Radio receiver

In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves (electromagnetic waves of radio frequency) and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters 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 data. 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 televisions, cell phones, wireless modems, radio clocks and other components of communications, remote control, and wireless networking systems.

Broadcast radio receivers

The most familiar form of radio receiver is a broadcast receiver, often just called a ''radio'', which receives audio programs intended for public reception transmitted by local radio stations. 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 batteries inside the radio or a power cord which plugs into an electric outlet. All radios have a 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 (AM) the strength of the radio signal is varied by the audio signal. AM broadcasting is allowed in the AM broadcast bands which are between 148 and 283 kHz in the longwave range, and between 526 and 1706 kHz in the medium frequency (MF) range of the radio spectrum. AM broadcasting is also permitted in shortwave bands, between about 2.3 and 26 MHz, which are used for long distance international broadcasting.

In frequency modulation (FM) the frequency of the radio signal is varied slightly by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 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 microphones. 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. 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)

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 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 rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band III (174–240 MHz) or L band (1.452–1.492 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, as well as any geographical obstructions such as hills between transmitter and receiver. AM broadcast band radio waves travel as ground waves 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 km), and can be blocked by hills between the transmitter and receiver. However FM radio is less susceptible to interference from radio noise ( RFI, sferics, static) and has higher fidelity; better frequency response and less audio distortion, than AM. So in many countries serious music is only broadcast by FM stations, and AM stations specialize in radio news, talk radio, and sports. Like FM, DAB signals travel by line of sight so reception distances are limited by the visual horizon to about 30–40 miles (48–64 km).

Types of broadcast receivers

Radios are or were 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. A typical table radio using vacuum tube technology was the All American Five, which has been superseded by radios using solid state technology. 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'' - A high fidelity AM/FM radio receiver in a component 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 batteries that can be carried with a person. Radios are now often integrated with other audio sources in CD players and portable media players. 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.
**''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 or Stereo receiver'' (in context often just called a ''receiver'') is a component in a hi-fi or 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 (e.g. 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 than an audio signal, from 600 kHz to 6 MHz.
**'' Terrestrial television receiver'', ''broadcast television'' or just ''television'' (TV) - Televisions contains an integral receiver (TV tuner) which receives free broadcast television from local television stations on TV channels in the VHF and 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 K_u band microwave downlink signal from a geostationary direct broadcast satellite 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 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 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 radio link, instead of being attached by a cord. Both the handset and the base station have radio receivers operating in the UHF band that receive the short range bidirectional duplex radio link.
**''Citizens band radio'' - a two-way half-duplex radio operating in the 27 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.
**'' Scanner'' - a receiver that continuously monitors multiple frequencies or radio channels 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.
**''Communications receiver'' or ''shortwave receiver'' - a general purpose audio receiver covering the LF, MF, shortwave ( HF), and 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 microphones used onstage by musical artists, public speakers, and television personalities.
**''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 (WiFi) modem'' - an automated short range digital data transmitter and receiver on a portable wireless device that communicates by microwaves with a nearby access point, a router or gateway, connecting the portable device with a local computer network (WLAN) to exchange data with other devices.
**''Bluetooth'' modem - a very short range (up to 10 m) 2.4-2.83 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, distances are limited by the visual horizon to 30–40 miles. Microwave links are used for private business data, wide area computer networks (WANs), and by telephone companies to transmit distance phone calls and television signals between cities.
*Satellite communications - Communication satellites 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 satellites and spacecraft can involve very long path lengths, from 35,786 km (22,236 mi) for geosynchronous satellites to billions of kilometers for interplanetary spacecraft. This and the limited power available to a spacecraft transmitter mean very sensitive receivers must be used.
**'' 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 exploration missions. These have large dish antennas around 85 ft (25 m) in diameter, and extremely sensitive radio receivers similar to radio telescopes. The RF front end of the receiver is often cryogenically cooled to −195.79 °C (−320 °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 systems, 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 shells.
**''VOR'' receiver - navigational instrument on an aircraft that uses the VHF signal from VOR navigational beacons between 108 and 117.95 MHz to determine the direction to the beacon very accurately, for air navigation.
**'' 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 sources in space like stars, nebulas and galaxies in radio astronomy. They are the most sensitive radio receivers that exist, having large parabolic (dish) antennas up to 500 meters in diameter, and extremely sensitive radio circuits. The RF front end of the receiver is often cryogenically cooled by liquid nitrogen to reduce radio noise.

