Automatic Frequency Control

2-27 Figure 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 signals around the crystal; therefore, some method must be provided to balance out this capacitance. You accomplish circuit balancing by applying out-of-phase voltages to neutralize any undesired signal voltages. 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 Control Afc circuits are used in situations where you must accurately control the frequency of an oscillator by some external signal. Basically, this type circuit does two things: It senses the difference between the actual oscillator frequency and the frequency that is desired and produces a control voltage proportional to the difference; it also uses the control voltage to change the oscillator to the desired frequency. Afc circuits are used to control the frequency of sinusoidal oscillators and nonsinusoidal oscillators. Only sinusoidal afc circuits will be covered here. Afc circuits are used in radio receivers, fm transmitters, and frequency synthesizers to maintain frequency stability. Figure 2-26 is a block diagram illustrating afc operation in a receiver. Let’s run through the applicable parts of this block diagram.