A Heterodyne is generated when two frequencies which are close together (and of similar level) interact to give a "beating" effect i.e. a noise which gets quieter and louder alternatively. An example of this is a twin propeller aeroplane whose engines (running at slightly different speeds) give a throbbing type of noise.
This is used in a superheterodyne radio receiver where it converts the incoming radio frequency by heterodyne action to a common intermediate frequency (such as the 455 kHz frequency used by many AM radios).
The heterodyne is also used to increase the resolution of time and frequency measurement systems, by converting the incoming signal to a lower frequency.
Subjects: Noise & Vibration