How receivers work

A radio receiver is connected to an 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 and magnetic fields of the radio wave push the electrons in the antenna back and forth, creating an oscillating voltage.

The antenna may be enclosed inside the receiver's case, as with the ferrite loop antennas of AM radios and the flat inverted F antenna of cell phones; attached to the outside of the receiver, as with whip antennas used on FM radios, or mounted separately and connected to the receiver by a cable, as with rooftop television antennas and satellite dishes.

Main functions of a receiver

Practical radio receivers perform three basic functions on the signal from the antenna: filtering, amplification, and demodulation:

Bandpass filtering

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 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 circuits (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 sidebands ''(SB)'' on either side of the carrier frequency ''(C)'', 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'' ''(PB)'', and the width of the passband in kilohertz is called the bandwidth ''(BW)''. 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'' determined by the filter. In modern receivers 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.

Amplification

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 picowatts or femtowatts. 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 transistors.

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'', which is the minimum signal strength of a station at the antenna, measured in microvolts, 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

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). Each type of modulation requires a different type of demodulator
*an AM receiver that receives an (amplitude modulated) radio signal uses an AM demodulator
*an FM receiver that receives a 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 waves by an earphone or loudspeaker. A video signal, representing moving images, as in a television receiver, is converted to light by a display. Digital data, as in a wireless modem, is applied as input to a computer or microprocessor, which interacts with human users.

:AM demodulation

:The easiest type of demodulation to understand is AM demodulation, used in AM radios to recover the audio modulation signal, which represents sound and is converted to sound waves by the radio's speaker. It is accomplished by a circuit called an envelope detector ''(see circuit)'', consisting of a diode ''(D)'' with a bypass capacitor ''(C)'' across its output.

:See graphs. The amplitude modulated radio signal from the tuned circuit is shown at ''(A)''. 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. ''(B)'' When this signal is applied as input ''V''_I to the detector, the diode ''(D)'' 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 rectifies the AC current to a pulsing DC current. The resulting voltage ''V''_O applied to the load ''R''_L no longer averages zero; its peak value is proportional to the audio signal. ''(C)'' The bypass capacitor ''(C)'' 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 filtering) function, removing the radio frequency carrier pulses, leaving the low frequency audio signal to pass through the load ''R''_L. The audio signal is amplified and applied to earphones or a speaker.

Tuned radio frequency (TRF) receiver

In the simplest type of radio receiver, called a tuned radio frequency (TRF) receiver, the three functions above are performed consecutively: Chapter 1 (1) the mix of radio signals from the antenna is filtered to extract the signal of the desired transmitter; (2) this oscillating voltage is sent through a radio frequency (RF) amplifier to increase its strength to a level sufficient to drive the demodulator; (3) 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; (4) 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. 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 section below describes how the superheterodyne receiver overcomes these problems.

The superheterodyne design

The superheterodyne receiver, invented in 1918 by Edwin Armstrong is the design used in almost all modern receiversWilliams, 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" (IF), before it is processed.Terman, Frederick E. (1943) ''Radio Engineers' Handbook'', p. 636-638
/ref> The incoming radio frequency signal from the antenna is mixed with an unmodulated signal generated by a ''local oscillator'' (LO) in the receiver. The mixing is done in a nonlinear circuit called the "'' 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 note. This lower frequency is called the '' intermediate frequency'' (IF). The IF signal also has the modulation sidebands that carry the information that was present in the original RF signal. The IF signal passes through filter and amplifier stages, then is demodulated 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 (IF) 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 factors, 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,.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. 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.

To achieve both good image rejection and selectivity, many modern superhet receivers use two intermediate frequencies; this is called a '' dual-conversion'' or ''double-conversion'' superheterodyne. 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.

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 oscillations that are encountered in receivers where most of the amplifier stages operate at the same frequency, as in the TRF receiver.

The most important advantage is that better selectivity can be achieved by doing the filtering at the lower intermediate frequency. One of the most important parameters of a receiver is its 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 $f_\text$, rather than the frequency of the original radio signal $f_\text$, 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.

