IFR FM/AM-500, Maintenance Manual

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counters only in the High Loop; the four right hand thumbwheels
(100 kHz, 10 kHz, 1 kHz, and 100 Hz) set counters only in the Low
Loop. Coding from the 1 MHz thumbwheel, though, is divided between
both these modules.
Three VCOs govern the RF frequency selection. The High Loop Synthe­
sizer controls the frequency of the first two VCOs, located in the
Dual VCO Module. The third VCO is part of the Low Loop Synthesizer.
The minimum frequency of the first VCO is 1300 MHz. The three left
hand thumbwheels (100 MHz, 10 MHz, and 1 MHz) increase the VCO fre­
quency in increments equal to the thumbwheel setting in excess of
000 MHz, except that the 1 MHz thumbwheel increases the frequency only
in even integer positions (O, 2, 4, 6, and 8). This produces a fre­
quency increase of up to 998 MHz, providing a VCO frequency of from
1300 MHz to 2298 MHz. The second VCO is preset to 1210 MHz, and is
not affected by the thumbwheel settings. The maximum frequency of the
third VCO is 79.3 MHz. As the four right hand thumbwheels (100 kHz,
10 kHz, 1 kHz, and 100 Hz) are rotated upward from 0000, the VCO fre­
quency is decreased in corresponding increments. Rotation of the 1 MHz
thumbwheel to any odd integer (1, 3, 5, 7, or 9) decreases the VCO
frequency 1 MHz. The combined effect of these five thumbwheels is to
reduce the third VCO frequency a maximum of 1.9999 MHz, or to a mini­
mum frequency of 77.3001 MHz. The difference frequency of all three
VCOs, then, is 10.7 MHz, or the IF frequency.
An RF signal received from the antenna or through the T/R jack, enters
the IF Module, where it passes through a 1000 MHz low pass filter. It
is then mixed with a selected frequency from the first VCO. A 1300 MHz
bandpass filter allows only a difference frequency of 1298-1300 MHz to
pass, thus rejecting all RF frequencies other than any within the 2 MHz
span which will produce the correct difference. This difference, or
IF, signal is mixed with the 1210 MHz from the second VCO and bandpass­
filtered again to pass the second IF of 88-90 MHz. The second IF is
mixed with the selected frequency from the third VCO to produce the
third IF of 10.7 MHz. In the Receiver/Generator, three 10.7 MHz fil­
ters (of 200 kHz, 15 kHz, and 6 kHz bandwidths) limit the 10.7 MHz IF
to be demodulated.
When generating RF signals, the modulated 10.7 MHz IF signal is fed
into the third mixer, through the IF Module in the reverse order of a
received signal, but goes to an Output Amplifier Module rather than to
the antenna.
2-4-3 AUDIO SYSTEM (Figure 2-5)
In the Receive mode, demodulated AM and FM audio from the Receiver/
Generator Module is bandwidth-filtered by the Receive Audio Module.
It is then furnished to the DEMOD jack on the Front Panel, and to the
Speaker through an audio amplifier in the Generate Audio Module. The
signal is also available to the Modulation Meter for monitoring modu­
lation. When the frequency of a received audio signal is being deter­
mined, the Digital Module compares a sample of the received signal with
a signal from the V�riable Tone Generator, using an up/down counter
2-6