Heterodyning

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In radio and signal processing, heterodyning is the generation of new frequencies by mixing (multiplying), two oscillating waveforms. It is useful for placing information of interest into a useful frequency range following modulation or prior to demodulation. The two frequencies are mixed in a vacuum tube, transistor, diode, or other signal processing device. Mixing two frequencies creates two new frequencies, according to the properties of the sine function; one is the sum of the two frequencies mixed, the other is their difference. These new frequencies are called heterodynes. Typically only one of the new frequencies is desired—the higher one after modulation and the lower one after demodulation. The other signal is filtered out of the output of the mixer.

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History

The word heterodyne is derived from the Greek roots hetero- "different", and dyn- "power" (cf. dynamis). The original heterodyne technique was pioneered by Canadian inventor-engineer Reginald Fessenden in 1901, but was not pursued very far because local oscillators were not very stable at the time.[1] Heterodyning was invented by Fessenden as a technique to make the Morse code radiotelegraph (CW) signals used during the wireless telegraphy era audible.[2] A "heterodyne" receiver had an oscillator circuit, called a local oscillator, that produced a radio signal that was adjusted to be close in frequency to the signal being received, so that when the two signals were mixed the difference or "beat" frequency was in the audible range. This produced a musical tone in the speaker, so the "dots" and "dashes" of Morse code were audible as beeping sounds. This technique is still used in radio telegraphy; the oscillator is now called a beat frequency oscillator.

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