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Heterodyne principle |
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This article contains too much jargon and may need simplification or further explanation. Please discuss this issue on the talk page, and/or remove or explain jargon terms used in the article. Editing help is available. (September 2007) |
In radio and signal processing, heterodyning is the generation of new frequencies by mixing, or multiplying, two oscillating waveforms. It is useful for modulation and demodulation of signals, or placing information of interest into a useful frequency range. This operation may be accomplished by a vacuum tube, transistor, or other signal processing device. Mixing two frequencies creates two new frequencies, according to the properties of the sine function: one at the sum of the two frequencies mixed, and the other at their difference. Typically only one of these frequencies is desired—the higher one after modulation and the lower one after demodulation. The other signal is either not passed by the tuned circuitry that follows, or may be filtered out.
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The word heterodyne is derived from the Greek roots hetero- "different", and -dyne "power". This technique was pioneered by Canadian inventor-engineer Reginald Fessenden.
A superheterodyne receiver converts an incoming frequency to an intermediate frequency, using heterodyne action. The incoming signal is mixed with a generated signal, producing sum and difference frequencies. The signal of interest is selected by using a bandpass filter. The filter is usually fixed, common choices being 455 kHz and 10.7 MHz. The receiver's operating frequency is tuned simply by changing the frequency of the generated signal.
Optical heterodyne detection receivers are a special case in part because unlike antennas, which detect electric fields, a photon receiver directly measures energy and thus the non-linear mixing element arises directly from the physics of the photo-electric effect and is not imposed at later stage in the electronic receiver. Additionally, in optical heterodyne detection the optical carrier bandwidth of the local oscillator and Signal beams are non-negligible in the analysis: these bandwidths are generally much wider (generally orders of magnitude) than the bandwidth of the mixed signal.
The term heterodyne is sometimes applied also to one of the new frequencies produced by heterodyne signal mixing.
Heterodyning is based on the simple trigonometric identity:
The product on the left hand side represents the multiplication of a sinusoidal signal waveform by another sine wave (the mixing frequency). This wave must have greater frequency than the bandwidth of the signal. The right hand side is the sum of two co-sinusoids, which can be considered to be separate signals in different frequency bands.
None of the sinusoids in the above equation is equivalent to a complete signal waveform. Any waveform may be converted to a combination of sinusoids by Fourier analysis, however. The equation then applies to the constituent frequencies. For example, if frequency φ is weak in the Fourier spectrum of the input signal, the output signal will also have low amplitudes at θ ± φ.
Since optical frequencies are far beyond any feasible electronic circuit bandwidth, all photon detectors are inherently energy detectors not oscillating electric field detectors. However since energy detection is inherently "square-law" detection, it intrinsically mixes any optical frequencies present on the detector. Thus sensitive detection of specific optical frequencies is possible by Optical heterodyne detection when two different (close-by) wavelengths of light illuminate the detector so that the oscillating electrical output corresponds to their difference frequency. This allows extremely narrow band detection (much narrower band than any possible color filter can achieve) as well as precision measurements of phase and frequency of a signal light relative to a reference light source.
This phase sensitive detection has been applied for Doppler measurements of wind speed, and imaging through dense media. The high sensitivity against background light is especially useful for LIDAR.
Many analog videotape systems relied on a downconverted color subcarrier in order to record color information in their limited bandwidth. These systems are referred to as "heterodyne systems" or "color-under systems". For instance, for NTSC systems, the VHS (and S-VHS) system converts the color subcarrier from the NTSC standard 3.58 MHz to ~629 kHz.1 PAL VHS color subcarrier is similarly downconverted (but from 4.43 MHz). The now-obsolete 3/4" U-matic systems used a heterodyned ~688 kHz subcarrier for NTSC recordings (as did Sony's Betamax), while PAL U-matic decks came in two mutually incompatible varieties, with different subcarrier frequencies, known as Hi-Band and Low-Band. Other videotape formats with heterodyne color systems include Video-8 and Hi8.2
The heterodyne system in these cases is used to convert quadrature phase-encoded and amplitude modulated sine waves from the broadcast frequencies to frequencies recordable on formats with less than 1 MHz bandwidth. On playback, the recorded color information was heterodyned back to the standard subcarrier frequencies for playback on televisions and for interchange with other standard video equipment.
Some U-matic (3/4") decks featured 7-pin mini-DIN connectors to allow dubbing of tapes without a heterodyne up-conversion and down-conversion, as did some industrial VHS, S-VHS, and Hi8 recorders.
The theremin, an electronic musical instrument, uses the heterodyne principle to produce a variable audio frequency in response to the movement of the musician's hands in the vicinity of some antennas. The output of a fixed radio frequency oscillator is mixed with that of an oscillator whose frequency is affected by the variable capacitance of the thereminist's hand as it moves near the pitch control antenna. The difference between the two oscillator frequencies produces a tone in the audio range.
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