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TM 11-6625-467-34
Figure 2-21. 3.975 MHz oscillutor circuit simplified schematic diagram.
detector output. Capacitor C18, and resistor R25
the emitter of Q1. The voltage at this point is in
form a decoupling network along with RF choke L1.
phase with the voltage at the collector and thereby
sustains oscillators in the circuit. Capacitors C2 and
2-27. 30 MHz and 68 MHz Oscillators
C3 shunting the 3.975-MHz crystal of C4 in series
with the crystal also have a slight effect upon the
The 30-megahertz and 68-megahertz oscillators are
frequency. LOW FREQ AMPL control R1 varies the
identical except for component values. For the pur-
dc voltage at the collector of the oscillator. This
voltage directly controls the output amplitude of the
poses of this discussion, only the 30-MHz oscillator is
oscillator. Capacitor C5 couples the oscillator output
considered. The oscillator circuit is a Colpitts-type
to the input of emitter follower Q2. Resistors R6 and
with collector-to-emitter feedback using a crystal in
R7 form a voltage divider network that educes the
the feedback path. Inductor L1 and capacitors C3
amplitude of the signal at the base of Q2. The emit-
and C4 in the collector of Q1 are resonant at 30
ter follower circuit current-amplifies the signal and
megahertz. Crystal Y1 couples the output from the
provides a low-impedance output. A level detector
tuned circuit back to the emitter circuit of Q1. The
consisting of diode CR1, resistor R5, and capacitor
crystal acts as a series-tuned circuit and offers
minimum impedence to a 30-megahertz resonant fre-
C6 samples the output of the oscillator and produces
a dc level that is proportional to the output ampli-
quency. At frequencies above or below 30-mega-
hertz, Y1 is reactive and causes a phase shift in the
tude of the oscillator. Resistors R8, R9, and R29
form an adjustable voltage divider for the detector
feedback voltage appearing at the emitter of Q1. At
a frequency of 30 megahertz, the reactive com-
output. The divider output connects to the meter
ponents of Y1 cancel and the feedback voltage to the
circuit in the test set. Resistor R9 calibrates the
2-62

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