Radio receiver

Source: Wikipedia, the free encyclopedia.
(Redirected from
Receiver (radio)
)
radio stations
.
A modern communications receiver, used in two-way radio communication stations to talk with remote locations by shortwave radio.
golden age of radio
, 1925–1955, families gathered to listen to the home radio receiver in the evening

In

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

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

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

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).

Reception

The

radio news, talk radio, and sports radio. 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

clock radio that combines a radio receiver with an alarm clock

Radios are manufactured in a range of styles and functions:

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.

How receivers work

Symbol for an antenna

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

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:

Bandpass filtering

Symbol for a bandpass filter used in block diagrams of radio receivers

Radio waves from many transmitters pass through the air simultaneously without interfering with each other and are received by the antenna. These can be separated in the receiver because they have different

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.

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 fC, 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 Vin, the transfer function
of the filter T, and the voltage at the output of the filter Vout as a function of frequency f. The transfer function T is the amount of signal that gets through the filter at each frequency:

Amplification

Symbol for an amplifier

The power of the radio waves picked up by a receiving antenna decreases with the square of its distance from the transmitting antenna. Even with the powerful transmitters used in radio broadcasting stations, if the receiver is more than a few miles from the transmitter the power intercepted by the receiver's antenna is very small, perhaps as low as

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

electronic noise
present in the circuit, which can drown out a weak radio signal.

Demodulation

Symbol for a demodulator

After the radio signal is filtered and amplified, the receiver must extract the information-bearing

demodulator (detector
). Each type of modulation requires a different type of 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

wireless modem, is applied as input to a computer or microprocessor
, which interacts with human users.

AM demodulation
Envelope detector circuit
How an envelope detector works
The easiest type of demodulation to understand is AM demodulation, used in
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
low pass filtering
) function, removing the radio frequency carrier pulses, leaving the low frequency audio signal to pass through the load RL. The audio signal is amplified and applied to earphones or a speaker.

Tuned radio frequency (TRF) receiver

Block diagram of a tuned radio frequency receiver. To achieve enough selectivity to reject stations on adjacent frequencies, multiple cascaded bandpass filter stages had to be used. The dotted line indicates that the bandpass filters must be tuned together.

In the simplest type of radio receiver, called a

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.[9] 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

Block diagram of a superheterodyne receiver. The dotted line indicates that the RF filter and local oscillator must be tuned in tandem.

The

Edwin Armstrong[10] is the design used in almost all modern receivers[11][9][12][13]
except a few specialized applications.

In the superheterodyne, the radio frequency signal from the antenna is shifted down to a lower "

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,.[18][13][17][19] 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.[13]
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.

Block diagram of a dual-conversion superheterodyne receiver

To achieve both good image rejection and selectivity, many modern superhet receivers use two intermediate frequencies; this is called a

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.[14]

The most important advantage is that better selectivity can be achieved by doing the filtering at the lower intermediate frequency.[9][12][14] 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 , rather than the frequency of the original radio signal , 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.[12]

Automatic gain control (AGC)

The

multipath interference; this is called fading.[20][9]
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.[9][21] 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

lowpass filter to smooth the variations and produce an average level.[21] 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

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.[22][23][24]
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

Guglielmo Marconi, who built the first radio receivers, with his early spark transmitter (right) and coherer receiver (left) from the 1890s. The receiver records the Morse code on paper tape
Generic block diagram of an unamplified radio receiver from the wireless telegraphy era[25]
siphon recorder
at RCA's New York receiving center in 1920. The translation of the Morse code is given below the tape.

The first

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.[24][27]

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".[24] 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:[9]

  • An antenna, to intercept the radio waves and convert them to tiny radio frequency electric currents.
  • 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
    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.[27][30] 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,[30] mainly by improving technology invented by others.[27][31][32][33]

Alexander Popov were also experimenting with similar radio wave receiving apparatus at the same time in 1894–5,[32][36] but they are not known to have transmitted Morse code during this period,[27][30]
just strings of random pulses. Therefore, Marconi is usually given credit for building the first radio receivers.

Coherer receiver


siphon recorder
(left) and transcribed later.
  • Circuit of Marconi's first coherer radio receiver from 1896
    Circuit of Marconi's first coherer radio receiver from 1896
  • Coherer from 1904 as developed by Marconi.
    Coherer from 1904 as developed by Marconi.

