Automatic frequency control: Difference between revisions

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{{Short description|Radio circuit}}

'''Automatic Frequency Control''' (AFC) is a method (or device) to automatically maintain a [[Tuner (electronics)|tuning]] of [[electromagnetic radiation]] (radio or [[microwave]]) signal to desired [[frequency]].

[[File:APCh2.svg|thumb|Basic automatic frequency control in a radio receiver. У = RF amplifier stages, Д = frequency discriminator stage]]

Assuming that a receiver is nearly tuned to the desired frequency, a [[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 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]].

In [[radio]] equipment, '''Automatic Frequency Control''' ('''AFC'''), also called '''Automatic Fine Tuning''' ('''AFT'''), is a method or circuit to automatically keep a [[resonant circuit]] [[Tuner (electronics)|tuned]] to the frequency of an incoming [[radio signal]].<ref>Rider 1937, pp. 2-3.</ref> It is primarily used in [[radio receiver]]s to keep the receiver tuned to the frequency of the desired station.{{citation needed|date=June 2025}}

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.{{citation needed|date=June 2025}}

Assuming that a receiver is nearly tuned to the desired frequency, athe 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 fed back to the tuning circuit in such a way that the tuning error is reduced.<ref>Rider 1937, p. 7.</ref> In most [[frequency modulation]] (FM) detectors, an error voltage of this type is easily available.{{citation needed|date=June 2025}} See [[Negative feedback]].

==Use==

{{Unreferenced section|date=June 2025}}

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 an ultra-stable quartz crystal. These maintained sufficiently stable frequencies that AFCs were no longer needed.

The AFC is also called Automatic Fine Tuning (AFT) in radio and TV receivers.

==See also==

* [[Frequency synthesizerdrift]]

* [[Phase-locked loop]] (PLL)

*[[Frequency drift]]

* [[Automatic gain control]] (AGC)

*[[PLL]]

==Notes==

[[Category:Communication circuits]]

{{reflist}}

==References==

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*{{Cite book |last=Rider |first=John F. |url=http://www.tubebooks.org/Books/afcs.pdf |title=Automatic Frequency Control Systems |publisher=John F. Rider |year=1937 |___location=New York City}}

{{electronics-stub}}

==External links==

*[http://www.radartutorial.eu/09.receivers/rx11.en.html Radar Tutorial]

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[[Category:Communication circuits]]

[[Category:Wireless tuning and filtering]]

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