Automatic frequency control: Difference between revisions

Revision as of 19:08, 12 March 2012

Revision as of 02:45, 13 March 2012

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{{Unreferenced|date=December 2009}}

In [[radio]] equipment, '''Automatic Frequency Control''' (AFC), also called '''Automatic Fine Tuning''' (AFT) is a method (or device)circuit to automatically keep a [[resonant circuit]] [[Tuner (electronics)|tuned]] to the [[frequency]] of an incoming [[radio signal]]. It is primarily used in [[radio receiver]]s to keep the receiver [[Tuner (electronics)|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.

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Assuming that a receiver is nearly tuned to the desired frequency, the AFC [[electrical network|circuit]] in the receiver develops an error [[voltage]] proportional to the degree to which the receiver is mistuned. This error voltage is then [[Feedback|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 was mainly used in radios and television sets around the mid-20th century. In the 1970s, receivers began to be designed using [[frequency synthesizer]] circuits, which synthesized the receiver's input frequency from a [[crystal oscillator]] using the vibrations of a precision [[quartz crystal]], which had a stable enough frequency that AFC was not needed.

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.

== See also ==