Heterodyne

[1][2][3] Heterodyning is used to shift signals from one frequency range into another, and is also involved in the processes of modulation and demodulation.

[2][4] The two input frequencies are combined in a nonlinear signal-processing device such as a vacuum tube, transistor, or diode, usually called a mixer.

[8] In a 1905 patent, Fessenden stated that the frequency stability of his local oscillator was one part per thousand.

This ringing would quickly decay, so the output of the transmitter would be a succession of damped waves.

When these damped waves were received by a simple detector, the operator would hear an audible buzzing sound that could be transcribed back into alpha-numeric characters.

With the development of the arc converter radio transmitter in 1904, continuous wave (CW) modulation began to be used for radiotelegraphy.

CW Morse code signals are not amplitude modulated, but rather consist of bursts of sinusoidal carrier frequency.

The direct-conversion (heterodyne) detector was invented to make continuous wave radio-frequency signals audible.

Fessenden coined the word heterodyne from the Greek roots hetero- "different", and dyn- "power" (cf.

[11] An important and widely used application of the heterodyne technique is in the superheterodyne receiver (superhet).

The regenerative radio receiver obtained more gain out of one gain device by using positive feedback, but it required careful adjustment by the operator; that adjustment also changed the selectivity of the regenerative receiver.

The superheterodyne provides a large, stable gain and constant selectivity without troublesome adjustment.

Besides its use in the superheterodyne circuit found in almost all radio and television receivers, it is used in radio transmitters, modems, satellite communications and set-top boxes, radar, radio telescopes, telemetry systems, cell phones, cable television converter boxes and headends, microwave relays, metal detectors, atomic clocks, and military electronic countermeasure (jamming) systems.

In large scale telecommunication networks such as telephone network trunks, microwave relay networks, cable television systems, and communication satellite links, large 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.

At the cable source or headend, electronic upconverters convert each incoming television channel to a new, higher frequency.

Some U-matic (3/4″) decks feature 7-pin mini-DIN connectors to allow dubbing of tapes without conversion, as do some industrial VHS, S-VHS, and Hi8 recorders.

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

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".

It is also being applied in the creation of more accurate atomic clocks based on directly measuring the frequency of a laser beam.

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.

[17] 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.

Some widely used mixer circuits, such as the Gilbert cell, operate in this way, but they are limited to lower frequencies.

Examples of nonlinear components that are used as mixers are vacuum tubes and transistors biased near cutoff (class C), and diodes.