Automatic Frequency Control

Automatic Frequency Control (AFC) is a method (or device) to automatically maintain a tuning of electromagnetic radiation (radio or microwave) signal to desired frequency. 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.
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A device or circuit that maintains the frequency of an oscillator within the specified limits with respect to a reference frequency.

automatic frequency control
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SINTONIA AUTOMATICA. REGOLAZIONE AUTOMATICA DELLA FREQUENZA