IFR FM/AM-500, Maintenance Manual

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is recharged. Transistors Q4005 and Q4007 are emitter-followers
which buffer the DC output, providing two diode gaps of ampli­
fication to the second sampler.
I. SECOND PHASE SAMPLER (On Analog PC Board)
The output from the first sampler isolation amplifier is applied
to the drain of FET Q4010. When gated by the spike from the
pulse generator through capacitor C4009, the output of Q4010 is
the instantaneous level of the signal at the drain. When the
system is phase locked, this level is constant. Capacitor
C4011 holds the output level, being recharged each time Q4010
is gated. At the same time Q4010 is gated, capacitor C4010 is
discharged to reduce output noise.
J. SECOND SAMPLER AMPLIFIER (On Analog PC Board)
The output from the second sampler is combined with an adjust­
able bias voltage through potentiometer R4045 and applied to
the base of transistor Q4012. Q4012 applies a voltage to the
base of transistor Q4011, whose output is divided, with one
branch connected to the first (1300-2298 MHz) VCO integrator
circuit and the second branch being fed to the second (1210 MHz)
VCO integrator circuit.
K. PHASE DETECTOR CIRCUIT (On Divider PC Board)
The 500 kHz reference signal from the reference signal amplifier
and the 500 kHz feedback signal from the second feedback ampli­
fier are fed through inverters U4113B and U4113A respectively,
to phase-locked loop U4112. U4112 has two outputs; one is to
transistor Q4103 in the phase lock indicator circuit; the other
is a correction signal to the first (1300-2298 MHz) VCO inte­
grator. The level of this signal, which is approximately
5.2 VDC when the system is phase locked, is limited to a mini­
mum of approximately 2.7 VDC and a maximum of approximately
8.1 VDC by a voltage limiting circuit consisting of zeners
CR4106 and CR4107, diodes CR4105 and CR4108, resistors R4126
and R4127 and capacitor C4133.
L. FIRST VCO INTEGRATOR CIRCUIT (On Analog PC Board)
The +40 VDC from the Power Supply provides the voltage to oper­
ate the first (1300-2298 MHz) VCO in the Dual VCO Module. The
first VCO integrator circuit controls this voltage, adjusting
it as necessary to operate the VCO at the desired frequency.
The output from the second sampler amplifier feeds into inte­
grator U4004A. As its input level increases, the integrator's
output decreases, pulling down on the VCO voltage through tran­
sistor Q4013 at a gain equal to the ratio of resistor R4039 and
resistors R4030 and R4031. Op amps U4005A and U4005B, with
potentiometers R4042 and R4047, establish high and low limits
respectively. A third limiter prevents the output to the VCO
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