Radio receiver

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.^[2][3][4] 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).

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.

The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. It consists of a component (C) at the carrier wave frequency f_C, with the modulation contained in narrow frequency bands called sidebands (SB) just above and below the carrier.

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_in, the transfer function of the filter T, and the voltage at the output of the filter V_out as a function of frequency f. The transfer function T is the amount of signal that gets through the filter at each frequency:
${\displaystyle V_{\text{out}}(f)={\text{T}}(f)V_{\text{in}}(f)}$

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.

Coherer receiver

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

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.

• 
  Circuit of Marconi's first coherer radio receiver from 1896
• 
  Coherer from 1904 as developed by Marconi.

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.^{[22][27][29][32][36][37][38]} The coherer was a glass tube with metal electrodes at each end, with loose metal powder between the electrodes.^[22][27][39] 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.^{[22][27][36][40]} 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.^[22][31][41] 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.^[22][31]

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.^[31][35] 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,^[39] until replaced by the crystal detector and electrolytic detector around 1907. In spite of much development work, it was a very crude unsatisfactory device.^[22][27] 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.^[27][39] 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.^[42][43]

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^[44] and even a human brain^[45] from a cadaver as detectors.^[22][46]

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.^[47] 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.^[48][49] 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.^{[50][51][52][53]} 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.^[54]

• 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.^{[22][51][55][56][57]} The electrolytic action caused current to be conducted in only one direction. The detector was used until about 1910.^[51] 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.^[22]

Early Fleming valve.

Marconi valve receiver for use on ships had two Fleming valves (top) in case one burned out. It was used on the RMS Titanic.

• 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.^{[29][58][59][60]} 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.^[58]

• 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^[47][48] and gave their name to the crystal radio receiver (below).^[51][61][62] 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.^[29] 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.^{[28][29][37][63]} 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.^[64][65] 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.^[28][63][66] Some method was needed to allow the receiver to select which transmitter's signal to receive.^[66][67] Multiple wavelengths produced by a poorly tuned transmitter caused the signal to "dampen", or die down, greatly reducing the power and range of transmission.^[68] In 1892, William Crookes gave a lecture^[69] 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.^[35][64][70] 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^[71] 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 transmission of power for lighting (mainly by what he thought was ground conduction^[72]) 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.^[32][70] Use of tuning in free space "Hertzian waves" (radio) was explained and demonstrated in Oliver Lodge's 1894 lectures on Hertz's work.^[73] 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.^[74][75][76]

By 1897 the advantages of tuned systems had become clear, and Marconi and the other wireless researchers had incorporated tuned circuits, consisting of capacitors and inductors connected together, into their transmitters and receivers.^{[28][32][35][37][63][75]} The tuned circuit acted like an electrical analog of a tuning fork. It had a high 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.^[29] The frequency of the station received f was determined by the capacitance C and inductance L in the tuned circuit: $${\displaystyle f={\frac {1}{2\pi {\sqrt {LC}}}}}$$

Inductive coupling

Braun receiving transformer from 1904

Crystal receiver from 1914 with "loose coupler" tuning transformer. The secondary coil (1) can be slid in or out of the primary (in box) to adjust the coupling. Other components: (2) primary tuning capacitor, (3) secondary tuning capacitor, (4) loading coil, (5) crystal detector, (8) headphones

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, allowing only a narrow band of frequencies through.^[28][29] 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 inductively-coupled circuit, or resonant transformer (oscillation transformer or RF transformer).^{[28][32][35][37][75][77]} 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.^[29] 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;^[63] 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.^[29] One was that by using the correct turns ratio, the 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.^[25][29] 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.^[28][77] 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 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.^[28][78] 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.^[29] Resonant transformers continued to be used as the bandpass filter in vacuum tube radios, and new forms such as the variometer were invented.^[78][79] 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.^[80] This allowed it to pass the sidebands of AM modulation on either side of the 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 IF transformers.

Crystal radio receiver

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.

