IFR FM/AM-500, Maintenance Manual

Page: 175 / 320

7-3 IF MODULE
7-3-1 THEORY OF OPERATION (Refer to Figure 7-2 and to IF Module
Circuit Schematic in Section 10)
A. GENERAL
An RF signal from the antenna first passes through a static
protector to an antenna enable relay. Anytime the set is oper­
ating in the Receive mode, the relay is energized by the Mode
Power Circuit in the Generate Audio Module and couples the sig­
nal to the IF network. When in the Generate mode, or if a
100 mW or greater signal is received at the T/R jack, the relay
is de-energized. The relay then switches the signal path
through the Output Amplifier to the T/R jack and, at the same
time, opens the antenna circuit.
From the antenna enable relay, the received RF signal passes
through a 1000 MHz lowpass filter to the first mixer. The
first mixer combines the received RF signal with the signal
(which is between 1300 and 2298 MHz) from the first VCO in the
Dual VCO Module. A 1300 MHz bandpass filter allows only a dif­
ference (first IF) frequency of 1298-1300 MHz, which contains
the modulation characteristics of the received RF signal, to
pass. The first IF signal is amplified, then mixed with the
1210 MHz, constant frequency from the second VCO in the Dual
VCO module. A second bandpass filter allows only the dif­
ference (second IF) signal of 88-90 MHz to pass. After further
amplification, the second IF is mixed with the signal (which is
between 79.3 and 77.3 MHz) from the third VCO in the Low Loop
Synthesizer Module. This third, or final IF, when the RF
thumbwheels are set to the actual RF frequency received, is the
10.7 MHz IF frequency used by the Receiver/Generator Module.
When the set is operating in the Generate mode, the IF module
functions basically in reverse of the Receive mode. However,
different interstage amplifiers are used. In the Receive mode,
the Generate Audio Module furnishes a REC command (+12 VDC) to
enable the receive amplifiers in the IF Module; in the Generate
Mode, a GEN command (+12 VDC) is applied to the generate ampli­
fiers.
B. STATIC PROTECT/ANTENNA ENABLE CIRCUIT
Resistors R2213 and R2214 and diodes CR2201 and CR2202 shunt to
ground any static charge developing in the antenna and its con­
netting coax. Diodes CR2203 through CR2206 form a diode switch
which, when forward biased by the REC command signal, conducts
the received frequency through antenna enable relay K2201 to
the 1000 MHz lowpass filter. Inductors L2202 and L2203, in the
diode switch control line, prevent the RF of a received signal
from entering the DC control circuitry. The relay, which is
energized by the ANT ENABLE command signal, -normally shunts any
7-11