Any device used to multiply signals which has a nonlinear response to an electric field.Possible devices include bolometers, photoconductors, classical (Schottky) diodes, and quantum non-linear devices (SIS receivers or Josephson junction mixers).Bolometers and photodetectors have limited response time and therefore limited IF bandwidths. Classical diodes cannot have gain and typically have I-V characteristics whose nonlinearity involves a semiconductor gap of V. Quantum nonlinear devices can have gain, and have a superconducting gap of a few mV, as well as having low power requirements (Phillips 1988).
Two diodes for which can be used as multipliers by inputing sums and differences of two voltages and differencing

Mixers combine a radio frequency [RF] signal and a local oscillator [LO]. The result of the multiplication for two cosinusoidal signals is


The high frequency term can be removed with a low pass filter.

The signal is then applied to a filter which only accepts a bandwidth , so the range of accepted frequencies is to , where is the intermediate frequency. The signals which are accepted satisfy

and the accepted RFs are


where LSB denotes the lower sideband,USB the upper sideband, and the LSB has frequencies in the opposite order.

For a few 10s of GHz, the signal is applied to low noise amplifiers and then passed through a filter which admits only one sideband (single-sideband mixer). For GHz, it is difficult to make low-noise amplifiers, so both sidebands are accepted for weak signals (double-sideband mixer).