Crosley]] offering various models.<ref name="Schiffer1991" /> The Sony TR-63 released in 1957 was the first mass-produced [[transistor radio]], leading to the mass-market penetration of transistor radios.<ref name="Skrabec">{{cite book | last1  = Skrabec | first1 = Quentin R. Jr. | title  = The 100 Most Significant Events in American Business: An Encyclopedia | publisher = ABC-CLIO  | date   = 2012 | pages  = 195–7 | url    = https://books.google.com/books?id=2kc69qrid9oC&pg=PA195 | isbn   = 978-0313398636 }}</ref>

====Digital technology====
{{See also|Digital radio}}
[[File:HA0478-006 (6011470974).jpg|thumb|A modern [[smartphone]] has several [[RF CMOS]] [[digital radio]] transmitters and receivers to connect to different devices, including a [[RF transceiver|cellular receiver]], [[wireless modem]], [[Bluetooth]] [[modem]], and [[GPS receiver]].<ref name="Kim">{{cite book |last1=Kim |first1=Woonyun |chapter=CMOS power amplifier design for cellular applications: an EDGE/GSM dual-mode quad-band PA in 0.18 μm CMOS |editor-last1=Wang |editor-first1=Hua |editor-last2=Sengupta |editor-first2=Kaushik |title=RF and mm-Wave Power Generation in Silicon |date=2015 |publisher=[[Academic Press]] |isbn=978-0-12-409522-9 |pages=89–90 |chapter-url=https://books.google.com/books?id=PzzLAwAAQBAJ&pg=PA89}}</ref>]]

The development of [[integrated circuit]] (<span class="ltr">IC</span>) chips in the 1970s created another revolution, allowing an entire radio receiver to be put on an IC chip. IC chips reversed the economics of radio design used with vacuum-tube receivers. Since the marginal cost of adding additional amplifying devices (transistors) to the chip was essentially zero, the size and cost of the receiver was dependent not on how many active components were used, but on the passive components; inductors and capacitors, which could not be integrated easily on the chip.<ref name="Lee" /> The development of RF [[CMOS]] chips, pioneered by [[Asad Ali Abidi]] at [[UCLA]] during the 1980s and 1990s, allowed low power wireless devices to be made.<ref name="O'Neill">{{cite journal |last1=O'Neill |first1=A. |title=Asad Abidi Recognized for Work in RF-CMOS |journal=IEEE Solid-State Circuits Society Newsletter |date=2008 |volume=13 |issue=1 |pages=57–58 |doi=10.1109/N-SSC.2008.4785694 |issn=1098-4232}}</ref>

The current trend in receivers is to use [[digital circuitry]] on the chip to do functions that were formerly done by [[analog circuit]]s which require passive components. In a digital receiver the IF signal is sampled and digitized, and the bandpass filtering and detection functions are performed by [[digital signal processing]] (DSP) on the chip. Another benefit of DSP is that the properties of the receiver; channel frequency, bandwidth, gain, etc. can be dynamically changed by software to react to changes in the environment; these systems are known as [[software-defined radio]]s or [[cognitive radio]].

Many of the functions performed by [[analog electronics]] can be performed by [[software]] instead. The benefit is that software is not affected by temperature, physical variables, electronic noise and manufacturing defects.<ref name="History of the Radio Receiver">{{cite web |url=http://www.radio-electronics.com/info/radio_history/radiohist/hstrx.php#top |title=History of the Radio Receiver |access-date=2007-11-23 |publisher=Radio-Electronics.Com |archive-url=https://web.archive.org/web/20070916234804/http://www.radio-electronics.com/info/radio_history/radiohist/hstrx.php#top |archive-date=2007-09-16 |url-status=dead }}</ref>

[[Digital signal processing]] permits signal processing techniques that would be cumbersome, costly, or otherwise infeasible with analog methods. A digital signal is essentially a stream or sequence of numbers that relay a message through some sort of medium such as a wire. DSP hardware can tailor the bandwidth of the receiver to current reception conditions and to the type of signal. A typical analog only receiver may have a limited number of fixed bandwidths, or only one, but a DSP receiver may have 40 or more individually selectable filters. DSP is used in [[cell phone]] systems to reduce the data rate required to transmit voice.

