Figure 3-4. FMFB effects on carrier deviation.
LEARNING EVENT 19: AUTOMATIC FREQUENCY CONTROL
1. During the receiving-demodulation process, the local-oscillator output is mixed with the incoming carrier
signal to produce an intermediate frequency. The difference between the local oscillator and carrier frequencies
is the IF.
2. If the carrier frequency or the local-oscillator frequency drifts, the average IF will change. If the average
frequency of the IF signal is permitted to drift, the extremes of the carrier deviation will exceed the limits of the
IF amplifier bandpass. As a result, distortion will appear in the demodulated signal. Therefore, it is necessary
to produce an IF signal whose average frequency is centered in the IF amplifier bandpass. This will ensure that
the deviation of the incoming signal will not exceed the limits of the IF amplifier bandpass.
3. To ensure the centering of the signal in the IF amplifier bandpass, it is necessary to have a circuit that will
sense the average frequency (or phase) changes and produce a voltage that will change the frequency of the
local oscillator. The automatic frequency control (AFC) circuit will produce this corrective voltage.