IFR FM/AM-500, Maintenance Manual

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(primarily the frequency error) are used to tune the first VCO
to 1300 MHz plus the selected RF frequency. The high frequency
components (primarily noise) of the error voltage tunes the
second VCO. The result is that while the first VCO may vary
slightly around its established frequency, the difference
between the first and second VCO frequencies is held constant,
producing a stable 90 MHz IF product.
B. FIRST (1300-2298 MHz) VCO CIRCUIT
Inductor L2102, varactor CR2101 and chip capacitors C2104 and
C2106, with transistor Q2102, inductor L2103 and resistor R2108
forming a regenerative feedback circuit, comprise the basic
oscillator. Capacitor C2122, resistors R2104, R2105, R2109 and
R2110, and inductors L2105 and L2106 provide a bias network for
transistor Q2102. Inductor L2101 and capacitor C2101 form a
lowpass L-type DC filter, and transistor Q2101, resistors R2101
and R2102, and capacitor C2102 make a voltage regulator. Tuning
voltage enters through J2101 and is filtered by resistors R2102,
R2106 and R2107, capacitors C2103, C2105 and C2123, and inductor
L2104. Exposed leads of varactor CR2101 and chip capacitors
C2104 and C2106 form an inductor, coupling the oscillator out­
put to inductor L2108, which feeds the buffer amplifiers.
C. FIRST VCO BUFFER AMPLIFIER CIRCUITS
The first VCO output is inductance-coupled through inductor
L2108 to two identical two-stage buffers. One buffer feeds the
first IF mixer through J2103, and the other goes to the mixer
furnishing VCO feedback to the High Loop Synthesizer. Since
both buffers are identical except for reference designators,
the buffer feeding the first IF mixer will be discussed in
detail. The first stage amplifier uses a common emitter, self­
biased, class B amplifier Q2105, with resistor R2118 establish­
ing proper bias and feedback. Inductor L2110 and capacitor
C2109 form a lowpass LC filter, and resistor R2115 is a load
resistor. The second stage is identical to the first. Capac­
itors C2110, C2114 and C2120 isolate the two stages, the input,
and the output of the amplifier.
D. SECOND (1210 MHz) VCO CIRCUIT
Transistor Q2002 functions as an oscillator with varactor CR2001
controlling the frequency in response to the applied voltage
level. Capacitor C2004 and inductor L2005, which is formed
from one lead of the capacitor, establish the tuned circuit.
Inductors L2002, L2003 and L2004 are isolation inductors.
Trimmer capacitor C2003 allows calibration of the output fre­
quency. Power is supplied through a lowpass LC filter comprised
of inductor, L2001 and capacitor C2002. Transistor Q2001 and
the associated resistors R2001 and R2002, and capacitor C2002
regulate the voltage. VCO output coupling is through inductor
L2006. Phase shift voltage enters through J2001.
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