Heterodyning

Heterodyning is a signal processing technique in which two signals are combined in a nonlinear device, producing new frequencies equal to the sum and difference of the original frequencies. When one input is a signal at frequency f_sig and the other is a locally generated reference at frequency f_LO, the output contains components at f_sig + f_LO and |f_LO − f_sig|, among harmonics depending on the nonlinearity.

In radio engineering, heterodyning is used to translate signals to a more convenient frequency range. A mixer or nonlinear element mixes the received RF signal with a local oscillator; the resulting intermediate frequency (IF) is easier to amplify or filter. The superheterodyne receiver, invented in the early 20th century, relies on this principle to convert a wide band of radio frequencies to a fixed IF for processing.

Beyond receivers, heterodyning appears in optical and microwave domains as heterodyne detection, where two optical fields interfere to produce a beat note in the electrical domain, enabling high-resolution spectroscopy and coherent sensing. In laser metrology and telecommunications, optical heterodyning and coherent detection offer high sensitivity and phase information.

In everyday acoustics, the term heterodyne is sometimes used to describe audible beating when two tones close in frequency interact, producing a fluctuating amplitude at the difference frequency. The concept underpins many signal-processing techniques, including frequency synthesis, demodulation, and spectrum analysis.