Automatic gain control (AGC)

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 propagation conditions along the path of the radio waves. 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''. 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. 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 gain of the amplifiers to give the optimum signal level for demodulation. 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. 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. 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 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

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 gaps attached to dipole and loop antennas to detect them. 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

The first radio transmitters, used during the initial three decades of radio from 1887 to 1917, a period called the ''spark era'', were spark gap transmitters which generated radio waves by discharging a capacitance through an electric spark. Each spark produced a transient pulse of radio waves which decreased rapidly to zero. These damped waves could not be modulated to carry sound, as in modern AM and 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.

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". The device which did this was called a "''detector''". Since there were no 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:

*An ''antenna'', to intercept the radio waves and convert them to tiny radio frequency electric currents.
*A ''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. Most receivers used a pair of tuned circuits with their coils magnetically coupled, called a resonant transformer (oscillation transformer) or "loose coupler".
*A ''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, 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 didn't 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. 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, mainly by improving technology invented by others.
Sarkar et al. (2006) ''History of Wireless'', p. 349-358archive
Oliver Lodge and Alexander Popov were also experimenting with similar radio wave receiving apparatus at the same time in 1894–5, but they are not known to have transmitted Morse code during this period, just strings of random pulses. Therefore, Marconi is usually given credit for building the first radio receivers.

Coherer receiver

The first radio receivers invented by Marconi, Oliver Lodge and Alexander Popov in 1894-5 used a primitive radio wave detector called a coherer, invented in 1890 by Edouard Branly and improved by Lodge and Marconi.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.McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 107-113
/ref> It initially had a high 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 DC circuit with a 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.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.Nahin, Paul J. (2001) ''The Science of Radio'', p. 53-56
/ref>Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 57-60
/ref> However it can be seen that it was essentially a bistable device, a radio-wave-operated switch, and so it did not have the ability to rectify the radio wave to demodulate the later amplitude modulated (AM) radio transmissions that carried sound.

In a long series of experiments Marconi found that by using an elevated wire monopole antenna instead of Hertz's dipole antennas 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, Cornwall to St. John's, Newfoundland, a distance of 3500 km (2200 miles), which was received by a coherer. 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, until replaced by the crystal detector and electrolytic detector around 1907. In spite of much development work, it was a very crude unsatisfactory device. It was not very sensitive, and also responded to impulsive radio noise ( RFI), such as nearby lights being switched on or off, as well as to the intended signal. 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.

Other early detectors

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 legs and even a human brain from a cadaver as detectors.Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 198-203
/ref>

By the first years of the 20th century, experiments in using amplitude modulation (AM) 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 (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.Phillips, Vivian 1980 ''Early Radio Wave Detectors'', p. 205-209
/ref> This rectified the alternating current radio signal, removing one side of the carrier cycles, leaving a pulsing 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. All except the magnetic detector could rectify and therefore receive AM signals:

*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. Stone, Ellery (1919) ''Elements of Radiotelegraphy'', p. 209-221
/ref>Fleming, John Ambrose (1910) ''The Principles of Electric Wave Telegraphy and Telephony'', p. 446-455
/ref>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 magnets. 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'' which was used to summon help during its famous 15 April 1912 sinking. copied on Stephenson's marconigraph.com personal website

*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.McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 115-119
/ref>Fleming, John Ambrose (1910) ''The Principles of Electric Wave Telegraphy and Telephony'', p. 460-464
/ref>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. Electrolytic detectors that Fessenden had installed on US Navy ships received the first AM radio broadcast on Christmas Eve, 1906, an evening of Christmas music transmitted by Fessenden using his new alternator transmitter.

*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 filament similar to that in an incandescent light bulb, and a metal plate anode.Lee, Thomas H. (2004) ''The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed.'', p. 9-11
/ref>McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 157-162
/ref>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.

*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 and gave their name to the '' crystal radio'' receiver ''(below)''.McNicol, Donald (1946) ''Radio's Conquest of Space'', p. 123-131
/ref>Fleming, John Ambrose (1910) ''The Principles of Electric Wave Telegraphy and Telephony'', p. 471-475
/ref> One of the first 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. 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.

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 of the receiver was equal to the broad bandwidth of the antenna. 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.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. Some method was needed to allow the receiver to select which transmitter's signal to receive. 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. In 1892, William Crookes gave a lecture 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 didn't interfere.Aitken, Hugh 2014 ''Syntony and Spark: The origins of radio'', p. 70-73
/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 and practical application of tuning showed up in the early to mid 1890s in wireless systems not specifically designed for radio communication. Nikola Tesla