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In electronics, the superheterodyne receiver (also known by its full name, the supersonic heterodyne receiver, or by the abbreviated form superhet) is a technique for selectively recovering the information from radio waves of a particular frequency. It is used in radio and television receivers and transmitters in order to tune them to a particular frequency. Electronics is the study of the flow of charge through various materials and devices such as, semiconductors, resistors, inductors, capacitors, nano-structures, and vacuum tubes. ...
History
The superheterodyne principle was originally conceived in 1918 by Edwin Armstrong during World War I as a means of overcoming the deficiencies of early vacuum triodes used as high-frequency amplifiers in radio direction finding (RDF) equipment. In a triode RF amplifier, if both the plate and grid are connected to resonant circuits tuned to the same frequency, stray capacitive coupling between the grid and the plate will cause the amplifier to go into oscillation if the stage gain is much more than unity. In early designs dozens of low-gain triode stages sometimes had to be connected in cascade to make workable designs, which drew enormous amounts of power in operation. However the strategic value was so high that British Admiralty felt it was money well spent. 1918 (MCMXVIII) was a common year starting on Tuesday of the Gregorian calendar (see link for calendar) or a common year starting on Wednesday of the Julian calendar. ...
Edwin Howard Armstrong (December 18, 1890 – January 31, 1954) was an American electrical engineer and inventor. ...
“The Great War †redirects here. ...
Simplified diagram of a triode. ...
Armstrong had realized that higher frequency equipment would allow them to detect enemy shipping much more effectively, but at the time no practical "short wave" (defined then as any frequency above 500 kHz) amplifier existed.
It had been noticed some time before that if a regenerative receiver was allowed to go into oscillation, other receivers nearby would suddenly start picking up stations on frequencies different from those they were actually transmitted on. Armstrong (and others) soon realized that this was caused by a "supersonic" heterodyne (or beat) between the station's carrier frequency and the oscillator frequency. For example, if a station were transmitting on 300kHz and the oscillator were set to 400kHz, as well as the original 300kHz, the same station would be also heard on 100kHz and 700kHz.
In a flash of insight, Armstrong suddenly realized that this was a potential solution to the "short wave" amplification problem. To monitor a frequency of 1500kHz, he could set up an oscillator to, say, 1560kHz, which would down-convert the signal to a 60kHz carrier, which was far more amenable to high gain amplification using triodes.
The first superheterodyne circuits used the self-resonance of iron-cored interstage coupling transformers to filter the intermediate frequency, and this is why the Intermediate Frequency tuned circuits were still referred to as IF "transformers", long after they had been replaced by proper tuneable coils. Early superhets used IFs as low as 20KHz, which made them extremely susceptible to image interference but at the time the main interest was sensitivity rather than selectivity.
Armstrong was able to put his ideas into practice quite quickly, and the technique was rapidly adopted by the military; however, it was less popular when radio broadcasting began in the 1920s, due both to the need for an extra tube for the oscillator, and the amount of technical knowledge required to operate it. For domestic radios, an alternative approach to Short Wave "Tuned RF" ("TRF") amplification called the Neutrodyne became more popular for reasons of simplicity and economy. The Tuned Radio Frequency Receiver (short TRF) was invented by Edwin Armstrong in 1918. ...
The Neutrodyne was a particular type of Tuned Radio Frequency (TRF) radio receiver, in which the inter-electrode capacitance of the triode RF tubes are neutralised with interstage variable trim capacitors connected between the grid and plate of each RF amplifier tube. ...
However, by the 1930s, improvements in vacuum tube technology rapidly eroded these advantages. First, the development of practical indirectly-heated cathodes allowed the mixer and oscillator functions to be combined in a single Pentode tube, in the so-called Autodyne mixer. This was rapidly followed by the introduction of low-cost multi-element tubes specifically designed for superheterodyne operation and by the mid-30s the TRF technique was rendered obsolete. Just about all radio receivers, including the receiver sections of television sets, now use the superheterodyne principle. In electronics, a vacuum tube (American English) or (thermionic) valve (British English) is a device generally used to amplify a signal. ...
Originally an autodyne circuit referred to a weakly oscillating receiver tube that made interrupted carrier wave (ICW) Morse Code transmissions audible. ...
Overview The superheterodyne receiver principle overcomes certain limitations of previous receiver designs. Tuned radio frequency (TRF) receivers suffered from poor selectivity, since even filters with a high Q factor have a wide bandwidth at radio frequencies. Regenerative and super-regenerative receivers offer better sensitivity than a TRF receiver, but suffer from stability and selectivity problems. The Tuned Radio Frequency Receiver (short TRF) was invented by Edwin Armstrong in 1918. ...
