In a superhet or superheterodyne receiver, the signal coming from the antenna is mixed with the signal from a tunable oscillator, and subtractive mixing in the mixer stage tube produces the intermediate frequency.
The intermediate frequency is amplified in one or more IF amplifier stages which are easier to design. Here the IF filters are installed as selection means to improve the selectivity. The signal then is feed to the demodulator and is passed on to an AF preamplifier and AF final amplifier.
The set designers choice of the intermediate frequency results from the frequency range of the receiver: for a longwave receiver, a low intermediate frequency is more favourable, since filters with sufficiently high quality can be constructed with simple L/C filters and the mirror images of the received signal are still sufficiently far off the actual operation frequency on the dial.
For shortwave receivers, where the double IF is dangerously close to the operating frequency on the dial, a higher IF is usually selected, but doing this, the quality of LC bandpass filters is increasingly poor, so that ceramic, quartz or mechanical filters have to be used. In addition, it tasks more effort to construct IF amplifiers working at higher frequencies.
A typical intermediate frequency of domestic broadcast receivers is 455 kHz, and many different wide and narrow filters are available for this IF.
In order to improve the image frequency rejection of high-quality double conversion receivers, the first step is to convert to a high first intermediate frequency. This is followed by the conversion to a second intermediate frequency, on which standardised IF amplifier circuits, filters and also additional accessories that require the IF (for example panoramic adaptors) operate.
For FM receivers, the usual intermediate frequency is 10.7 MHz.