LEARNING EVENT 20: FREQUENCY-SENSITIVE AFC LOOP
1. The block diagram of an AFC circuit is shown in Figure 3-5. Here the mixer combines the local-oscillator
and incoming carrier frequencies and generates a difference (intermediate) frequency. The IF amplifier
increases the amplitude of the signal to a level sufficient for demodulation by the discriminator.
Figure 3-5. Frequency-sensitive AFC circuit.
2. The discriminator is a frequency-sensitive device that converts the IF changes that are above or below the
desired IF into positive or negative DC (baseband) signals. If there are no frequency changes in the IF, there is
no output from the discriminator.
3. Since the incoming signal is frequency modulated, the IF varies at the baseband frequency rate. If these
variations are fed back to the local oscillator, the local oscillator's frequency will change and cause the mixer to
reduce the deviations in the IF signal. It is generally undesirable to use the baseband to control the local
oscillator. Therefore, AFC circuits use a low-pass filter to prevent the baseband frequency from being fed back
to the local oscillator. The low-frequency variations representing the drift of the IF average value will be
passed by the low-pass filter to the local-oscillator stage.
4. The local oscillator is a VCO whose output is combined in the mixer to produce the IF. A DC voltage is
applied to the local oscillator from the discriminator through the low-pass filter to control the operating
frequency.
5. The AFC loop will attempt to maintain the IF at a constant value regardless of whether the IF tends to
increase or decrease.
a. Assume that the increases and that, as a result, the discriminator produces an average output which is
positive. (The output polarity depends on the actual circuit configuration and the requirements-of the local
oscillator.) The positive voltage, when applied through the low-pass filter, causes the local-oscillator frequency
to increase. As the frequency of the local oscillator increases, the IF decreases toward the desired value. As the
IF decreases, the output voltage from the discriminator will also decrease. When the average IF is at the proper
value, the output from the discriminator has an average value of zero.
3-19
SS0031
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