Assuming that a receiver is nearly tuned to the desired frequency, a circuit in the receiver develops an error voltage proportional to the degree to which the receiver is mistuned. This error voltage is then fed back to the tuning circuit in such a way that the tuning error is reduced. In most frequency modulation (FM) detectors an error voltage of this type is easily available. See Negative feedback.
The AFC is also called Automatic Fine Tuning (AFT) in radio and TV receivers.