Vertex VXR-1000 Service Manual page 28

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Circuit Description
sizer, and is composed of crystal X2001 (12.8
MHz), the temperature compensation circuit
which includes D2007 (1SS353) and thermostats
TH2001 and TH2002, and the transmit (DCS)
modulation circuit D2005/ D2006 (HVU350 × 2).
Transmitter
Voice audio from the microphone or external
inputs passes through a low-pass filter Q1005
(NJM2902M), and a limiter amplifier and low-
pass filter at Q1006 (NJM2902M), then is adjust-
ed for optimum deviation level and delivered
to the next stage.
Voice input from the microphone, or external
inputs and CTCSS, are frequency-modulated at
the VCO of the synthesizer, while DCS audio is
modulated by the reference frequency oscillator
of the synthesizer. Synthesizer output, after pass-
ing through diode switch D2010 (1SS184), is
amplified by driver Q2015 (µPC2710) and pow-
er module Q2003 (M68739M) to obtain full RF
output. The RF energy then passes through an-
tenna switch D2001/ D2002 and a low-pass filter
circuit and finally to the antenna connector.
RF output power from the final amplifier is
sampled by C2117 and C2118 and is rectified by
D2017 (1SS321). The resulting DC is fed through
Automatic Power Controller Q2018 (TA75S01F)
to transmitter power module Q2003.
Generation of spurious products by the trans-
mitter is minimized by the fundamental carrier
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frequency being equal to the final transmitting
frequency, modulated directly in the transmit
VCO. Additional harmonic suppression is pro-
vided by a low-pass filter consisting of L2001,
L2002, and L2024 and C2002, C2004, C2009,
C2111, C2112, C2113, and C2114, resulting in
more than 60 dB of harmonic suppression prior
to delivery to the RF energy to the antenna.
DCS Demodulator
DCS signals are demodulated on the RF-
UNIT, and are applied to low-pass filter in sec-
tions 3 and 4 of Q1040 (NJM2902M), as well as
the limiter comparator in section 1 of Q1040.
CTCSS encoder/decoder
Generation, demodulation, and detection of
the CTCSS tones are carried out by IC Q1014
(MX165C).
MPU
Operation is controlled by 8-bit MPU IC
Q1039 (HD64F3334YTF). The system clock uses
a 9.8304 MHz crystal for a time base. IC Q1015
(RH5VL45AA) resets the MPU when the power
is on, and monitors the voltage of the regulated
5V power supply line.
EEPROM
The EEPROM retains Tx and Rx data for all
16 channels, CTCSS data, DCS data, prescaler
dividing, IF frequency, local oscillator injection
side (upper/lower), and REF oscillator data (in-
ternal/external).
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