4-2
VX-4000 VHF Low-band Service Manual
Circuit Description
Transmitter
Voice audio from the microphone is delivered via
the Mic (Jack) Unit to the MAIN Unit, after passing
through amplifier Q3039/Q2108 (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 syn-
thesizer, while DCS audio is modulated in the ref-
erence frequency oscillator of the synthesizer.
Synthesizer output, after passing through diode
switch D1024 (1SS321), is amplified by drivers
Q1020 (2SK3074)/Q2131 (2SC3133) and final tran-
sistor Q1013 (2SC3240) to obtain full RF output. The
RF energy then passes through antenna switch
D1007/D1008 and a low-pass filter circuit and final-
ly to the antenna connector.
RF output power from the final amplifier is sam-
pled by D1011 and D1014(HSM88AS x 2). The re-
sulting DC is fed through Automatic Power Con-
troller Q1007 (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. Addition-
al harmonic suppression is provided by a low-pass
filter consisting of L1012, L1003, L1002 and C1037,
C1033, C1023, C1013, C1009, and C1006, 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 Q2110
(NJM2902V), as well as the limiter comparator
Q2110.
CTCSS Encoder/Decoder
The CTCSS code is generated and encoded by
MPU IC Q2019 (MB90F583B).
Demodulation and detection of the CTCSS tones
are carried out by IC Q2013 (MX165C).
MPU
Operation is controlled by 16-bit MPU IC Q2019
(MB90F583B). The system clock uses a 16.000 MHz
crystal for a time base. IC Q2027 (S-80735SN) re-
sets 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).
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