A superheterodyne receiver is an electronic device which uses frequency mixing to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency. Virtually all modern radio and television receivers use the superheterodyne principle. The superheterodyne receiver has three elements: the local oscillator, a frequency mixer that mixes the local oscillator's signal with the received signal, and a tuned amplifier.
Reception starts with an antenna signal, optionally amplified, including the frequency the user wishes to tune, fd. The local oscillator is tuned to produce a frequency close to fd, fLO. The received signal is mixed with the local oscillator signal. This stage does not just linearly add the two inputs, like an audio mixer. Instead it multiplies the input by the local oscillator, producing four frequencies in the output; the original signal, the original fLO, and the two new frequencies fd+fLO and fd-fLO. The output signal also generally contains a number of undesirable mixtures as well. These are 3rd- and higher-order intermodulation products. If the mixing were performed as a pure, ideal multiplication, the original fd and fLO would also not appear; in practice they do appear because mixing is done by a nonlinear process that only approximates true ideal multiplication.
The amplifier portion of the system is tuned to be highly selective at a single frequency, fIF. By changing fLO, the resulting fd-fLO (or fd+fLO) signal can be tuned to the amplifier's fIF. In typical amplitude modulation ("AM radio" in the U.S., or MW) receivers, that frequency is 455 kHz; for FM receivers, it is usually 10.7 MHz; for television, 45 MHz. Other signals from the mixed output of the heterodyne are filtered out by the amplifier.
The original heterodyne technique was pioneered by Canadian inventor Reginald Fessenden, but it was not pursued far because local oscillators available at the time were unstable in their frequency output, and vacuum tubes were not yet available. The superheterodyne principle was revisited in 1918 by U.S. Army Major Edwin Armstrong in France during World War I. He invented this receiver as a means of overcoming the deficiencies of early vacuum tube triodes used as high-frequency amplifiers in radio direction finding equipment.