TM 11-6625-486-14&P
EL6625-486-14-TM-15
combination of R3 and C6 and (not in AN/USM-159) C13
5-4. High Frequency Oscillator
adjusts the feedback signal while R3 and R1 provide
forward base bias. The collector current and operating
stability of Q1 are maintained at close limits by
connection of the emitter through R2 and R4 to the
transistorized oscillator, is the vfo for range C.
It
emitter of Q2. This series connection between the two
produces fundamental frequencies that may be varied
emitters constitutes a current feedback loop. Since the
from 65 to 130 MHz and harmonics up to 1,000 MHz.
emitter-follower output signal of Q1 is applied through C7
The variable capacitors, labeled TUNING and
to the base of Q2, the phase of the input signal (positive
CORRECTOR, are mechanically linked to similar
or negative) will tend to decrease or increase the emitter
capacitors in the low frequency vfo. A segment of the
current flowing through R8. This current change will be
filmstrip is calibrated in fundamentals for range C. When
reflected on the emitter bias of Q1 and cause the correct
this range is in use, a mask covers the other two ranges.
change in bias to stabilize collector current. The oscillator
output signal that appears at the collector of Q2 will be at
b. The high frequency oscillator circuit consists of
a constant amplitude.
Potentiometer R4 sets the
Colpitts oscillator Q1 followed by buffer amplifier Q2.
minimum value of emitter bias and collector current for
Positive feedback is obtained between the collector and
Q1. Inductor L3 and C2 remove spurious frequencies.
base circuits formed by the tank circuit. For tuning
Actually, inductor L3 is not a coil but a short length of wire
purposes, the oscillator tank circuit is formed by the
inside a piece of ferrite material which contributes the
parallel-resonant combination of coil L1, and split-stator
required inductance. At the frequencies to be rejected, L3
capacitor C701B. The midpoint of this tuning capacitor
has minimum reactance (equal to a straight piece of wire)
electrically forms a center tap on coil L1. The effective
and permits these frequencies to be bypassed through
tuning range is padded by capacitors C3, C4, and C5.
C2. For desired frequencies, L3 behaves as an iron-core
Variable capacitor C702B (labeled CORRECTOR) and L1
rf choke and presents reactance.
form a series-resonant circuit to adjust tuning over a
limited range.
d. Buffer amplifier Q2 isolates the oscillator circuit
and amplifies the signal. The collector of Q2 receives
c. The collector of Q1 is supplied voltage through
voltage through S703, power supply decoupling resistor
S703A, rf choke L2, and L1. Bypass capacitor C8 and
R5, and high frequency peaking coil L4. Capacitor C10
choke L2 form a low-pass filter to keep rf out of the power
bypasses rf and base bias is established by R6 and R7.
supply. Choke L2 presents a high reactance to rf and
Emitter resistor R8 prevents emitter current from
prevents grounding of the feedback signal at the junction
exceeding a safe value and capacitor C11 bypasses rf.
of L1 and L2.
The resistance-capacitance (rc)
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