Automatic frequency control: Difference between revisions

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{{Short description|Radio circuit}}

{{other uses2|AFC}}

{{Unreferenced stub|auto=yes|date=December 2009}}

In [[radio]] equipment, '''Automatic Frequency Control''' (AFC) is a method (or device) 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.

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

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 [[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]].

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]].<ref>Rider 1937, pp. 2-3.</ref> It is primarily used in [[radio receiver]]s to keep the receiver [[Tuner (electronics)|tuned]] to the frequency of the desired station.{{citation needed|date=June 2025}}

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.

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}}

AFC in radio receivers

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.<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==

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.

{{Unreferenced stub|auto=yessection|date=DecemberJune 20092025}}

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.

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.

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==

* [[Frequency drift]]

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

*[[PLL]]

* [[de:Automatic frequencygain control]] (AGC)

==Notes==

{{reflist}}

==References==

*{{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}}

==External links==

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

{{DEFAULTSORT:Automatic Frequency Control}}

[[Category:Communication circuits]]

[[Category:Wireless tuning and filtering]]

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[[da:Automatisk frekvenskontrol]]

[[de:Automatic frequency control]]

[[hu:Automatikus frekvenciaszabályozás]]

[[ja:自動周波数制御]]

[[pl:Automatyczna regulacja częstotliwości]]

[[ru:Автоматическая подстройка частоты]]

[[sv:Automatisk frekvenskontroll]]