Vertex Standard VX-4000L, Service Manual, Page: 20 / 80

4-1
VX-4000 VHF Low-band Service Manual
Circuit Description
Reception and transmission are switched by “RX”
and “TX” lines from the microprocessor unit (MPU).
Signal Path Overview
The receiver uses double-conversion superhet-
erodyne circuitry, with a 17.7 MHz 1st IF and 450
kHz 2nd IF. The 1st LO, produced by a PLL synthe-
sizer, yields the 17.7 MHz 1st IF.
The 2nd LO uses a 17.25 MHz (17.7 MHz-450
kHz) signal generated by a crystal oscillator. The 2nd
mixer and other circuits use a custom IC to convert
and amplify the 2nd IF, and detect FM to obtain de-
modulated signals. During transmit, the PLL syn-
thesizer oscillates at the desired frequency directly,
for amplification to obtain RF power output. Dur-
ing transmit, voice modulation and CTCSS (or DCS)
modulation are applied to this synthesizer. Trans-
ceiver functions, such as Tx/Rx control, PLL synthe-
sizer settings, and channel programming, are con-
trolled using the MPU.
Receiver
Incoming RF signals from the antenna connector
are delivered to the MAIN Unit, and pass through a
low-pass filter (LPF) antenna switching network
consisting of coils L1002, L1003, L1006, and L1001,
capacitors C1006, C1009, C1013, C1023, and C1001,
and antenna switching diodes D1006, D1007, and
D1008 for delivery to the receiver front end.
Signals within the frequency range of the trans-
ceiver are then passed through a varactor-tuned
band-pass filter consisting of L1008/L1009 and
L1024/L1025 before RF amplification by Q1012
(
3SK240
).
The amplified RF is then band-pass filtered again
by varactor-tuned resonators L1018/L1019 and
L1038/L1039 to ensure pure in-band input to 1st
mixer Q1025 (
3SK240
).
Buffered output from the VCO Unit is amplified
by Q1021 (
2SC5415E
) and low-pass filtered by
L1042/L1046 and C1132/C1139/C1142/C1250, to pro-
vide a pure 1st local signal between 47.4 and 67.7
MHz to the 1st mixer.
The 17.7 MHz 1st mixer product then passes
through dual monolithic crystal filters XF1001 and
XF1002 (7.5 kHz BW), and is amplified by Q1029
(
2SC4215Y
) and delivered to the input of the FM IF
subsystem IC Q1026 (
TA31136FN
).
This IC contains the 2nd mixer, 2nd local oscilla-
tor, limiter amplifier, FM detector, noise amplifier,
and squelch gates.
The 2nd LO in the IF-IC is produced from crys-
tal X1001 (17.25 MHz), and the 1st IF is converted to
450 kHz by the 2nd mixer and stripped of unwant-
ed components by ceramic filter CF1002. After pass-
ing through a limiter amplifier, the signal is demod-
ulated by the FM detector.
Demodulated receive audio from the IF-IC is
amplified by Q2014 (
CXA1846N
). After volume ad-
justment by the AF power amplifier Q2029
(
TDA7240AV
), the audio signal is passed to the op-
tional headphone jack or 4-Ohm loudspeaker.
PLL Synthesizer
The 1st LO maintains stability from the PLL syn-
thesizer by using a 17.250 MHz reference signal from
crystal X1001. PLL synthesizer IC Q1024
(
SA7025DK
) consists of a prescaler, reference
counter, swallow counter, programmable counter,
a serial data input port to set these counters based
on the external data, a phase comparator, and charge
pump. The PLL-IC divides the 17.250 MHz refer-
ence signal by 690 using the reference counter (25.0
kHz comparison frequency). The phase detector
comparison frequency is configured to be eight times
the channel spacing (5 kHz). The VCO output is di-
vided by the prescaler, swallow counter, and pro-
grammable counter. These two signals are compared
by the phase comparator and sent to the input of
the charge pump. A voltage proportional to their
phase difference is delivered to the low-pass filter
circuit, then fed back to the VCO as a voltage with
phase error, controlling and stabilizing the oscillat-
ing frequency. This synthesizer also operates as a
modulator during transmit.
The RX-VCO is composed of Q1015 (
2SK508
)
and D1017/D1018 (
HVU306A
x 2), and oscillates be-
tween 47.4 MHz and 67.7 MHz according to the pro-
grammed receiving frequency. And the TX-VCO is
composed of Q1014 (
2SC4226-R24
) and D1015/
D1016/D1034/D1035 (
HVU300A
x 4), and oscillates
between 29.700 MHz and 50.000 MHz according to
the programmed transmit frequency. The VCO out-
put passes through buffer amplifier Q1018
(
2SC5107-O
), and a portion is fed to the buffer am-
plifier Q1019 (
2SC4215Y
) of the PLL IC, and at the
same time amplified by Q1021 (
2SC5415E
) to ob-
tain stable output. The VCO DC supply is regulated
by Q1008 (
2SC4154E
). Synthesizer output is fed to
the 1st mixer by diode switch D1024 (
1SS321
) dur-
ing receive, and to drive amplifier Q1020 (
2SK3074
)
for transmit. The reference oscillator feeds the PLL
synthesizer, and is composed of crystal X1001 (17.25
MHz), the temperature compensation circuit which
includes D1033 (
MC2850
), thermostats TH1003 and
TH1002, and transmit (DCS) modulation circuit
D1029 (
HVU306A
).