Transmit Inhibit - Vertex Standard VX-410 Series Service Manual

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Circuit Description
ode D1048 (HSC277TRF) to the buffer amplifier Q1024
(2SC5005), pre driver amplifier Q1020 (2SC3357),
driver amplifier Q1014 (RD01MUS1), and then the am-
plified transmit signal is applied to the final amplifier
Q1008 (RD07MVS1), which delivers up to 5 watts of
output power.
The transmit signal then passes through the antenna switch
D1004 (RLS135) and is low-pass filtered, to suppress har-
monic spurious radiation before delivery to the antenna.
Automatic Transmit Power Control
Current from the final amplifier is sampled by R1015,
R1038 and R1040, and is rectified by Q1003 (IMZ2A).
The resulting DC is fed back through Q1002 (UMX1) to
the drive amplifier Q1014 and final amplifier Q1008, for
control of the power output.
The microprocessor selects "High" or "Low" power levels.

Transmit Inhibit

When the transmit PLL is unlocked, pin 7 of PLL IC
Q1005 (MB15A01PFV) goes to a logic "Low." The re-
sulting DC unlock control voltage is passed to pin 10 of
the microprocessor Q1047. While the transmit PLL is un-
locked, pin 31 of Q1047 remains high, which then turns
off Q1012 (CPH6102) and the Automatic Power Con-
troller Q1002 (UMX1) to disable the supply voltage to
the pre-drive amplifier Q1024, drive amplifier Q1014/
Q1020 and final amplifier Q1008, thereby disabling the
transmitter.
Spurious Suppression
Generation of spurious products by the transmitter is
minimized by the fundamental carrier frequency being
equal to final transmitting frequency, modulated directly
in the transmit VCO. Additional harmonic suppression is
provided by a low-pass filter consisting of coils L1001,
L1002, and L1003 plus capacitors C1001, C1002, C1021,
C1022, C1023, C1024, C1025, C1026, and C1027, re-
sulting in more than 60 dB of harmonic suppression prior
to delivery of the RF signal to the antenna.
PLL Frequency Synthesizer
The PLL circuitry on the Main Unit consists of VCO
Q1030, Q1033 (both 2SK508-K52), VCO buffer Q1026
( 2 S C 5 0 0 5 ) , a n d P L L s u b s y s t e m I C Q 1 0 5 1
(MB15A01PFV1), which contains a reference divider,
serial-to-parallel data latch, programmable divider, phase
comparator and charge pump, and TCXO unit X1002
(TTS05VS) which yields frequency stability of ±2.5ppm
@ –22°F to +140°F (–30°C to +60°C).
While receiving, VCO Q1030 oscillates between
399.15 and 439.15 MHz according to the transceiver ver-
sion and the programmed receiving frequency. The VCO
output is buffered by Q1026, then applied to the prescaler
section of Q1051. There the VCO signal is divided by 64
or 65, according to a control signal from the data latch
section of Q1051, before being sent to the programmable
divider section of Q1051.
The data latch section of Q1051 also receives serial
dividing data from the microprocessor Q1047, which
causes the pre-divided VCO signal to be further divided
in the programmable divider section, depending upon the
desired receive frequency, so as to produce a 5.0 kHz or
6.25 kHz derivative of the current VCO frequency.
Meanwhile, the reference divider section of Q1051 di-
vides the 16.80 MHz crystal reference from the reference
oscillator Q1051, by 3360 (or 2688) to produce the 5.0
kHz (or 6.25 kHz) loop references (respectively).
The 5.0 kHz (or 6.25 kHz) signal from the program-
mable divider (derived from the VCO) and that derived
from the reference oscillator are applied to the phase de-
tector section of Q1051, which produces a pulsed output
with pulse duration depending on the phase difference be-
tween the input signals.
This pulse train is filtered to DC and returned to
varactors D1021, D1022, D1023, and D1024 (all
HVC355B). Changes in the level of the DC voltage are
applied to the varactors, affecting the reference in the tank
circuit of the VCO according to the phase difference be-
tween 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 Q1030 (2SK508), af-
ter buffering by Q1026, is applied to the first mixer as
described previously.
For transmission, the VCO Q1033 (2SK508) oscil-
lates between 450.00 and 490.00 MHz according to the
model version and programmed transmit frequency. 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). Also,
the VCO is modulated by the speech audio applied to
D1017 (HVC350B), as described previously.
Receive and transmit buses select which VCO is made
active, using Q1028, Q1031, Q1032 (all RT1N441U).
Miscellaneous Circuits
Push-To-Talk Transmit Activation
The PTT switch on the microphone is connected to pin
22 of microprocessor Q1047, so that when the PTT switch
is closed, pin 27 of Q1047 goes high. This signal disables
the receiver by disabling the 5V supply bus at Q1022
(DTB123EK) to the front-end, FM IF subsystem IC
Q1048 and the receiver VCO circuitry.
At the same time, Q1013 (UMX1N) and Q1012
(CPH6102) activate the transmit 5V supply line to en-
able the transmitter.
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