IFR FM/AM-500, Maintenance Manual

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circuit, which forward-biases the isolation diodes and allows
the filter to function.
E. AM DEMODULATOR CIRCUIT
When operating in the Receive AM or Receive SSB mode, the modu­
lated 10.7 MHz IF from the final amplifier coupler transformer
FL4305 is applied to the base of RF transistor Q4312, which
drives tuned filter FL4307. Diode CR4304 and capacitor C4327
form a high level AM detector. Resistor R4374, capacitor C4330
and diode CR4315 provide bias for the detector, which is buf­
fered by op amp U4303B. The signal is then filtered by induc­
tor L4305 and capacitor C4344 to remove any RF which may have
passed diode CR4304, then is fed to the Receiver Audio Module
as a signal varying only in voltage. When in SSB mode, a
10.7 MHz BFO, taken from the 21.4 MHz VCO, is added to the
received signal at the base of transistor Q4312.
F. FM DEMODULATOR CIRCUIT
In the Receive FM mode, the 10.7 MHz IF from the secondary wind­
ing of final amplifier coupler transformer FL4305 is applied to
RF transistor Q4313. This transistor and associated circuitry
form a limiter amplifier which strips off any AM and amplifies
the remaining carrier and FM slightly. Diodes CR4317 and CR4318
limit the output. U4302 is an IF amplifier with quadrature
detector. Choke L4320, resistor R4378 and capacitors C4341,
C4342 and C4380 form a tuned circuit for the detector. One
output of the detector is a hard-limited 10.7 MHz signal fed to
the Digital Module. The other output is a varying DC, which is
the demodulated FM audio signal. This is amplified by op amp
U4303A and filtered by inductor L4311 and capacitor C4343 to
remove any residual RF, then is fed to the Receiver Audio
Module.
G. 21.4 MHz VCO CIRCUIT
Field-effect transistor Q4305 and associated components comprise
the basic oscillator circuit. Zener diode CR4316 regulates the
supply voltage to +12 VDC, with decoupling from the power supply
accomplished by inductor L4317 and capacitor C4354. A tuned
circuit, consisting of diode CR4309, capacitors C4360, C4362
and C4374, and inductor L4312, determines the oscillator fre­
quency. An FM modulation voltage, supplied by the Generate
Audio Module, is combined with the tuning voltage through
varactor CR4309 to produce a frequency-modulated signal cen­
tered on 21.4 MHz. Transistor Q4306 amplifies the signal. The
signal is then split, with one branch going to the Digital
Module at 21.4 MHz. Another branch is divided to 10.7 MHz by
flip-flop U4301A to be used in the AM demodulator as a beat fre­
quency for SSB reception. A third branch is divided to 10.7 MHz
by flip-flop U4301B, and becomes the IF signal. This signal,
which contains the frequency modulation originating in the
Generate Audio Module, passes through the AM modulator circuit.
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