IFR FM/AM-500, Maintenance Manual

Page: 180 / 320

7-4 DUAL VCO
7-4-1 THEORY OF OPERATION (Refer to Figure 7-3 and Dual VCO Circuit
Schematic in Section 10)
A. GENERAL
The Dual VCO furnishes the first and second local oscillator
signals to the IF Module. The frequency of the first oscilla­
tor is controlled by the High Loop Synthesizer to between 1300
and 2298 MHz, depending upon the setting of the RF FREQUENCY
Thumbwheels. This provides the first mixer in the IF Module
with the frequency required for selection of the RF frequency.
The second oscillator frequency is fixed at 1210 MHz, and fur­
nishes the second mixer in the IF Module with the frequency
required to produce the second stage IF frequency.
The Dual VCO Module consists of two similar VCO circuits, a
1400 MHz lowpass filter, and a mixer. An 1120 MHz Lowpass
Filter is not a physical part of the module, but is required
to allow only the difference frequency from the mixer to reach
the High Loop.
The High Loop Synthesizer generates an error voltage which con­
trols the frequencies of the two VCOs. To do this, a sample of
each VCO frequency is mixed, then filtered by the 1120 MHz Low­
pass Filter to allow the High Loop Synthesizer to compare only
the difference frequency with the selected RF frequency. The
High Loop then generates an error voltage when this difference
frequency does not equal the selected RF frequency plus 90 MHz.
The DC and low frequency components of the error voltage
0-40
voe
FROM
HIGH LOOP SYNTHESIZER
-
.
D-40
voe
FROM
HIGH LOOP SYNTHESIZER
-
r{)
FIRST
vco
-
SECOND -
-
VCO
-
L{:::
1400 MHz
_1�1
I
r----...,
1300-2298 MHz TO FIRST IF MIXER
1120 MHz
90-1088 MHz TO
- I
r-,,,..___J
l
HIGH LOOP SYNTHESIZER
�7�1
/DUAL VCO MODULE/
1210 MHz TO SECOND IF MIXER
Figure 7-3 Dual VCO Block Diagram
7-17
-
-