Radio receiver
In
Radio receivers are essential components of all systems that use
Broadcast radio receivers
The most familiar form of radio receiver is a broadcast receiver, often just called a radio, which receives
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
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.
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)
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
so reception distances are limited by the visual horizon to about 30–40 miles (48–64 km).Types of broadcast receivers
Radios are manufactured in a range of styles and functions:
- Console radio - A self-contained radio with speaker designed to stand on the floor.
- Table radio also called a "Mantel radio" - A self-contained radio with speaker designed to sit on a table, cabinet, or fireplace mantel.[5][6] Table radios typically plug into a wall outlet, although some "cordless" battery powered table radios exist.
- Clock radio - A bedside table radio that also includes an alarm clock. The alarm clock can be set to turn on the radio in the morning instead of an alarm, to wake the owner.
- Tuner - A high fidelity AM/FM radio receiver in a component home audio system. It has no speakers but outputs an audio signal which is fed into the system and played through the system's speakers.
- batteries that can be carried with a person. Radios are now often integrated with other audio sources in CD players and portable media players. Portable radios typically are small enough to be hand held, or, for larger radios, have a handle or carrying strap. Portable radios may have an arrangement for powering from an outlet, conserving the batteries when an outlet is available. Portable "emergency" radios may be solar and/or hand crank powered.[7]
- Boom box - a portable battery-powered high fidelitystereo sound system in the form of a box with a handle, which became popular during the mid-1970s.
- Transistor radio - an older term for a portable pocket-sized broadcast radio receiver. Made possible by the invention of the transistor and developed in the 1950s, transistor radios were hugely popular during the 1960s and early 1970s, and changed the public's listening habits.
- Car radio - A radio integrated into the dashboard of a vehicle, used for entertainment while driving. Virtually all modern cars and trucks are equipped with radios, which usually also includes a CD player.
- direct broadcast satellite. The subscriber must pay a monthly fee. They are mostly designed as car radios.
- Shortwave receiver - This is a broadcast radio that also receives the shortwave bands. It is used for shortwave listening.
- An AV or Stereo receiver (in context often just called a receiver) is a component in a hi-fi or home theatre system combining a radio and audio amplifier in one unit that connects to the speakers and often to other input and output components (e.g. turntable, television, tape deck, and CD and DVD players)
Other applications
Radio receivers are essential components of all systems that use radio. Besides the broadcast receivers described above, radio receivers are used in a huge variety of electronic systems in modern technology. They can be a separate piece of equipment (a radio), or a subsystem incorporated into other electronic devices. A transceiver is a transmitter and receiver combined in one unit. Below is a list of a few of the most common types, organized by function.
- Broadcast television reception - Televisions receive a video signal representing a moving image, composed of a sequence of still images, and a synchronized audio signal representing the associated sound. The television channel received by a TV occupies a wider bandwidththan an audio signal, from 600 kHz to 6 MHz.
- UHFbands.
- direct broadcast satellite 22,000 miles (35,000 km) above the Earth, and the signal is converted to a lower intermediate frequencyand transported to the box through a coaxial cable. The subscriber pays a monthly fee.
- Two-way voice communications - A full duplex, a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone.
- GPS receiver. The cell tower has sophisticated multichannel receivers that receive the signals from many cell phones simultaneously.
- UHF band that receive the short range bidirectional duplexradio link.
- Citizens band radio - a two-way half-duplex radio operating in the 27 MHz band that can be used without a license. They are often installed in vehicles and used by truckers and delivery services.
- Walkie-talkie - a handheld short range half-duplex two-way radio.
- One-way (simplex) voice communications
- Wireless microphone receiver - these receive the short range signal from wireless microphones used onstage by musical artists, public speakers, and television personalities.
- Baby monitor - this is a cribside appliance for parents of infants that transmits the baby's sounds to a receiver carried by the parents, so they can monitor the baby while they are in other parts of the house. Many baby monitors now have video cameras to show a picture of the baby.
- Data communications
- WLAN) to exchange data with other devices.
- Bluetooth modem - a very short range (up to 10 m) 2.4-2.83 GHz data transceiver on a portable wireless device used as a substitute for a wire or cable connection, mainly to exchange files between portable devices and connect cellphones and music players with wireless earphones.
- Microwave relay - a long-distance high bandwidth point-to-point data transmission link consisting of a dish antenna and transmitter that transmits a beam of microwaves to another dish antenna and receiver. Since the antennas must be in line-of-sight, distances are limited by the visual horizon to 30–40 miles. Microwave links are used for private business data, wide area computer networks (WANs), and by telephone companiesto transmit distance phone calls and television signals between cities.
- geosynchronous satellites to billions of kilometers for interplanetaryspacecraft. This and the limited power available to a spacecraft transmitter mean very sensitive receivers must be used.
- direct broadcast satellite the transponder broadcasts a stronger signal directly to satellite radio or satellite televisionreceivers in consumer's homes.
- cryogenically cooled to −195.79 °C (−320 °F) by liquid nitrogen to reduce radio noisein the circuit.
