English: Graph showing how a superheterodyne radio receiver receives a radio signal, shifting it down to the IF frequency. The frequency spectrum of the desired radio transmission is shown at RF. it consists of a carrier wave at the center frequency (dark blue) with sidebands representing the modulation (light blue) on either side. This is mixed with the signal from the local oscillator (LO) at a lower frequency. The result is a heterodyne (beat) signal called the intermediate frequency at the difference of these two frequencies IF = RF − LO. This is amplified and bandpass (red) filtered in the IF amplifier, and passed through a detector to extract the modulation (M).

The receiver can also receive signals at the image frequency LO − IF, so an image rejection filter (red) is needed on the input .to reject this frequency