Automatic frequency control: Difference between revisions

Revision as of 16:01, 28 September 2011

Revision as of 17:39, 28 September 2011

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{{other uses2|AFC}}

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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.

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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 ==

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==

*[[Frequency drift]]

*[[PLL]]

== External links ==

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

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[[Category:Wireless tuning and filtering]]

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