Selectivity is a measure of the performance of a radio receiver to respond only to the tuned transmission (such as a radio station) and reject other signals nearby, such as another broadcast on an adjacent channel. ...
Television signal splitter consisting of a hi-pass and a low-pass filter. ...
This article or section does not cite any references or sources. ...
This article does not cite any references or sources. ...
In biology, regeneration is the ability to recreate lost or damaged tissues, organs and limbs. ...
In receivers using the superheterodyne principle, a signal at variable frequency  is converted to a fixed lower frequency,  , before detection. is called the intermediate frequency, or "IF". In typical AM (Amplitude Modulation, e.g as used on medium wave) home receivers, that frequency is usually 455 kHz; for FM VHF receivers, it is usually 10.7 MHz. An intermediate frequency (IF) is a frequency to which a carrier frequency is shifted as an intermediate step in transmission or reception. ...
Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. ...
Frequency modulation (FM) is a form of modulation that represents information as variations in the instantaneous frequency of a carrier wave. ...
Very high frequency (VHF) is the radio frequency range from 30 MHz (wavelength 10 m) to 300 MHz (wavelength 1 m). ...
Heterodyne receivers "mix" all of the incoming signals with an internally generated waveform called the local oscillator. The user tunes the radio by adjusting the set's oscillator frequency,  In the mixer stage of a receiver, the local oscillator signal multiplies with the incoming signals, which shifts them all down in frequency. The one that shifts to is passed on by tuned circuits, amplified, and then demodulated to recover the original audio signal. The oscillator also shifts a "copy" of each incoming signal up in frequency by amount Those very high frequency "images" are all rejected by the tuned circuits in the IF stage. The Superheterodyne receiver (or to give it its full name, The Supersonic Heterodyne Receiver – usually these days shortened to superhet) was invented by Edwin Armstrong in 1918. ...
High-side and low-side injection The amount that a signal is down-shifted by the local oscillator depends on whether its frequency () is higher or lower than That is because its new frequency is  in either case. Therefore, there are potentially two signals that could both shift to the same  one at  and another at  . One or the other of those signals has to be filtered out prior to the mixer to avoid aliasing. When the upper one is filtered out, it is called high-side injection, because  is above the frequency of the received signal. The other case is called low-side injection. High-side injection also reverses the order of a signal's frequency components. Whether or not that actually changes the signal depends on whether it has spectral symmetry or not. The reversal can be undone later in the receiver, if necessary. Properly sampled image of brick wall. ...
Design and its evolution The diagram below shows the basic elements of a single conversion superhet receiver. In practice not every design will have all these elements, nor does this convey the complexity of other designs, but the essential elements of a local oscillator and a mixer followed by a filter and IF amplifier are common to all superhet circuits. Cost-optimized designs may use one active device for both local oscillator and mixer—this is sometimes called a "converter" stage. One such example is the pentagrid converter. NOTE: This article is currently being extensively rewritten and expanded off line. ...
The advantage to this method is that most of the radio's signal path has to be sensitive to only a narrow range of frequencies. Only the front end (the part before the frequency converter stage) needs to be sensitive to a wide frequency range. For example, the front end might need to be sensitive to 1–30 MHz, while the rest of the radio might need to be sensitive only to 455 kHz, a typical IF. Only one or two tuned stages need to be adjusted to track over the tuning range of the receiver; all the intermediate-frequency stages operate at a fixed frequency which need not be adjusted. Superheterodyne receiver block diagram – A smaller and cleaned-up PNG version of Image:Superhet. ...
Sometimes, to overcome obstacles such as image response, more than one IF is used. In such a case, the front end might be sensitive to 1–30 MHz, the first half of the radio to 5 MHz, and the last half to 50 kHz. Two frequency converters would be used, and the radio would be a "Double Conversion Super Heterodyne"—a common example is a television receiver where the audio information is obtained from a second stage of intermediate frequency conversion. Occasionally special-purpose receivers will use an intermediate frequency much higher than the signal, in order to obtain very high image rejection. Image Response (or more correctly, Image Response Rejection Ratio, or IMRR) is a measure of performance of a radio receiver, particularly one that operates on the super-heterodyne principle. ...
