The Super Heterodyne (sometimes shortened to superhet) principle as used in radio receivers allows certain obstacles to high performance radio design to be overcome. Tuned Radio Frequency (TRF) receivers suffered from poor frequency stability, and poor selectivity, as even filters with a high Q factor have a wide bandwidth at radio frequencies.
In radios using the principle, all signal frequencies are converted typically to a constant lower frequency before detection. This constant frequency is called the Intermediate Frequency, or IF. In a typical AM (Medium Wave) home receivers, that frequency is 455 kHz, for FM VHF receivers, it is usually 10.7 MHz.
Super Heterodyne receivers "beat" or heterodyne a frequency from a local oscillator (within the receiver) with the incoming signal. The user tunes the radio by adjusting the set's oscillator frequency and/or the tuning of the incoming signals. This heterodyning results in a higher and a lower frequency than that of the incoming frequency. Either the higher or the lower (typically) is chosen as the IF, which is amplified and then demodulated (reduced to just audio frequencies through a speaker).
Almost all receivers in use today utilize this method. The diagram below shows the basic elements of a single conversion superhet receiver. In practice not every design will have all these elements, nor does this convey the complexity of other designs, but the essential elements of a local oscillator and a mixer followed by a filter and IF amplifier are common to all superhet circuits.
The advantage to this method is that most of the radio's signal path has to be sensitive to only a narrow range of frequencies. Only the front end (the part before the frequency converter stage) needs to be sensitive to a wide frequency range. For example, the front end might need to be sensitive to 1-30 MHz, while the rest of the radio might need to be sensitive only to 455 kHz, a typical IF frequency.
Sometimes, to overcome obstacles such as image response, more than one IF is used. In such a case, the front end might be sensitive to 1-30 MHz, the first half of the radio to 5 MHz, and the last half to 50 kHz. Two frequency converters would be used, and the radio would be a "Double Conversion Super Heterodyne".
Super Heterodyne receivers have superior characteristics in both frequency stability and selectivity. It is much easier to stabilize an oscillator than a filter, especially with modern frequency synthesiser technology, and IF filters can give much narrower passbands at the same Q factor than an equivalent RF filter. A fixed IF also allows the use of a crystal filter in very critical designs such as radiotelephone receivers which have exceptionally high selectivity.
Radio transmitters also use Super Heterodyne technology. The design of a Super Heterodyne transmitter is similar to that of a Super Heterodyne receiver which each stage of the signal path reversed.