Advantages of intermediate frequency

The intermediate frequency, or IF, in a superhetrodyne receiver is used to tune the desired signal.

The IF stage is tuned to a specific, fixed frequency, usually 455 kHz for AM and 10.7 MHz for FM. Since the IF stage does not need to be adjustable, it can be designed for high performance, tight bandpass operation.

This is far easier to do than creating an adjustable RF stage with tight bandpass operation. Instead, the RF stage is broadband and the IF stage is narrow band.

The actual tuning of the receiver is done by varying the local oscillator that is mixed with the radio frequency (RF) signal just upstream of the IF stage. The difference frequency of the output of the mixer is then amplified by the IF stage.

The audio information, to be fed to an amplifier and be heard on the speaker, is extracted after the last IF stage - either by a detector in AM or discriminator in FM.