The first radio receivers invented by Marconi,

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.

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,

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

Experiment to use human brain as a radio wave detector, 1902

The coherer's poor performance motivated a great deal of research to find better radio wave detectors, and many were invented. Some strange devices were tried; researchers experimented with using

frog 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

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
  • RMS Titanic which was used to summon help during its famous 15 April 1912 sinking.[54]
Electrolytic detector
  • Electrolytic detector ("liquid barretter") - Invented in 1903 by Reginald Fessenden, this consisted of a thin silver-plated platinum wire enclosed in a glass rod, with the tip making contact with the surface of a cup of nitric acid.[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.
  • 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]
A galena cat's whisker detector from a 1920s crystal radio
  • 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.

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

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

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:

Inductive coupling
Marconi's inductively coupled coherer receiver from his controversial April 1900 "four circuit" patent no. 7,777.
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

resonant frequency
.

This circuit had two advantages.

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

IF transformers
.

Patent disputes

Marconi's initial radio system had relatively poor tuning limiting its range and adding to interference.

US Supreme Court ruling on the Marconi Company's ability to sue the US government over patent infringement during World War I. The Court rejected the Marconi Company's suit saying they could not sue for patent infringement when their own patents did not seem to have priority over the patents of Lodge, Stone, and Tesla.[32][70]

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.

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

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

Radio receiver with Poulsen "tikker" consisting of a commutator disk turned by a motor to interrupt the carrier.

Beginning around 1905

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.

). 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]

Rudolph Goldschmidt
, a wheel spun by a motor with contacts spaced around its circumference, which made contact with a stationary brush.

Fessenden's heterodyne radio receiver circuit

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 fO offset from the incoming radio wave carrier fC was applied to a rectifying detector such as a crystal detector or electrolytic detector, along with the radio signal from the antenna. In the detector the two signals mixed, creating two new heterodyne (beat) frequencies at the sum fC + fO and the difference fC − fO between these frequencies. By choosing fO correctly the lower heterodyne fC − fO 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 fO with the required stability.

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]

Vacuum tube era

Unlike today, when almost all radios use a variation of the superheterodyne design, during the 1920s vacuum tube radios used a variety of competing circuits.
During the "Golden Age of Radio" (1920 to 1950), families gathered to listen to the home radio in the evening, such as this Zenith console model 12-S-568 from 1938, a 12-tube superheterodyne with pushbutton tuning and 12-inch cone speaker.

The

Lee De Forest in 1906 was the first practical amplifying device and revolutionized radio.[58] Vacuum tube transmitters replaced spark transmitters and made possible four new types of modulation: continuous wave (CW) radiotelegraphy, amplitude modulation (AM) around 1915 which could carry audio (sound), frequency modulation (FM) around 1938 which had much improved audio quality, and single sideband
(SSB).

The amplifying vacuum tube used energy from a battery or electrical outlet to increase the power of the radio signal, so vacuum tube receivers could be more sensitive and have a greater reception range than the previous unamplified receivers. The increased audio output power also allowed them to drive

earphones, permitting more than one person to listen. The first loudspeakers were produced around 1915. These changes caused radio listening to evolve explosively from a solitary hobby to a popular social and family pastime. The development of amplitude modulation (AM) and vacuum-tube transmitters during World War I, and the availability of cheap receiving tubes after the war, set the stage for the start of AM broadcasting
, which sprang up spontaneously around 1920.

The advent of

tuning indicators and automatic gain control (AGC) were added.[103][105] The receiver market was divided into the above broadcast receivers and communications receivers, which were used for two-way radio communications such as shortwave radio.[107]

A vacuum-tube receiver required several power supplies at different voltages, which in early radios were supplied by separate batteries. By 1930 adequate rectifier tubes were developed, and the expensive batteries were replaced by a transformer power supply that worked off the house current.[103][104]

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

A defect in many early vacuum-tube receivers was that the amplifying stages could oscillate, act as an oscillator, producing unwanted radio frequency alternating currents.[29][108][109] These parasitic oscillations mixed with the carrier of the radio signal in the detector tube, producing audible beat notes (heterodynes); annoying whistles, moans, and howls in the speaker. The oscillations were caused by feedback in the amplifiers; one major feedback path was the capacitance between the plate and grid in early triodes.[108][109] This was solved by the Neutrodyne circuit, and later the development of the tetrode and pentode around 1930.

superregenerative receiver, the superheterodyne receiver, and modern frequency modulation
(FM).