Family listening to the first broadcasts around 1920 with a crystal receiver. The mother and father have to share an earphone

After vacuum-tube receivers appeared around 1920, the crystal set became a simple cheap alternative radio used by youth and the poor.

Simple crystal radio. The capacitance of the wire antenna connected to the coil serves as the capacitor in the tuned circuit.

Typical "loose coupler" crystal radio circuit

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.^[88] 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 broadcasts, which began around 1920.^[89] 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.^[88] 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.^[29][51][90] It consisted of a mineral crystal, usually galena, which was lightly touched by a fine springy wire (the "cat whisker") on an adjustable arm.^[51][91] 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:

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.^[92]

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 earphones; it could not drive a loudspeaker.^[29][91] 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.^[93][94] However, when used to receive broadcast stations a typical home crystal set had a more limited range of about 25 miles.^[95] In sophisticated crystal radios the "loose coupler" inductively coupled tuned circuit was used to increase the Q. However it still had poor selectivity compared to modern receivers.^[91]

Heterodyne receiver and BFO

Beginning around 1905 continuous wave (CW) 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.^[24]

The continuous wave radiotelegraphy signals produced by these transmitters required a different method of reception.^[96][97] The radiotelegraphy signals produced by spark gap transmitters consisted of strings of damped waves 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 (sine waves). 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.^{[48][96] [98]} 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.^[11] 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.

In 1901 Reginald Fessenden had invented a better means of accomplishing this.^{[96][98][99][100]} In his heterodyne receiver an unmodulated sine wave radio signal at a frequency f_O offset from the incoming radio wave carrier f_C was generated by a local oscillator and 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) frequencies at the sum f_C + f_O and the difference f_C − f_O between these frequencies. By choosing f_O correctly the lower heterodyne f_C − f_O 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".^[98]

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_O with the required stability.^[101] Fessenden first used his large radio frequency alternator,^[11] 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^[98] invented by Edwin Armstrong and Alexander Meissner in 1913.^[48][102] 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 receivers 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).^[98][11]

The first vacuum-tube receivers

This section's factual accuracy is disputed. (October 2020)

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, 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 for radio receivers, by adding a third electrode to the thermionic diode detector, the Fleming valve.^{[58][79][110][111]} It was not widely used until its amplifying ability was recognized around 1912.^[58] 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 rectified and amplified the radio signal.^{[79][108][112]} There was uncertainty about the operating principle of the Audion until Edwin Armstrong explained both its amplifying and demodulating functions in a 1914 paper.^{[113][114][115]} The grid-leak detector circuit was also used in regenerative, TRF, and early superheterodyne receivers (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.^[79] 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 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.^[116][117] This made it a more sensitive detector^[116] but also caused its electrical characteristics to vary during use.^[79][110] 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 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 GE realized that the residual gas was not necessary; the Audion could operate on electron conduction alone.^{[110][116][117]} They evacuated tubes to a lower pressure of 10⁻⁹ 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.^[110] 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.

Regenerative (autodyne) receiver

Homemade Armstrong regenerative receiver, 1922. The "tickler" coil (L3) is visible on the front panel, coupled to the input tuning coils.

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.

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.

The regenerative receiver, invented by Edwin Armstrong^[118] in 1913 when he was a 23-year-old college student,^[119] 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-talkies. In the regenerative receiver the 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.^{[29][108][120][121][122]} 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 of the receiver by the same factor, improving selectivity greatly.^{[108][120][121]} The receiver had a control to adjust the feedback. The tube also acted as a grid-leak detector to rectify the AM signal.^[108]

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 CW radiotelegraphy transmissions audible.^{[108][120][121]} 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 circuit, in which a "tickler" coil in the plate circuit was coupled to the tuning coil in the grid circuit, to provide the feedback.^{[29][108][122]} 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.^[120] This was done by an adjustable air core transformer called a variometer (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 (Q) of the tuned circuit could be too sharp, attenuating the AM sidebands, thus distorting the audio modulation.^[123] 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 (RFI) in nearby receivers.^{[29][108][120][121][122][124]} 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 heterodynes, (beats), howls and whistles.^[29] 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.^[108][120] But by the mid-1920s "regens" were no longer sold by the major radio manufacturers.^[29]

Superregenerative receiver

This was a receiver invented by Edwin Armstrong in 1922 which used regeneration in a more sophisticated way, to give greater gain.^{[109][125][126][127][128]} 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-talkies and garage door openers.

