Heterodyne detection is a method of extracting information encoded as modulation of the phase and/or frequency of an oscillating signal, by comparing that signal with a standard or reference oscillation that would have a fixed offset in frequency and phase from the signal if it carried null information. "Heterodyne" signifies more than one frequency, in contrast to the single frequency employed in homodyne detection. The heterodyne technique is commonly used in doppler radar, continuous-wave radar, telecommunications and astronomy.
The processed signals are most commonly produced by the reception of radiation in the form of either radio waves (see superheterodyne receiver) or light (see Optical heterodyne detection or interferometry). The reference signal is known as the local oscillator. The signal and the local oscillator are compared in the receiver using a type of technology suitable for the wavelength of the radiation. For radio frequency signals, a frequency mixer may be used, while for optical heterodyne detection, a common mixer is a photodiode, which has a response that is linear in energy, and hence quadratic in amplitude.