Automatic frequency control
Encyclopedia
In radio
equipment, Automatic Frequency Control (AFC) is a method (or device) to automatically keep a resonant circuit tuned to the frequency
of an incoming radio signal. It is primarily used in radio receivers to keep the receiver 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.
Assuming that a receiver is nearly tuned to the desired frequency, the AFC 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
.
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.
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
equipment, Automatic Frequency Control (AFC) is a method (or device) to automatically keep a resonant circuit tuned to the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of an incoming radio signal. It is primarily used in radio receivers to keep the receiver 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
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
, 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 circuit
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
in the receiver develops an error voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
proportional to the degree to which the receiver is mistuned. This error voltage is then fed back
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
to the tuning circuit in such a way that the tuning error is reduced. In most frequency modulation
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
(FM) detectors an error voltage of this type is easily available. See Negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
.
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
Frequency synthesizer
A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator. They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems,...
method became cheaper and more widespread.