IFR FM/AM-500, Maintenance Manual

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LED on the Low Loop is grounded, allowing a continuous forward
bias to illuminate the LED. The grounding of the LED also
enables a 2 Hz oscillator in the Generate Audio Module, causing
the LED on the Front Panel to flash at approximately half­
second intervals.
B. VCO.CIRCUIT
The Low Loop output of 79.3 to 77.3 MHz is produced by voltage
controlled oscillator Q4206. Variable inductor L4203 provides
calibration adjustment. Inductor L4202 and capacitor C4212
decouple the oscillator from the power supply circuitry.
Oscillator tuning bandwidth is established by capacitors C4213,
C4214 and C4217, inductor L4203, and diode CR4203. Diode
CR4202, resistor R4217, and capacitor C4215 control the ampli­
tude.
C. BUFFER AMPLIFIERS
The VCO output is buffered by transistor Q4205, then split and
applied to two independent buffers. One second stage buffer,
consisting of transistors Q4203 and Q4204, feeds the third
mixer of the IF Module. The other second stage buffer, which
uses transistors Q4201 and Q4202, furnishes feedback to the
programmable divider network.
D. PROGRAMMABLE DIVIDER NETWORK
The divider network consists of a tlO/tll prescaler and chained
dividers U4201, U4203, U4204, U4205, U4207 and U4208, which are
programmed by the X.XXXX RF FREQUENCY thumbwheel switches. The
network divides the VCO output frequency by a number equal to
793000 minus the setting of the .XXXX thumbwheels (minus 10,000
when the X. thumbwheel setting is an odd integer). The output
of the divider network goes to the phase detector circuit.
E. PHASE DETECTOR CIRCUIT
Dual flip-flop U4212 compares the programmable divider network
output frequency with the 100 Hz reference signal from the
Digital Counter Module. NANO gate U4213D allows the Q outputs
of U4212 to reset the flip-flops. Diodes CR4204 and CR4205
allow the flip-flops to charge or discharge capacitor C4224
when a phase error exists. This provides a ripple DC to drive
op amp U4214B. U4214B and capacitor C4215 function as an inte­
grator, furnishing analog DC to the VCO circuit to steer the
VCO frequency as required to correspond with the reference
signal. The� outputs of U4212 are applied to NANO gate U4213C.
When the two Q signals are not in phase, U4213C causes tran­
sistor Q4207 to conduct, grounding LED CR4207, causing it to
illuminate.
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