IFR FM/AM-500, Maintenance Manual

Page: 200 / 320

7-7 RECEIVER/GENERATOR
7-7-1 THEORY OF OPERATION (Refer to Figure 7-8 and Receiver/Generator
Circuit Schematic in Section 10)
A. GENERATE MODE
In the Generate mode, the Receiver/Generator utilizes a VCO
tuned to 21.4 MHz to provide the carrier for RF signal genera­
�ion and SSB reception. The output of the VCO is sampled by
the Digital Module, which returns an analog tuning voltage to
maintain the frequency at 21.4 MHz. When FM modulation is
required, an audio modulation voltage from the Generate Audio
Module is combined with the tuning voltage to produce the fre­
quency modulated VCO output signal. A flip-flop divides the
VCO output signa� in half to produce the IF 10.7 MHz AM carrier
or the FM signal. This signal passes through an attenuator
which, in the Generate AM mode, modulates the carrier with the
audio modulation voltage from the Generator Audio Module. The
signal is then sent to the third mixer in the IF Module. The
Generate Audio Module also supplies the GEN command signal
(+12 VDC) which enables the flip-flop and, through an inverter,
opens the receive enable circuit, preventing the generated
signal from entering the receiver circuit at full strength.
This attenuated signal is fed into the receiver circuit to allow
the Receive Audio Module to monitor and display the signal on
the Modulation and Frequency Error Meters.
B. RECEIVE MODE
In the Receive mode, the modulated 10.7 MHz signal from the IF
Module passes through the receive enable circuit, two AGC
amplifiers, and one of three bandpass filters. These filters
have a center frequency of 10.7 MHz with bandwidths of 200 kHz
(FM Wide and Mid), 15 kHz (FM Narrow and AM Norm) and 6 kHz (AM
Narrow and SSB). The signal is amplified again by a third AGC
amplifier, then demodulated, with the AM and FM audio going to
the Receive Audio Module and the 10.7 MHz carrier going to the
Digital Module. When in the Receive SSB mode, a flip-flop
divides the 21.4 MHz output of the VCO to 10.7 MHz and, through
a BFO level potentiometer, injects the beat frequency into the
modulated signal just ahead of the AM demodulator.
C. RECEIVED SIGNAL AMPLIFIER CIRCUITS
The 10.7 MHz IF signal from the IF Module first enters the
Receive Enable circuit. This circuit consists of diode CR4301,
resistor R4384 and capacitors C4336 and C4348. When forward
biased by transistor Q4307, diode CR4301 conducts, coupling the
RF signal through coupling transformer FL4301 to the first
amplifier circuit. When the mode power circuit in the Generate
Audio Module applies the GEN command, the +12 VDC command volt­
age is applied to transistor Q4307, which shuts off diode
7-37