In [[digital radio]] broadcasting systems such as [[Digital Audio Broadcasting]] (DAB), the analog audio signal is [[digital audio|digitized]] and [[Audio compression (data)|compressed]], typically using a [[modified discrete cosine transform]] (MDCT) [[audio coding format]] such as [[AAC+]].<ref>{{cite book |last1=Britanak |first1=Vladimir |last2=Rao |first2=K. R. |title=Cosine-/Sine-Modulated Filter Banks: General Properties, Fast Algorithms and Integer Approximations |date=2017 |publisher=Springer |isbn=9783319610801 |page=478 |url=https://books.google.com/books?id=cZ4vDwAAQBAJ&pg=PA478}}</ref>

"PC radios", or radios that are designed to be controlled by a standard PC are controlled by specialized PC software using a serial port connected to the radio. A "PC radio" may not have a front-panel at all, and may be designed exclusively for computer control, which reduces cost.

Some PC radios have the great advantage of being field upgradable by the owner. New versions of the DSP [[firmware]] can be downloaded from the manufacturer's web site and uploaded into the [[flash memory]] of the radio. The manufacturer can then in effect add new features to the radio over time, such as adding new filters, DSP noise reduction, or simply to correct bugs.

A full-featured radio control program allows for scanning and a host of other functions and, in particular, integration of databases in real-time, like a "TV-Guide" type capability. This is particularly helpful in locating all transmissions on all frequencies of a particular broadcaster, at any given time. Some control software designers have even integrated [[Google Earth]] to the shortwave databases, so it is possible to "fly" to a given transmitter site location with a click of a mouse. In many cases the user is able to see the transmitting antennas where the signal is originating from.

Since the [[Graphical user interface|Graphical User Interface]] to the radio has considerable flexibility, new features can be added by the software designer. Features that can be found in advanced control software programs today include a band table, GUI controls corresponding to traditional radio controls, local time clock and a [[UTC]] clock, signal strength meter, a database for shortwave listening with lookup capability, scanning capability, or [[text to speech|text-to-speech]] interface.

The next level in integration is "[[software-defined radio]]", where all filtering, modulation and signal manipulation is done in software. This may be a PC soundcard or by a dedicated piece of DSP hardware. There will be a [[Radio frequency|RF]] front-end to supply an intermediate frequency to the software defined radio. These systems can provide additional capability over "hardware" receivers. For example, they can record large swaths of the radio spectrum to a hard drive for "playback" at a later date. The same SDR that one minute is demodulating a simple AM broadcast may also be able to decode an HDTV broadcast in the next. An open-source project called [[GNU Radio]] is dedicated to evolving a high-performance SDR.

All-digital radio transmitters and receivers present the possibility of advancing the capabilities of radio.<ref>[http://www.eejournal.com/archives/news/20161219-04/ Pizzicato Comes of Age]</ref>

==See also==
{{Portal|Radio}}
* [[Batteryless radio]]
* [[Dielectric wireless receiver]]
* [[Digital Audio Broadcast]] (DAB)
* [[Direct conversion receiver]]
* {{annotated link|List of radios}}
* [[Minimum detectable signal]]
* [[Radiogram (furniture)]]
* [[Receiver (information theory)]]
* [[Distortion]]
* [[Telecommunication]]
* [[Television receive-only]]
* [[Radio transmitter design]]
* [[Radio receiver design]]

==References==
{{reflist|2}}

==Further reading==
{{Commons category|Radio receivers}}
*Communications Receivers, Third Edition, Ulrich L. Rohde, Jerry Whitaker, McGraw Hill, New York, 2001, {{ISBN|0-07-136121-9}}
* {{cite book
| last       = Buga 
| first      = N.
|author2=Falko A. |author3=Chistyakov N.I.
 | editor        = Chistyakov N.I.
| others        = Translated from the Russian by Boris V. Kuznetsov
| title         = Radio Receiver Theory
| year      = 1990
| publisher = [[Mir Publishers]]
| location  = [[Moscow]]
| isbn      = 978-5-03-001321-3
| postscript    = First published in Russian as «Радиоприёмные устройства»
}}
{{Electronic systems}}
{{Telecommunications}}
{{Authority control}}

[[Category:Receiver (radio)| ]]

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radio receiver: Sitelink, Title, Some statements, Description: en, Miscellaneous (e.g. aliases, entity existence)
Category:Receiver (radio): Sitelink

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