IFR FM/AM-500, Maintenance Manual

Page: 220 / 320

A DC signal from the Power Detector Circuit in the Output Ampli­
fier Module, which is representative of the signal level enter­
ing the T/R jack, passes through the Power Monitor Circuit.
The Power Monitor Circuit sends two samples of the power detec­
tor signal, one of which is amplified for the low power level,
to the function select multiplexer U3313 for Power Meter read­
ings. Another sample from the Power Monitor Circuit goes to
the Generate Audio Module for automatic change over when trans­
mitter power is present at the T/R jack.
B. INCOMING SIGNAL SELECTION
Multiplexer U3301A selects between FM and AM audio signals
according to the position of the Modulation Mode Selector Switch
on the Front Panel. The squelch gate, multiplexer U3322B, con­
ducts when the signal level received by squelch amplifier
U3302B exceeds the level established by the SQUELCH Control on
the Front Panel.
C. LOWPASS FILTERS
The three lowpass filters are identical circuits except for
resistor and capacitor values. Each filter utilizes a dual op
amp (U3319, U3320 and U3321) configured in two identical serial
stages. The circuit with U3319 forms the 80 kHz filter. The
one with U3320 makes the 8 kHz filter, while the 250 Hz filter
uses U3321. Multiplexer U3322A selects the 250 Hz filter when
a frequency of less than 400 Hz is selected on the optional
Variable Tone Generator for audio frequency counting. If the
Modulation Mode Selector Switch is in the "FM WIDE" position,
multiplexer U3322C directs the signal from the 80 kHz filter to
the Speaker and Modulation Meter circuits. When the control
voltage to either or both of these multiplexers is low, the
audio signal through the 8 kHz filter is fed into the corres­
ponding circuits.
D. RECEIVE AGC CIRCUIT
The signal from the AM demodulator (in the Receiver/Generato�
Module), which represents the detected level of the received
10.7 MHz IF signal, is amplified, ·then used for feedback to the
Receiver AGC circuit in the Receiver/Generator. Multiplexer
U3301C changes the speed of integrator U3302A according to the
position of the Modulation Mode Selector Switch. Also, when
SSB is selected, multiplexer U3301B switches the integrator
from average operation to peak operation by allowing fast
charging of the integrator through resistor R3306 and diode
CR3301, and slow discharging throu9h resistor R3301. One out­
put from integrator U3302A is to the Receiver/Generator AGC
circuit. The other output is rectified by diode CR3302 and
amplified by op amp U3303A. One output of U3303A is coupled to
the Modulation Meter through the meter function select multi­
plexer U3313 to provide an indication of relative signal
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