The first vacuum-tube receivers

De Forest's first commercial Audion receiver, the RJ6 which came out in 1914. The Audion tube was always mounted upside down, with its delicate filament loop hanging down, so it did not sag and touch the other electrodes in the tube.
rectifying
the radio carrier.

The first amplifying vacuum tube, the

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

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

Block diagram of regenerative receiver
Circuit of single tube Armstrong regenerative 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

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

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

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

Armstrong presenting his superregenerative receiver, June 28, 1922, Columbia University

This was a receiver invented by

Edwin Armstrong in 1922 which used regeneration in a more sophisticated way, to give greater gain.[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

thumbwheels
instead of knobs which can be turned with a finger, so a third hand is not needed.

The

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,

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

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

Block diagram of simple single tube 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

The first superheterodyne receiver built at Armstrong's Signal Corps laboratory in Paris during World War I. It is constructed in two sections, the mixer and local oscillator (left) and three IF amplification stages and a detector stage (right). The intermediate frequency was 75 kHz.
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

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.

Semiconductor era

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

Portable radios

A Zenith transistor based portable radio receiver

Companies first began manufacturing radios advertised as portables shortly after the start of commercial broadcasting in the early 1920s. The vast majority of tube radios of the era used batteries and could be set up and operated anywhere, but most did not have features designed for portability such as handles and built in speakers. Some of the earliest portable tube radios were the Winn "Portable Wireless Set No. 149" that appeared in 1920 and the Grebe Model KT-1 that followed a year later. Crystal sets such as the Westinghouse Aeriola Jr. and the RCA Radiola 1 were also advertised as portable radios.[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 1951. 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

GPS receiver.[145]

The development of

UCLA during the 1980s and 1990s, allowed low power wireless devices to be made.[146]

The current trend in receivers is to use

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]

cell phone
systems to reduce the data rate required to transmit voice.

In

"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

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]

