Push-To-Talk (Ptt) Transmit Activation - Vertex Standard VX-2000V Service Manual

(2SB1201STP-FA), Q1040 (2SC4116GR), and
D1029 (02CZ5.6Y), with temperature compen-
sation provided by thermistors TH1003/TH1004
and capacitors associated with the 17.25 MHz
reference crystal, X1002.
In the receive mode, VCO Q1046 oscillates
between 130.3 and 156.3 MHz, according to the
transceiver version and the programmed receiv-
ing frequency. The VCO output is buffered by
Q1044, and applied to the prescaler section of
Q1050. There the VCO signal is divided by 64 or
65, according to a control signal from the data
latch section of Q1050, before being applied to
the programmable divider section of Q1050. The
data latch section of Q1050 also receives serial
dividing data from the microprocessor, Q1039,
which causes the pre-divided VCO signal to be
further divided in the programmable divider sec-
tion, depending on the desired receive frequen-
cy, so as to produce either a 5.0 kHz or 6.25 kHz
derivative of the current VCO frequency.
Meanwhile, the reference divider section of
Q1050 divides the 17.25 MHz crystal reference
frequency by 3450 (or 2760) to produce the 5 kHz
(or 6.25 kHz) loop reference (respectively). The
5 kHz (or 6.25 kHz) signal from the programma-
ble divider (derived from the VCO) and that de-
rived from the reference oscillator are applied to
the phase detector section of Q1050, which pro-
duces a pulsed output with pulse duration de-
pending on the phase difference between these
input signals. The pulse train is filtered to DC
and returned to varactors D1035/D1036/D1038/
D1039 (all 1SV276).
Changes in the level of the DC voltage applied
to the varactors affect the reactance in the tank
circuit of the VCO, changing the oscillating fre-
Circuit Description
quency of the VCO according to the phase dif-
ference 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 is then deliv-
ered to the first mixer via buffer amplifier Q1051.
For transmission, the VCO Q1046 oscillates
between 134 and 174 MHz, according to the mod-
el version and the programmed transmit frequen-
cy. The remainder of the PLL circuitry is shared
with the receiver section. However, the dividing
data from the microprocessor is such that the
VCO frequency is at the actual transmitting fre-
quency (rather than being offset by the IF, as in
the receiving case). Also, the VCO is modulated
by the speech audio applied to D1041 (1SV276),
as described previously.
Receive and transmit buses select which VCO
is made active via Q1048 (DTC124EU). FET
Q1043 (2SK880GR) buffers the VCV line for ap-
plication to the tracking bandpass filters in the
receiver front end.

Push-To-Talk (PTT) Transmit Activation

The PTT switch on the microphone is connect-
ed to pin 24 of microprocessor Q1039, such that
when the PTT switch is closed, pin 24 of Q1039
goes low . This signals the microprocessor's pin
35 to activate the T /R
X
which in turn disables the receiver by disconnect-
ing the 9 Volt supply bus at Q1006 (DTB123EK)
to the receiver front end, FM subsystem IC, and
receiver VCO circuitry. At the same time, Q1005
(DTB123YK) activates the T
to enable the transmitter.
controller Q1004 (IMH6),
X
9 Volt supply line
X
17