4-5.
AFC
The-AFC circuit is used to control the center frequency of oscillator Q2. The
circuit compensates for any variations that might be caused by temperature changes,
humidity, or vibrations. The circuit contains a crystal oscillator stage, a mixer
stage, and a discriminator stage.
a. Crystal Oscillator.
Transistor Q8 and its associated components make up a
crystal-controlled Colpitts oscillator.
The inductance required to resonate with
the tapped capacitors is provided by crystal Y1.
b. Mixer.
Mixer stage Q9 heterodynes the two signals and produces an out put
that is analyzed in the discriminator stage.
c. Discriminator. This phase discriminator produces a dc output voltage that is
proportional to the frequency error of the input signal.
The polarity of the dc
signal depends on whether the input signal is higher or lower than the desired
frequency of oscillator stage Q2.
LESSON EXERCISES
In each of the following exercises, select the ONE answer that BEST completes
the statement or answers the question.
Indicate your solution by circling the
letter opposite the correct answers in the subcourse booklet.
1. Figure 82 of TM 11-668 is a functional block diagram of an FM transmitter and
shows an oscillator corrector as part of the AFC system.
The electron tube in
figure 100 of TM 11-668 that functions as this corrector is
a. V2.
c.
V4.
b. V3.
d.
V5.
2. To obtain a zero potential between point A and ground in the circuit shown in
figure 84 of TM 11-668, the frequency of the signal in T1 must be equal to the
a. resonant frequency of T2.
b. resonant frequency of T3.
c. difference between the resonant frequencies of T2 and T3.
d. sum of the resonant frequencies of T2 and T3 divided by 2.
3. An electron tube with a single cathode can be used in the double-tuned
discriminator shown in figure 84 of TM 11-668.
However, the same tube CANNOT be
used in the phase discriminator shown in figure 87 because
323 L4
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