Editing Heterodyne (section)

==Applications==
Heterodyning, also called ''frequency conversion'', is used very widely in [[communications engineering]] to generate new frequencies and move information from one frequency channel to another. Besides its use in the superheterodyne circuit found in almost all radio and television receivers, it is used in [[radio transmitter]]s, [[modem]]s, [[satellite]] communications and set-top boxes, [[radar]], [[radio telescope]]s, [[telemetry]] systems, cell phones, cable television converter boxes and [[Cable television headend|headends]], [[microwave relay]]s, [[metal detector]]s, [[atomic clock]]s, and military [[electronic countermeasure]] (jamming) systems.

===Up and down converters===
In large scale [[telecommunication network]]s such as [[telephone network]] trunks, [[microwave relay]] networks, cable television systems, and [[communication satellite]] links, large [[bandwidth (signal processing)|bandwidth]] capacity links are shared by many individual communication channels by using heterodyning to move the frequency of the individual signals up to different frequencies, which share the channel. This is called [[frequency division multiplexing]] (FDM).

For example, a [[coaxial cable]] used by a cable television system can carry 500 television channels at the same time because each one is given a different frequency, so they do not interfere with one another. At the cable source or [[Cable television headend|headend]], electronic upconverters convert each incoming television channel to a new, higher frequency.  They do this by mixing the television signal frequency, ''f<sub>CH</sub>'' with a [[local oscillator]] at a much higher frequency {{math|''f<sub>LO</sub>''}}, creating a heterodyne at the sum {{math|''f<sub>CH</sub>''&nbsp;+&nbsp;''f<sub>LO</sub>''}}, which is added to the cable. At the consumer's home, the cable [[set top box]] has a downconverter that mixes the incoming signal at frequency {{math|''f<sub>CH</sub>''&nbsp;+&nbsp;''f<sub>LO</sub>''}} with the same local oscillator frequency {{math|''f<sub>LO</sub>''}} creating the difference heterodyne frequency, converting the television channel back to its original frequency: {{math|1=(''f<sub>CH</sub>''&nbsp;+&nbsp;''f<sub>LO</sub>'')&nbsp;−&nbsp;''f<sub>LO</sub>'' =&nbsp;''f<sub>CH</sub>''}}.  Each channel is moved to a different higher frequency.  The original lower basic frequency of the signal is called the [[baseband]], while the higher channel it is moved to is called the [[passband]].

===Analog videotape recording===
<!-- [[Color under]] redirects here. Please fix it if the name of this section changes :) -->
Many analog [[videotape]] systems rely on a downconverted color subcarrier 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&nbsp;MHz to ~629&nbsp;kHz.<ref name = lionlamb>[http://www.lionlmb.org/quad/format.html#12incomposite Videotape formats using {{convert|1/2|in|mm|adj=mid|-wide}} tape] {{Webarchive|url=https://web.archive.org/web/20060616104342/http://lionlmb.org/quad/format.html#12incomposite |date=June 16, 2006 }} ; Retrieved 2007-01-01</ref> [[PAL]] VHS color subcarrier is similarly downconverted (but from 4.43&nbsp;MHz). The now-obsolete 3/4" [[U-matic]] systems use a heterodyned ~688&nbsp;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]].<ref name = poynton>{{cite book|first=Poynton |last=Charles |author-link=Charles Poynton |title=Digital Video and HDTV: Algorithms and Interfaces |year=2003 |publisher=Morgan Kaufmann Publishers |location=San Francisco |pages=582–3 |isbn=978-1-55860-792-7 |url=https://books.google.com/books?id=ra1lcAwgvq4C}}</ref>

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&nbsp;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 connector]]s to allow dubbing of tapes without conversion, as do some industrial VHS, S-VHS, and Hi8 recorders.

===Music synthesis===
The [[theremin]], an [[electronic musical instrument]], traditionally uses the heterodyne principle to produce a variable [[audio frequency]] in response to the movement of the musician's hands in the vicinity of one or more antennae, which act as capacitor plates. 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 musician's hand as it is moved near the pitch control antenna. The difference between the two oscillator frequencies produces a tone in the audio range.

The [[ring modulator]] is a type of [[frequency mixer]] incorporated into some synthesizers or used as a stand-alone audio effect.