See also

References

  1. ^ Radio-Electronics, Radio Receiver Technology
  2. .
  3. .
  4. .
  5. ^ Marianne Fedunkiw, Inventing the Radio, Crabtree Publishing Company, 2007, page 17
  6. ^ Radio Attic Gallery of Table/Mantle Radios
  7. ^ Solar/Hand Crank Powered Radio
  8. .
  9. ^ . Chapter 1
  10. ^ a b Armstrong, Edwin H. (February 1921). "A new system of radio frequency amplification". Proceedings of the Institute of Radio Engineers. 9 (1): 3–11. Retrieved December 23, 2015.
  11. ^ a b c d e f Lee, Thomas H. (2004) The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed., p. 14-15
  12. ^ .
  13. ^ a b c d Williams, Lyle Russell (2006) The New Radio Receiver Building Handbook, p. 28-30
  14. ^ a b c d e Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 195-197
  15. ^ a b c McNicol, Donald (1946) Radio's Conquest of Space, p. 272-278
  16. ^ a b Terman, Frederick E. (1943) Radio Engineers' Handbook, p. 636-638
  17. ^ .
  18. ^ .
  19. ^ Terman, Frederick E. (1943) Radio Engineers' Handbook, p. 645
  20. ^ .
  21. ^ .
  22. ^ .
  23. ^ Appleyard, Rollo (October 1927). "Pioneers of Electrical Communication part 5 - Heinrich Rudolph Hertz" (PDF). Electrical Communication. 6 (2): 67. Retrieved December 19, 2015.
  24. ^ .
  25. ^ .
  26. .
  27. ^ .
  28. ^ .
  29. ^ .
  30. ^ .
  31. ^ a b c d Nahin, Paul J. (2001) The Science of Radio, p. 53-56
  32. ^ .
  33. .
  34. .
  35. ^ a b c d e Sarkar et al. (2006) History of Wireless, p. 349-358, archive Archived 2016-05-17 at the Portuguese Web Archive
  36. ^ a b c Fleming, John Ambrose (1910). The Principles of Electric Wave Telegraphy and Telephony, 2nd Ed. London: Longmans, Green and Co. pp. 420–428.
  37. ^ a b c d Stone, Ellery W. (1919). Elements of Radiotelegraphy. D. Van Nostrand Co. pp. 203–208.
  38. ^ Phillips, Vivian 1980 Early Radio Wave Detectors, p. 18-21
  39. ^ a b c McNicol, Donald (1946) Radio's Conquest of Space, p. 107-113
  40. ^ Phillips, Vivian 1980 Early Radio Wave Detectors, p. 38-42
  41. ^ Phillips, Vivian 1980 Early Radio Wave Detectors, p. 57-60
  42. ^ Maver, William Jr. (August 1904). "Wireless Telegraphy To-Day". American Monthly Review of Reviews. 30 (2): 192. Retrieved January 2, 2016.
  43. .
  44. ^ Worthington, George (January 18, 1913). "Frog's leg method of detecting wireless waves". Electrical Review and Western Electrician. 62 (3): 164. Retrieved January 30, 2018.
  45. ^ Collins, Archie Frederick (February 22, 1902). "The effect of electric waves on the human brain". Electrical World and Engineer. 39 (8): 335–338. Retrieved January 26, 2018.
  46. ^ Phillips, Vivian 1980 Early Radio Wave Detectors, p. 198-203
  47. ^ a b Phillips, Vivian 1980 Early Radio Wave Detectors, p. 205-209
  48. ^
    S2CID 51644366
    . Retrieved 2010-01-19.
  49. ^ Secor, H. Winfield (January 1917). "Radio Detector Development". Electrical Experimenter. 4 (9): 652–656. Retrieved January 3, 2016.
  50. .
  51. ^ a b c d e f Stone, Ellery (1919) Elements of Radiotelegraphy, p. 209-221
  52. ^ Fleming, John Ambrose (1910) The Principles of Electric Wave Telegraphy and Telephony, p. 446-455
  53. ^ Phillips, Vivian 1980 Early Radio Wave Detectors, p. 85-108
  54. ^ Stephenson, Parks (November 2001). "The Marconi Wireless Installation in R.M.S. Titanic". Old Timer's Bulletin. 42 (4). Retrieved May 22, 2016. copied on Stephenson's marconigraph.com personal website
  55. ^ McNicol, Donald (1946) Radio's Conquest of Space, p. 115-119
  56. ^ Fleming, John Ambrose (1910) The Principles of Electric Wave Telegraphy and Telephony, p. 460-464
  57. ^ Phillips, Vivian 1980 Early Radio Wave Detectors, p. 65-81
  58. ^ a b c d e Lee, Thomas H. (2004) The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed., p. 9-11
  59. ^ McNicol, Donald (1946) Radio's Conquest of Space, p. 157-162
  60. ^ Fleming, John Ambrose (1910) The Principles of Electric Wave Telegraphy and Telephony, p. 476-483
  61. ^ McNicol, Donald (1946) Radio's Conquest of Space, p. 123-131