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.^[125] 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.^[109][12] This allowed a single tube to produce gains of over 10⁶.

TRF receiver

Early 6 tube TRF receiver from around 1920. The 3 large knobs adjust the 3 tuned circuits to tune in stations

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.

Tuning all 3 stages of a TRF set in unison. This 1925 Grebe Synchrophase receiver has thumbwheels instead of knobs which can be turned with a finger, so a third hand is not needed.

The tuned radio frequency (TRF) receiver, invented in 1916 by Ernst Alexanderson, improved both sensitivity and selectivity by using several stages of amplification before the detector, each with a tuned circuit, all tuned to the frequency of the station.^{[29][109][12][129][130]}

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.^[29][109] 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.^[12][131] Therefore, the knobs had to be turned simultaneously. For this reason most TRF sets had no more than three tuned RF stages.^[108][123]

A second problem was that the multiple radio frequency stages, all tuned to the same frequency, were prone to oscillate,^[131][132] and the parasitic oscillations mixed with the radio station's carrier in the detector, producing audible heterodynes (beat notes), whistles and moans, in the speaker.^{[29][108][109][130]} 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.^[130]

Today the TRF design is used in a few integrated (IC) 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.^[132] Since the bandwidth of a filter with a given Q is proportional to the frequency, as the receiver is tuned to higher frequencies its bandwidth increases.^[14][18]

Neutrodyne receiver

Hazeltine's prototype Neutrodyne receiver, presented at a March 2, 1923 meeting of the Radio Society of America at Columbia University.

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.

The Neutrodyne receiver, invented in 1922 by Louis Hazeltine,^[133][134] 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.^{[29][109][130][131][135]} 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.^[108] The Neutrodyne was popular until the advent of cheap tetrode tubes around 1930.

Reflex receiver

The reflex receiver, invented in 1914 by Wilhelm Schloemilch and Otto von Bronk,^[136] and rediscovered and extended to multiple tubes in 1917 by Marius Latour^[136][137] and William H. Priess, was a design used in some inexpensive radios of the 1920s^[138] which enjoyed a resurgence in small portable tube radios of the 1930s^[139] and again in a few of the first transistor radios in the 1950s.^[109][140] 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, demodulated in a detector, then the resulting audio signal is passed again though the same amplifier stages for audio amplification.^[109] 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".^[140] 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.^[140]

Superheterodyne receiver

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^[10] when he was in the Signal Corps, is the design used in almost all modern receivers, except a few specialized applications.^[11][12][13] 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.^[15] 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" (IF), before it is processed.^[14][15][16] Its operation and advantages over the other radio designs in this section are described above in 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.

Portable radios

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.^[141]

Thanks to miniaturized vacuum tubes first developed in 1940, smaller portable radios appeared on the market from manufacturers such as 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.^[141] But some, like the Privat-ear and Dyna-mite pocket radios, were small enough to fit a pocket.^[142][143]

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 1954. The era of true, shirt-pocket sized portable radios followed, with manufacturers such as Sony, Zenith, RCA, DeWald, and Crosley offering various models.^[141] The Sony TR-63 released in 1957 was the first mass-produced transistor radio, leading to the mass-market penetration of transistor radios.^[144]

Digital technology

The development of integrated circuit (IC) 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.^[22] 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.^[146]

The current trend in receivers is to use digital circuitry on the chip to do functions that were formerly done by analog circuits 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 radios 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.^[147]

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 digitized and compressed, typically using a modified discrete cosine transform (MDCT) audio coding format such as AAC+.^[148]

"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 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 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 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.^[149]