Vertex Standard VXA-710 SPIRIT, Service Manual

Page: 9 / 40

9
Circuit Description
Meanwhile, the reference divider section of Q1013 divides
the 17.475 MHz crystal reference from the reference oscil-
lator
Q2030 by 3495 to produce the 5 kHz loop reference.
The 5 kHz signal from the programmable divider (derived
from the VCO) and that derived from the reference oscil-
lator are applied to the phase detector section of
Q1013 ,
which produces a pulsed output with pulse duration de-
pending on the phase difference between the input sig-
nals. This pulse train is filtered to DC and returned to the
varactor
D1007 ( HVC350 ).
Changes in the level of the DC voltage applied to the var-
actors affect the reactance in the tank circuit of the VCO,
changing the oscillating frequency of the VCO according
to the phase difference between the signals derived from
the VCO and the crystal reference oscillator. The VCO is
thus phase-locked to the crystal reference oscillator.
The output of the VCO Q1014 is buffered by Q1017 be-
fore application to the 1st mixer, as described previously.
For transmission, the VCO Q1014 oscillates between 118
and 137 MHz. The remainder of the PLL circuitry is shared
with the receiver. However, the dividing data from the
microprocessor is such that the VCO frequency is at the
actual transmit frequency (rather than offset for IFs, as in
the receiving case).
Receive and transmit buses select which VCO is made
active by
Q1010 ( RT1N241M ). FET Q1019 ( 2SK880GR )
buffers the VCV line for application to the tracking band-
pass filters in the receiver front end.
When the power saving feature is active, the micropro-
cessor periodically signals to the PLL IC
Q1013 to con-
serve power, and to shorten its lock-up time.
Push-To-Talk Transmit Activation
The PTT switch on the microphone is fed through the PTT
controller,
Q2001 ( UMZ2N ), to pin 28 of microprocessor
Q3026 , so that when the PTT switch is closed, pin 25 of
Q3026 goes high. This sends the signal to cut off the re-
ceiver, by disabling the 3.5 V supply bus at Q1007
( DTA143EE ) which feeds the front-end, FM IF subsystem
IC Q2039 , and receiver VCO circuitry. At the same time,
Q1018 ( UMW1 ) and Q1020 ( DTA143EE ) activate the trans-
miter’s 3.5 V supply line to enable the transmitter.