Automatic frequency control (AFC) is a circuit used in some electronic devices to maintain a particular frequency automatically. Modern devices that are digitally tuned do not use AFC. A familiar use of AFC is to keep a radio tuned to a specific station. In some FM receivers, a device called an oscillator helps zero in on the frequency of the desired station. However, the oscillator can drift off the frequency as the radio heats up. The AFC circuit detects this frequency drift and develops a correction voltage in response. This voltage acts on the oscillator to reverse the drift and hold it on frequency. Without an AFC circuit, a listener would have to retune the radio manually each time a station began to fade.
AFC circuits are also used in some television and radar sets. In a TV set, the AFC circuit helps maintain the operating frequency of the horizontal oscillator. This oscillator produces an electronic signal that, when amplified, causes electron beams in the picture tube to scan the TV screen horizontally. For proper scanning to occur, the horizontal oscillator must have a constant frequency. Whenever the oscillator’s frequency varies, the AFC circuit produces a correction voltage. In a radar set, AFC corrects variations in the frequency of the signal generated by the set’s oscillator.
