the TX 5V line is activated , biasing Q2020
and Q2021 on, which then brings t/r
switch D2005 into conduction. Otherwise,
when the P T T switch is open, the RX 5V
line is activated via Q2022 ( 2 SA733AP ) .
When low power i s selected b y opening
High/Low power switch SW2003 , DC bias at
RF amplifiers Q2013 and Q20 14 is altered to
reduce their gain, with VR200 1 allowing
fine adj ustment of the bias of Q2013 to set
the low power level.
PLL
T he first local signal for the receiver (21.4
MHz b elow the channel frequency ) , and the
carrier for the transmitter (at the transmit
frequency ) are generated by VCO Q2005, as
already mentioned , under control of the
P L L circuit. The oscillating frequency is
determined primarily by the level of DC
voltage fed from the loop lowpass filter to
varactor diodes D200 1/2002. A sample of
the VCO oscillating frequency is taken at
T 2001 and b uffered by Q2007 (2SC2786L)
for
application
to
prescaler
Q2004
( M C 12017P ) , which divides the sampled VCO
frequency by 64 or 65 according to a
control signal fed back from the prescaler
control
logic section
of
programmable
divider
IC
Q2001 (TC9198P ) .
T he divided signal from the prescaler is
fed to the programmable divider where it
is further divided down to 5 kHz according
to data read from the diodes installed on
the Matrix U nit and selected via Q3001/3002
(both T C4503BF ) by the channel selector.
T his 5 kHz signal is applied to phase com
parator
Q2003
(TC5081AP ) .
Meanwhile,
reference oscillator/divider Q2002 (TC-
5082P ) generates a 5 kHz signal from 5.12
MHz crystal X2001, which signal is also
applied to Q2003.
Q2003 compares the phase of the 5 kHz
signal from the programmable divider with
that of the reference signal , and any
phase difference appears as a pulse train,
the pulse width of
.
which varies in accor
dance with the difference in phase between
the input signals. This pulse train is then
-
33
-
lowpass filtered by L2001 , L 200 2 , R2001 ,
R2002 , R2016 and C2004- 2006 t o D C , w hich
is applied to varactors D2001/2002, phase
locking the
vco
to the crystal reference.
When the VCO is unlocked, the voltage at
pin 4 of Q2003 drops, turning Q2023
( BN1L4M ) on, and Q2024 ( 2 S A 1 1 1 5 E ) and
Q2025 ( BA1L3Z) off, which biases the trans
mitter RF amplifier stages off to prevent
transmission.
