|
|
|
|
Heterodyne
|
| |
|
| |
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.
Discussion
Ask a question about 'Heterodyne' Start a new discussion about 'Heterodyne' Answer questions from other users |
Encyclopedia
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.
Origin and use of term
The word heterodyne is derived from the Greek roots hetero- "different", and -dyne "power". The original heterodyne technique was pioneered by Canadian inventor-engineer Reginald Fessenden but was not pursued far because local oscillators were not very stable at the time.
Later, the superheterodyne receiver (superhet) was invented by Edwin Howard Armstrong in 1918, converting the incoming Radio Frequency (RF) to a fixed Intermediate Frequency (IF), using the heterodyne technique. The difference between the superhet and the original heterodyne is the use of a "sloppy" but tunable RF filter on the front end, an electronic mixer circuit, a stable local oscillator (LO) and a fixed frequency high-gain amplifier. The original heterodyne technique tried to accomplish all of this in one stage thus producing an unstable amplifier.
In a superhet, the incoming signal of interest (the radio frequency) is selected by using a "sloppy" but tunable bandpass filter on the front end. This RF signal is then mixed (i.e., multiplied) with an LO signal (the reference), thus producing two new output frequencies at the sum (RF+LO) and difference (RF-LO) of the original frequencies. One of these two new frequencies is discarded, usually the higher one (sum=RF+LO) in a receiver. The remaining frequency (difference=RF-LO) is called the IF (intermediate frequency). The IF is passed to the high-gain amplifier. Common choices for the IF frequency are 455 kHz inside some AM radios and 10.7 MHz in some FM radios. This process of shifting the RF signal down to the IF frequency is called "down conversion".
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.
Mathematical principle
Heterodyning is based on the simple trigonometric identity:
The product on the left hand side represents the multiplication (aka "mixing") 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 f is weak in the Fourier spectrum of the input signal, the output signal will also have low amplitudes at ? ± f.
Applications
Sensitive Optical detection
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.
Analog videotape recording
Many analog videotape systems rely 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 video systems, the VHS (and S-VHS) recording system converts the color subcarrier from the NTSC standard 3.58 MHz to ~629 kHz. PAL VHS color subcarrier is similarly downconverted (but from 4.43 MHz). The now-obsolete 3/4" U-matic systems use a heterodyned ~688 kHz subcarrier for NTSC recordings (as does Sony's Betamax, which is at its basis a 1/2" consumer version of U-matic), 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.
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 in less than 1 MHz bandwidth. On playback, the recorded color information is heterodyned back to the standard subcarrier frequencies for display on televisions and for interchange with other standard video equipment.
Some U-matic (3/4") decks feature 7-pin mini-DIN connectors to allow dubbing of tapes without a heterodyne up-conversion and down-conversion, as do some industrial VHS, S-VHS, and Hi8 recorders.
Music synthesis
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 between the antenna and the thereminist as that person moves her or his hand near the pitch control antenna. The difference between the two oscillator frequencies produces a tone in the audio range.
Amplitude Modulation
Amplitude Modulation uses the heterodyne principle to move a signal from baseband, centered at 0 Hz, to passband, centered at the carrier frequency, at the transmitter. The process is reversed at the receiver, moving the signal from passband to baseband. There may be indirect frequencies used in the conversion process, known as intermediate frequencies.
See also
Notations
- Glinsky, Albert. Theremin: Ether Music and Espionage. Urbana: University of Illinois Press, 2000. ISBN 0-252-02582-2.
- Nahin, Paul J. The Science of Radio. New York: Springer-Verlag, AIP Press, 2001. ISBN 0-387-95150-4.
Footnotes
|
| |
|
|