===Optical heterodyning===
[[Optical heterodyne detection]] (an area of active research) is an extension of the heterodyning technique to higher (visible) frequencies. Guerra<ref>{{Cite journal |last=Guerra |first=John M. |date=1995-06-26 |title=Super-resolution through illumination by diffraction-born evanescent waves |url=http://aip.scitation.org/doi/10.1063/1.113814 |journal=Applied Physics Letters |language=en |volume=66 |issue=26 |pages=3555–3557 |doi=10.1063/1.113814 |bibcode=1995ApPhL..66.3555G |issn=0003-6951}}</ref> (1995) first published the results of what he called a "form of optical heterodyning" in which light patterned by a 50 nm pitch grating illuminated a second grating of pitch 50 nm, with the gratings rotated with respect to each other by the angular amount needed to achieve magnification. Although the illuminating wavelength was 650 nm, the 50 nm grating was easily resolved. This showed a nearly 5-fold improvement over the Abbe resolution limit of 232 nm that should have been the smallest obtained for the numerical aperture and wavelength used. This super-resolution microscopic imaging through optical heterodyning later came to be know by many as "structured illumination microscopy".  

In addition to super-resolution optical microscopy, optical heterodyning could greatly improve [[optical modulator]]s, increasing the density of information carried by [[optical fiber]]s. It is also being applied in the creation of more accurate [[atomic clock]]s based on directly measuring the frequency of a laser beam.{{refn|group=notes|See [[NIST]] subtopic 9.07.9-4.R for a description of research on one system to do this.<ref>[http://tsapps.nist.gov/ts_sbir/sbirrss/index.cfm?action=contractdetails&id=78 Contract Details: Robust Nanopopous Ceramic Microsensor Platform<!-- Bot generated title -->]</ref><ref>[http://tsapps.nist.gov/ts_sbir/sbirrss/index.cfm?action=contractdetails&id=147 Contract Details: High Pulsed Power Varactor Multipliers for Imaging<!-- Bot generated title -->]</ref>}}

Since optical frequencies are far beyond the manipulation capacity of any feasible electronic circuit, all visible frequency photon detectors are inherently energy detectors not oscillating electric field detectors. However, since energy detection is inherently "[[square-law detector|square-law]]" detection, it intrinsically mixes any optical frequencies present on the detector. Thus, sensitive detection of specific optical frequencies necessitates optical heterodyne detection, in which two different (close by) wavelengths of light illuminate the detector so that the oscillating electrical output corresponds to the difference between their frequencies. This allows extremely narrow band detection (much narrower than any possible color filter can achieve) as well as precision measurements of phase and frequency of a light signal relative to a reference light source, as in a [[laser Doppler vibrometer]].

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]].

In [[optical Kerr effect]] (OKE) spectroscopy, optical heterodyning of the OKE signal and a small part of the probe signal produces a mixed signal consisting of probe, heterodyne OKE-probe and homodyne OKE signal. The probe and homodyne OKE signals can be filtered out, leaving the heterodyne frequency signal for detection.

Heterodyne detection is often used in [[interferometry]] but usually confined to single point detection rather than widefield interferometry, however, widefield heterodyne interferometry is possible using a special camera.<ref>{{cite journal|last=Patel|first=R.|author2=Achamfuo-Yeboah, S. |author3=Light R.|author4=Clark M.|title=Widefield heterodyne interferometry using a custom CMOS modulated light camera|journal=Optics Express|date=2011|volume=19|issue=24|pages=24546–24556|url=https://www.osapublishing.org/oe/abstract.cfm?uri=oe-19-24-24546|doi=10.1364/oe.19.024546|pmid=22109482|bibcode=2011OExpr..1924546P |doi-access=free}}</ref>   Using this technique which a reference signal extracted from a single pixel it is possible to build a highly stable widefield heterodyne interferometer by removing the piston phase component
caused by [[microphonics]] or vibrations of the optical components or object.<ref>{{cite journal|last=Patel|first=R.|author2=Achamfuo-Yeboah, S. |author3=Light R.|author4=Clark M.|title=Ultrastable heterodyne interferometer system using a CMOS modulated light camera|journal=Optics Express|date=2012|volume=20|issue=16|pages=17722–17733|url=https://www.osapublishing.org/oe/abstract.cfm?uri=oe-20-16-17722|doi=10.1364/oe.20.017722|pmid=23038324|bibcode=2012OExpr..2017722P |doi-access=free}}</ref>