Vertex Standard VX-5500L, Service Manual

Page: 21 / 94

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, Q2041 (
NJM2902V
), a pre-
emphasis network, a limiter (IDC: instantaneous
deviation control) , and LPF Q2001 (
NJM2902V
); it
then is adjusted for optimum deviation level, and
delivered to the next stage.
Voice input from the microphone (and CTCSS, if
activated) are modulated in the VCO of the synthe-
sizer, while DCS audio is modulated in the reference
frequency oscillator of the synthesizer.
Synthesizer output, after passing through diode
switch D1022 (
1SS321
), is amplified by drivers
Q1004 (
2SK3074
), Q1002 (
RD16HHF1
) and final
transistor Q1001 (
2SC2879
: Type A,
SD1405
: Type
B) to obtain full RF output. The RF energy then pass-
es through antenna switch D1033, D1034 and a low-
pass filter circuit and finally to the antenna connec-
tor.
RF output power from the final amplifier is sam-
pled by D1036 and D1037 (
MA729
x 2). The result-
ing DC is fed through Automatic Power Controller
Q1038 (
NJM2902V
) to the transmitter RF amplifier,
allowing control of the power output.
Generation of spurious products in the transmit-
ter is minimized by the fundamental carrier frequen-
cy being equal to the final transmitting frequency,
modulated directly in the transmit VCO. Additional
harmonic suppression is provided by a low-pass fil-
ter consisting of L1017, L1011, L1012 and C1102,
C1088, C1034, C1030, C1029, and C1024, resulting in
more than 60dB of harmonic suppression prior to
delivery of 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
Q2110.
CTCSS Encoder/Decoder
The CTCSS code is generated and encoded by
MPU IC Q2025 (
MB90F583B
).
Demodulation and detection of the CTCSS tones
are carried out by IC Q2017 (
MX165CDW
).
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 (
RN5VL35AA
) re-
sets the MPU when the power is on, and monitors
the voltage of the regulated 5V power supply line.
EEPROM
The EEPROM Q2036 (
AT24C256-10TI2.7
) retains
TX and RX data for all memory channels and CTC-
SS data, DCS data, prescaler dividing, and REF os-
cillator data (internal/external).