Automatic frequency control

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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.

AFC in radio receivers

AFC circuits are used in situations where you must accurately control the frequency of an oscillator by some external signal. The AFC circuit senses the difference between the actual oscillator frequency and the frequency that is desired and produces a control voltage proportional to the difference. A varicap is used to keep the IF stable. The varicap application here produces an apparent reactance, which is included in the oscillator frequency control circuitry. For example, let's assume the IF is 10,7 megahertz and the local oscillator (LO) is tracking below the incoming frequency. When the LO output decreases slightly in frequency, the IF will rise. This causes the output of the discriminator to increase the capacitive reactance of the varicap, which increases the oscillator frequency to the desired value. Now let's assume the LO output increases. The IF will then decrease. This causes the discriminator output to decrease the capacitive reactance of the varicap. This will cause the oscillator frequency to decrease. This variant of AFC circuits are used in radio receivers, fm transmitters, and frequency synthesizers to maintain frequency stability. It requires a relatively constant amplitude of the (received) input-signal. For pulse-radar sets this form isn't practicable therefore.

See also