  62. ^ Fleming, John Ambrose (1910) The Principles of Electric Wave Telegraphy and Telephony, p. 471-475
  63. ^ .
  64. ^ a b Aitken, Hugh 2014 Syntony and Spark: The origins of radio, p. 70-73
  65. ^ Beauchamp, Ken (2001) History of Telegraphy, p. 189-190
  66. ^ a b Kennelly, Arthur E. (1906). Wireless Telegraphy: An Elementary Treatise. New York: Moffatt, Yard and Co. pp. 173–183. selective signaling.
  67. ^ Aitken, Hugh 2014 Syntony and Spark: The origins of radio, p. 31-48
  68. ^ Jed Z. Buchwald, Scientific Credibility and Technical Standards in 19th and early 20th century Germany and Britain, Springer Science & Business Media - 1996, page 158
  69. ^ Crookes, William (February 1, 1892). "Some Possibilities of Electricity". The Fortnightly Review. 51: 174–176. Archived from the original on September 29, 2018. Retrieved August 19, 2015.
  70. ^ .
  71. ^ Cecil Lewis Fortescue, Wireless Telegraphy, Read Books Ltd - 2013, chapter XIII
  72. .
  73. ^ Peter Rowlands, Oliver Lodge and the Liverpool Physical Society, Liverpool University Press - 1990, page 117
  74. ^ Jed Z. Buchwald, Scientific Credibility and Technical Standards in 19th and early 20th century Germany and Britain, Springer Science & Business Media - 1996, pages 158-159
  75. ^ .
  76. ^ Thomas H. Lee, The Design of CMOS Radio-Frequency Integrated Circuits, Cambridge University Press - 2004, page 35
  77. ^ a b McNicol, Donald (1946) Radio's Conquest of Space, p. 242-253
  78. ^ a b Marx, Harry J.; Van Muffling, Adrian (1922). Radio Reception. New York: G. Putnam's Sons. pp. 95–103. loose coupler variometer variocoupler.
  79. ^ a b c d e McNicol, Donald (1946) Radio's Conquest of Space, p. 254-259
  80. ^ Terman, Frederick E. (1943). Radio Engineers' Handbook (PDF). New York: McGraw-Hill Book Co. p. 170.
  81. ^ a b c Hong, Sungook (2001). Wireless: From Marconi's Black-box to the Audion. MIT Press. pp. 91-99
  82. ^ a b Howard B. Rockman, Intellectual Property Law for Engineers and Scientists, John Wiley & Sons - 2004, page 198
  83. ^ U.S. Patent No. 649,621, 3/15/1900, and part of 645,576, 3/20/1900 (filed Sept. 2, 1897) Marconi Wireless Telegraph Co. of America v. United States. United States v. Marconi Wireless Telegraph Co. of America. 320 U.S. 1 (63 S.Ct. 1393, 87 L.Ed. 1731)
  84. ^ US Patent no. 714,756, John Stone Stone Method of electric signaling, filed: February 8, 1900, granted: December 2, 1902
  85. ^ Marconi Wireless Telegraph Co. of America v. United States. United States v. Marconi Wireless Telegraph Co. of America. 320 U.S. 1 (63 S.Ct. 1393, 87 L.Ed. 1731)
  86. ^ Hong, Sungook (2001). Wireless: From Marconi's Black-box to the Audion. MIT Press. p. 48
  87. ^ Susan J. Douglas, Listening in: Radio and the American Imagination, U of Minnesota Press, page 50
  88. ^ .
  89. .
  90. ^ Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers. US Dept. of the Army. 1952. pp. 167–169.
  91. ^ .
  92. .
  93. .
  94. ^ Bucher, Elmer Eustice (1917). Practical Wireless Telegraphy. New York: Wireless Press. pp. 306.
  95. ^ Lescarboura, Austin C. (1922). Radio for Everybody. New York: Scientific American Publishing Co. pp. 93–94.
  96. ^ a b c Lauer, Henri; Brown, Harry L. (1920). Radio Engineering Principles. McGraw-Hill. pp. 135–142. tikker heterodyne.
  97. ^ Phillips, Vivian 1980 Early Radio Wave Detectors, p. 172-185
  98. ^ .
  99. ^ US patent no. 1050441, Reginald A. Fessenden, Electrical signaling apparatus, filed July 27, 1905; granted January 14, 1913
  100. ^ Hogan, John V. L. (April 1921). "The Heterodyne Receiver". The Electric Journal. 18 (4): 116–119. Retrieved January 28, 2016.
  101. ^ Nahin, Paul J. (2001) The Science of Radio, p. 91
  102. ^ McNicol, Donald (1946) Radio's Conquest of Space, p. 267-270
  103. ^ a b c McNicol, Donald (1946) Radio's Conquest of Space, p. 341-344
  104. ^ .
  105. ^ .
  106. ^ McNicol, Donald (1946) Radio's Conquest of Space, p. 336-340
