IFR FM/AM-500, Maintenance Manual

Page: 187 / 320

pulses are nulled, the signal is amplified, then sent to the
integrator circuit for the first VCO and to a gain control and
digital-to-analog circuit for the second VCO.
One amplifier in the integrator circuit varies the voltage con­
trolling the first VCO frequency. Two comparators limit this
voltage to preset low and high levels, restricting the VCO to
between 1275 MHz and 2330 MHz. A third comparator is enabled
when the RF FREQUENCY Thumbwheels are set at less than 200 MHz.
This ensures that the VCO frequency slews sufficiently to be
within the range of the frequency detector circuit when making
extreme frequency changes on the thumbwheels.
A quad-bilateral switch, controlled by the BCD signal from the
100 MHz RF FREQUENCY Thumbwheel, adjusts the gain of the second
VCO error signal from the phase detector. The AC components of
the error signal pass through the selected switch or switches
and corresponding paralleled resistors to an amplifier. A choke
smooths the amplifier output, which then goes to the second VCO
to effect a phase shift.
90-1088 MHz FROM
:{>--+
PHASE LOCK
DUAL-VCO MIXER
+4
TO GEN AUDIO MODULE
PRESCALER
INDICATOR
-
'
I
,,
f. '" -{>
THUMBWHEEL
PROGRAMMABLE
-
PHASE
INTEGRATOR
0-40V TO FIRST VCO _
SWITCHES
'
DIVIDER
-
DETECTOR
MHz TO 100MHz
-
......
PULSE
GENERATOR
I
'
�
V
�
500 KHz
F IRST PHASE SECOND PHASE
REFERENCE SIGNAL
SAMPLER SAMPLER
-
-
-
FAST TUNE
AMPLIFIER
0-40V TO SECOND
vco-
Figure 7-5 High Loop Synthesizer Block Diagram
7-24