Automatic frequency control

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.

2-27Figure 2-25.—Crystal filter used in the IF section of a superheterodyne receiver.The capacitance that exists between the crystal holder plates (C3) may bypass unwanted signalsaround the crystal; therefore, some method must be provided to balance out this capacitance. Youaccomplish circuit balancing by applying out-of-phase voltages to neutralize any undesired signalvoltages. The tap on L3 permits proper impedance matching and is the balanced input to the next stage.Q23. What does a tone control circuit in a receiver do to the audio signal?Q24. What is the function of a crystal filter in a receiver?Automatic Frequency ControlAfc circuits are used in situations where you must accurately control the frequency of an oscillatorby some external signal. Basically, this type circuit does two things: It senses the difference between theactual oscillator frequency and the frequency that is desired and produces a control voltage proportionalto the difference; it also uses the control voltage to change the oscillator to the desired frequency. Afccircuits are used to control the frequency of sinusoidal oscillators and nonsinusoidal oscillators. Onlysinusoidal afc circuits will be covered here. Afc circuits are used in radio receivers, fm transmitters, andfrequency synthesizers to maintain frequency stability. Figure 2-26 is a block diagram illustrating afcoperation in a receiver. Let’s run through the applicable parts of this block diagram

See also

• Frequency drift
• PLL