Superheterodyne receivers have superior characteristics to simpler receiver types in frequency stability and selectivity. It is much easier to stabilize a tuneable oscillator than a tuneable filter, especially with modern frequency synthesizer technology. IF filters can give much narrower passbands at the same Q factor than an equivalent RF filter. A fixed IF also allows the use of a crystal filter in very critical designs such as radiotelephone receivers which have exceptionally high selectivity. A frequency synthesiser is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator. ...
This article or section does not cite any references or sources. ...
A Crystal Filter is a special form of quartz crystal used in electronics systems, in particular communications devices. ...
A radiotelephone is a communications device that allows two or more people to talk using radio. ...
In the case of modern television receivers, no other technique was able to produce the precise bandpass characteristic needed for vestigial sideband reception, first used with the original NTSC system introduced in 1941. This originally involved a complex collection of tuneable inductors which needed careful adjustment, but since the early 1980s these have been replaced with precision electromechanical surface acoustic wave (SAW) filters. Fabricated by precision laser milling techniques, SAW filters are much cheaper to produce, can be made to extremely close tolerances, and are extremely stable in operation. Single-sideband modulation (SSB) is a refinement of the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. ...
A surface acoustic wave (SAW) is a kind of wave used in piezoelectric devices called SAW devices in electronics circuits. ...
Television signal splitter consisting of a hi-pass and a low-pass filter. ...
The next evolution of Superheterodyne receiver design is the software defined radio architecture, where the IF processing after the initial IF filter is implemented in software. This technique is already in use in the latest design analog television receivers and digital set top boxes, where there are no coils or other resonant circuits used at all. The antenna simply connects via a small capacitor to a pin on an integrated circuit and all the signal processing is carried out digitally. Similar techniques are used in the tiny FM radios incorporated into Mobile phones and MP3 players. A software-defined radio (SDR) system is a radio communication system which uses software for the modulation and demodulation of radio signals. ...
Computer software (or simply software) refers to one or more computer programs and data held in the storage of a computer for some purpose. ...
Integrated circuit of Atmel Diopsis 740 System on Chip showing memory blocks, logic and input/output pads around the periphery Microchips with a transparent window, showing the integrated circuit inside. ...
Radio transmitters may also use a mixer stage to produce an output frequency, working more or less as the reverse of a superheterodyne receiver. In communications and information processing, a transmitter (sometimes abbreviated XMTR) is an object (source) which sends information to an observer (receiver). ...
Drawbacks to the superheterodyne receiver include the cost of the mixer and local oscillator stages. Receivers become vulnerable to interference from signals other than the desired signal. A strong signal at the intermediate frequency may overcome the desired signal; regulatory authorities will prevent licensed transmitters from operating on these frequencies. In urban environments with many strong signals, the signals from multiple transmitters may combine in the mixer stage to interfere with the desired signal. A superheterodyne receiver may pick up a so-called "image frequency" signal that also produces a mixer output at the desired intermediate frequency; this phenomenon is sometimes used for scanner reception of transmissions outside of the receiver's official capabilities. Intermodulation distortion: Nonlinear distortion characterized by the appearance, in the output of a device, of frequencies that are linear combinations of the fundamental frequencies and all harmonics present in the input signals. ...
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See also The H2X radar, nicknamed the Mickey set, provided a ground mapping capability for both navigation and in daylight when overcast (and at night) for the USAAF during World War II. The H2X system replaced the British H2S radar. ...
Automatic gain control (AGC) is an electronic system found in many types of devices. ...
A demodulator is an electronic circuit used to recover the information content from the carrier wave of a signal. ...
Direct conversion receivers, (DCR) also known also known as Homodyne, Synchrodyne, or zero-IF receivers, is a system of demodulating amplitude modulated signals; it uses a local oscillator, synchronized in frequency to the carrier of the wanted signal, to modulate the received signal. ...
VFO is an acronym for Variable Frequency Oscillator. ...
Single-sideband modulation (SSB) is a refinement of the technique of amplitude modulation designed to be more efficient in its use of electrical power and bandwidth. ...
Directly amplifying receiver. ...
Reflectional radio receiver. ...
Interference of two circular waves - Wavelength (decreasing bottom to top) and Wave centers distance (increasing to the right). ...
In telecommunications, to heterodyne is to generate new frequencies by mixing two or more signals in a nonlinear device such as a vacuum tube, transistor, or diode mixer. ...
External links - Who Invented the Superheterodyne? An article giving the history of the various inventors working on the superheterodyne method.
- [1]An in depth introduction to superhetrodyne receivers
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