Heterodyne

A heterodyne is the result of mixing two signals in a nonlinear device to produce new frequency components, notably sum and difference frequencies of the original signals. This process, called heterodyning, is fundamental to signal processing and radio systems. The term originated with Reginald Fessenden, who in 1908 described a “heterodyne receiver” in which a locally generated oscillation interacts with the received signal to produce “beats of an audible frequency.” In that paper he also referred to his 1902 patent describing such a system, although the term heterodyne was not used there.

In a typical implementation, signals at frequencies f₁ and f₂ are applied to a nonlinear element such as a frequency mixer, producing components at f₁ + f₂ and f₁ − f₂, along with additional intermodulation products arising from the device nonlinearity. One of these components is selected by filtering for further use, while the others are rejected.

Heterodyning is not limited to radio frequencies; the same principle applies across the electromagnetic spectrum, including infrared and optical systems, where it is used in coherent detection techniques. Heterodyne frequencies are related to the phenomenon of beats in acoustics.

A major application of the heterodyne process is in the superheterodyne radio receiver circuit, which is used in virtually all modern radio receivers.