  107. ^ Terman, Frederick E. (1943) Radio Engineers' Handbook, p. 656
  108. ^ .
  109. ^ a b c d e f g h i j Lee, Thomas H. (2004) The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed., p. 15-18
  110. ^ .
  111. . Retrieved March 30, 2021. The link is to a reprint of the paper in the Scientific American Supplement, Nos. 1665 and 1666, November 30, 1907 and December 7, 1907, p.348-350 and 354-356.
  112. ^ Terman, Frederick E. (1943). Radio Engineers' Handbook (PDF). New York: McGraw-Hill Book Co. pp. 564–565.
  113. S2CID 85101768
    . Retrieved May 14, 2017.
  114. ^ McNicol, Donald (1946) Radio's Conquest of Space, p. 180
  115. ^ Lee, Thomas H. (2004) The Design of CMOS Radio Frequency Integrated Circuits, 2nd Ed., p. 13
  116. ^ . Retrieved January 12, 2016.
  117. ^ a b Tyne, Gerald F. J. (December 1943). "The Saga of the Vacuum Tube, Part 9" (PDF). Radio News. 30 (6): 30–31, 56, 58. Retrieved June 17, 2016.
  118. S2CID 2116636
    . Retrieved August 29, 2012.
  119. ^ Armstrong, Edwin H. (April 1921). "The Regenerative Circuit". The Electrical Journal. 18 (4): 153–154. Retrieved January 11, 2016.
  120. ^ a b c d e f Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 187-190
  121. ^ a b c d Terman, Frederick E. (1943) Radio Engineers' Handbook, p. 574-575
  122. ^ a b c McNicol, Donald (1946) Radio's Conquest of Space, p. 260-262
  123. ^ a b Langford-Smith, F. (1953). Radiotron Designer's Handbook, 4th Ed (PDF). Wireless Press for RCA. pp. 1223–1224.
  124. ^ In the early 1920s Armstrong, David Sarnoff head of RCA, and other radio pioneers testified before the US Congress on the need for legislation against radiating regenerative receivers. Wing, Willis K. (October 1924). "The Case Against the Radiating Receiver" (PDF). Broadcast Radio. 5 (6): 478–482. Retrieved January 16, 2016.
  125. ^ a b Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 190-193
  126. ^ Terman, Frederick E. (1943). Radio Engineers' Handbook (PDF). New York: McGraw-Hill Book Co. pp. 662–663.
  127. ^ Williams, Lyle Russell (2006) The New Radio Receiver Building Handbook, p. 31-32
  128. ^ McNicol, Donald (1946) Radio's Conquest of Space, p. 279-282
  129. ^ Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 170-175
  130. ^ a b c d McNicol, Donald (1946) Radio's Conquest of Space, p. 263-267
  131. ^ a b c Army Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers, 1952, p. 177-179
  132. ^ a b Terman, Frederick E. (1943). Radio Engineers' Handbook (PDF). New York: McGraw-Hill Book Co. pp. 438–439.
  133. ^ US Patent No. 1450080, Louis Alan Hazeltine, "Method and electric circuit arrangement for neutralizing capacity coupling"; filed August 7, 1919; granted March 27, 1923
  134. ^ Hazeltine, Louis A. (March 1923). "Tuned Radio Frequency Amplification With Neutralization of Capacity Coupling" (PDF). Proc. of the Radio Club of America. 2 (8): 7–12. Retrieved March 7, 2014.[permanent dead link]
  135. ^ Terman, Frederick E. (1943). Radio Engineers' Handbook (PDF). New York: McGraw-Hill Book Co. pp. 468–469.
  136. ^ a b Grimes, David (May 1924). "The Story of Reflex and Radio Frequency" (PDF). Radio in the Home. 2 (12): 9–10. Retrieved January 24, 2016.
  137. ^ US Patent no. 1405523, Marius Latour Audion or lamp relay or amplifying apparatus, filed December 28, 1917; granted February 7, 1922
  138. ^ McNicol, Donald (1946) Radio's Conquest of Space, p. 283-284
  139. ^ "Reflexing Today: Operating economy with the newer tubes" (PDF). Radio World. 23 (17): 3. July 8, 1933. Retrieved January 16, 2016.[permanent dead link]
  140. ^ a b c Langford-Smith, F. (1953). Radiotron Designer's Handbook, 4th Ed (PDF). Wireless Press for RCA. pp. 1140–1141.
  141. ^ .
  142. ^ The Portable Radio in American Life
  143. ^ Popular Mechanics aug 1953
  144. .
  145. .
  146. .
  147. ^ "History of the Radio Receiver". Radio-Electronics.Com. Archived from the original on 2007-09-16. Retrieved 2007-11-23.
  148. .
  149. ^ Pizzicato Comes of Age

Further reading