- keyless entrysystems.
- Radiolocation - This is the use of radio waves to determine the location or direction of an object.
- Radar - a device that transmits a narrow beam of microwaves which reflect from a target back to a receiver, used to locate objects such as aircraft, spacecraft, missiles, ships or land vehicles. The reflected waves from the target are received by a receiver usually connected to the same antenna, indicating the direction to the target. Widely used in aviation, shipping, navigation, weather forecasting, space flight, vehicle collision avoidance systems, and the military.
- artillery shells.
- VOR receiver - navigational instrument on an aircraft that uses the VHF signal from VOR navigational beacons between 108 and 117.95 MHz to determine the direction to the beacon very accurately, for air navigation.
- Wild animal tracking receiver - a receiver with a directional antenna used to track wild animals which have been tagged with a small VHF transmitter, for wildlife management purposes.
- Other
- oil and gas drilling, and unmanned scientific instruments in remote locations.
- Measuring receiver - a calibrated, laboratory grade radio receiver used to measure the characteristics of radio signals. Often incorporates a spectrum analyzer.
- cryogenically cooled by liquid nitrogen to reduce radio noise.
How receivers work
A radio receiver is connected to an antenna which converts some of the energy from the incoming radio wave into a tiny radio frequency AC voltage which is applied to the receiver's input. An antenna typically consists of an arrangement of metal conductors. The oscillating electric and magnetic fields of the radio wave push the electrons in the antenna back and forth, creating an oscillating voltage.
The
Main functions of a receiver
Practical radio receivers perform three basic functions on the signal from the antenna:
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
The bandpass filter consists of one or more
- Bandwidth and selectivity: See graphs. The information (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 tuned circuit. When the resonant frequency is equal to the radio transmitter's frequency the tuned circuit oscillates in sympathy, passing the signal on to the rest of the receiver.
Amplification
The power of the radio waves picked up by a receiving antenna decreases with the square of its distance from the transmitting antenna. Even with the powerful transmitters used in radio broadcasting stations, if the receiver is more than a few miles from the transmitter the power intercepted by the receiver's antenna is very small, perhaps as low as
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
Demodulation
After the radio signal is filtered and amplified, the receiver must extract the information-bearing
- 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
- AM demodulation
- 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
In the simplest type of radio receiver, called a
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
The
except a few specialized applications.In the superheterodyne, the radio frequency signal from the antenna is shifted down to a lower "
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
The RF filter on the front end of the receiver is needed to prevent interference from any radio signals at the
To achieve both good image rejection and selectivity, many modern superhet receivers use two intermediate frequencies; this is called a
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
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
In certain receiver designs such as modern digital receivers, a related problem is
History
Radio waves were first identified in German physicist
Spark era
The first
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] just strings of random pulses. Therefore, Marconi is usually given credit for building the first radio receivers.
Coherer receiver
-
Circuit of Marconi's first coherer radio receiver from 1896
-
Coherer from 1904 as developed by Marconi.
The first radio receivers invented by Marconi,
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.
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,
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
By the first years of the 20th century, experiments in using
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:
- 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]
- 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]
- 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
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.
Tuning was used in Hertz's original experiments
By 1897 the advantages of tuned systems had become clear, and Marconi and the other wireless researchers had incorporated
Inductive coupling
In order to reject
This circuit had two advantages. 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
Patent disputes
Marconi's initial radio system had relatively poor tuning limiting its range and adding to interference.
Crystal radio receiver
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.
The crystal radio used a
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
Heterodyne receiver and BFO
Beginning around 1905
The continuous wave radiotelegraphy signals produced by these transmitters required a different method of reception.
The first crude device that did this was the tikker, invented in 1908 by Valdemar Poulsen.[48][96]
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.
Armstrong later used Fessenden's heterodyne principle in his superheterodyne receiver (below).[98][11]
Vacuum tube era
The
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
The advent of
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.
The first vacuum-tube receivers
This section's factual accuracy is disputed. (October 2020) |
The first amplifying vacuum tube, the
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
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
The
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
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
Superregenerative receiver
This was a receiver invented by
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
The
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,
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
The Neutrodyne receiver, invented in 1922 by
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
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
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
The development of
The current trend in receivers is to use
Many of the functions performed by
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
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
- Batteryless radio
- Dielectric wireless receiver
- Digital Audio Broadcast(DAB)
- Direct conversion receiver
- List of radios – List of specific models of radios
- Minimum detectable signal
- Radiogram (furniture)
- Receiver (information theory)
- Distortion
- Telecommunication
- Television receive-only
- Radio transmitter design
- Radio receiver design
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Further reading
- Communications Receivers, Third Edition, Ulrich L. Rohde, Jerry Whitaker, McGraw Hill, New York, 2001, ISBN 0-07-136121-9
- Buga, N.; Falko A.; Chistyakov N.I. (1990). Chistyakov N.I. (ed.). Radio Receiver Theory. Translated from the Russian by Boris V. Kuznetsov. ISBN 978-5-03-001321-3First published in Russian as «Радиоприёмные устройства»)
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