4-2
VX-4000 UHF 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
), 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 D1024 (
1SS321
), is amplified by driver Q1020
(
2SC5107
) / Q1022 (
2SC5415E
) / Q1031 (
2SC2954
)
and power module Q1013 (
M68759
) to obtain full
RF output.
The RF energy then passes through antenna
switch D1007 / D1008 and a low-pass filter circuit
and finally to the antenna connector.
RF output power from the final amplifier is sam-
pled by CM coupler and is rectified by D1011,D1014
(
HSM88AS
x2). The resulting DC is fed through Au-
tomatic Power Controller Q1007 (
NJM2904V
), Q1001
(
2SC4154E
), Q1002 (
2SB1143S
) to transmitter RF
amplifier and thus the power output.
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 L1001, L1003, L1004 and C1004,
C1008, C1009, C1011 and C1014, resulting in more
than 60dB of harmonic suppression prior to deliv-
ery to 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 generation and encoding 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).