Vertex Standard VX-5500V, Service Manual

Page: 21 / 80

4-2
Circuit Description
Transmitter
Voice audio from the microphone is delivered via
the MIC (Jack) Unit to the MAIN Unit, after passing
through amplifier Q3039/Q2040 (
NJM2902V
), pre-
emphasis, limiter (IDC instantaneous deviation con-
trol), and LPF Q2001 (
NJM2902V
), is adjusted for
optimum deviation level and delivered to the next
stage.
Voice input from the microphone and CTCSS are
FM-modulated to the VCO of the synthesizer, while
DCS audio is modulated by the reference frequency
oscillator of the synthesizer.
Synthesizer output, after passing through diode
switch D1025 (
1SS321
), is amplified by driver Q1031
(
2SC3356
) / Q1025 (
2SC3357
) and power module
Q1017 (
RA60H1317M
) to obtain full RF output. The
RF energy then passes through antenna switch
D2015 / D2016 and a low-pass filter circuit and fi-
nally to the antenna connector.
RF output power from the final amplifier is sam-
pled by CM coupler and is rectified by D1037, D1038
(
HSM198S x 2
). The resulting DC is fed through Au-
tomatic Power Controller Q1007 (
NJM2904V
) to
transmitter RF amplifier and thus the power out-
put.
Generation of spurious products by the transmit-
ter is minimized by the fundamental carrier frequen-
cy being equal to the final transmitting frequency,
modulated directly in the transmit VCO. Addition-
al harmonic suppression is provided by a low-pass
filter consisting of L1008 L1004, L1002, L1001 and
C1338, C1336, C1337, C1335, C1334, C1333, C1332
and C1326, resulting in more than 60 dB of harmon-
ic suppression prior to delivery to the RF energy to
the antenna.
DCS Demodulator
DCS signals are demodulated on the MAIN-
UNIT, and are applied to low-pass filter Q2040
(
NJM2902V
), as well as the limiter comparator
Q2003.
CTCSS encoder/decoder
The CTCSS code is generation and encoding by
MPU IC Q2025 (
MB90F583B
).
Demodulation and detection of the CTCSS tones
are carried out by IC Q2017 (
MX165C
).
MPU
Operation is controlled by 16-bit MPU IC Q2025
(
MB90F583B
). The system clock uses a 16.000 MHz
crystal for a time base. IC Q2035 (
RN5V35AA
) 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 mem-
ory channels and CTCSS data, DCS data, prescaler
dividing, and REF oscillator data (internal/external).