In radio equipment, Automatic Frequency Control (AFC) is a method (or device) to automatically keep a resonant circuit tuned to the frequency of an incoming radio signal. It is primarily used in radio receivers to keep the receiver tuned to the frequency of the desired station.
In radio communication AFC is needed because, after the bandpass frequency of a receiver is tuned to the frequency of a transmitter, the two frequencies may drift apart, interrupting the reception. This can be caused by a poorly controlled transmitter frequency, but the most common cause is drift of the center bandpass frequency of the receiver, due to thermal or mechanical drift in the values of the electronic components.
Assuming that a receiver is nearly tuned to the desired frequency, the AFC 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.
AFC is also called Automatic Fine Tuning (AFT) in radio and TV receivers. It became rare in this application, late in the 20th century, as the more effective frequency synthesizer method became cheaper and more widespread.