(1) The oscillator's tuned circuit consists of transformer T1, trimmer
capacitor C4, capacitors C2 and C3, and varactor diodes CR2 and CR3.
Capacitors C2 and C3 also serve to match the output impedance of the
oscillator to the input impedance of the following stage.
Capacitor C7
couples the output back to the emitter to sustain oscillations. Emitter-
to-base bias is developed across resistor R4 and C5.
Base bias is
developed at the junction of R5, zener diode CR4, and C6.
effective capacitance of the diodes to change. These changes in effective
capacitance, in accordance with the audio frequency, cause the oscillator
frequency to change.
Therefore, the output of the oscillator is an FM
signal that varies in accordance with the audio frequency from the speech
amplifier.
c. Reactance Modulation. The speech amplifier performs the same basis function
as the reactance-tube modulator described in TM 11-668. The amplifier changes the
incoming amplitude variations of the audio frequency signal into a varying
reactance, which is injected into the oscillator's tank circuit. These variations
cause changes in the capacitance of CR2 and CR3, which result in capacitive
reactance changes in the oscillator's tank circuit.
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. Assume that the circuit shown in figure 37 of TM 11-668 is used to produce an
FM signal. When the. value of inductance L is 0.9 microhenry (uh) and capacitance
C is 0.002 microfarad, the center frequency is 3.75 MHz.
If the microphone
diaphragm is moved s, that the capacitance is raised to 0.004 microfarad, the
frequency deviation will be approximately
a. 0.10 MHz.
c.
2.65 MHz.
b. 1.10 MHz.
d.
3.70 MHz.
2. The Hartley oscillator circuit shown in figure 36 of TM 11-668 is of the type
used as the master oscillator in a portable FM transmitter. When the value of coil
L is 30 microhenries and the value of capacitor C is 94 picofarads (94 x 10-12
farads), the operating frequency is 3 MHz. If the value of inductance is changed
to decrease the frequency to 2.5 MHz, the increase in inductive reactance is
approximately
a. 50 ohms.
c.
300 ohms.
b. 110 ohms.
d.
560 ohms.
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