Automatic frequency control (AFC)
Helgert, Hermann J. Department of Electrical and Computer Engineering, George Washington University, Washington, DC.
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The automatic control of the intermediate frequency in radio, television, or radar receivers, to correct for variations of the frequency of the transmitted carrier or the local oscillator in the receiver. For many reasons the frequency of the oscillators at the transmitter or receiver may fluctuate in time about its nominal value. Some of the more pronounced effects include variations in temperature or humidity, internal noise and external interference with the operation of the circuitry, and Doppler shifts of the frequency of the received signal due to movement of the transmitter or moving obstacles in a multipath channel. Automatic frequency control (AFC) is employed in radio and television receivers, FM transmitters, radar systems, satellite transceivers, and frequency synthesizers to maintain frequency stability. See also: Communications satellite; Doppler effect; Intermediate-frequency amplifier; Mobile communications; Radar; Radio receiver; Radio transmitter; Television receiver
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