Circuit Description - Vertex Standard VX-4000U Service Manual

Reception and transmission are switched by "RX"
and "TX" lines from the microprocessor unit (MPU).
Main Receiver Signal Path
The receiver uses double-conversion superhet-
erodyne circuitry, with a 43.95MHz 1st IF and 450
kHz 2nd IF. The 1st LO, produced by a PLL synthe-
sizer, yields the 43.95MHz 1st IF.
The 2nd LO uses a 43.5 MHz (43.95 MHz-450 kHz)
signal generated by a crystal oscillator. The 2nd mix-
er and other circuits use a custom IC to convert and
amplify the 2nd IF, and detect FM to obtain demod-
ulated signals.
During transmit, the PLL synthesizer oscillates
at the desired frequency directly, for amplification
to obtain RF power output. During transmit, voice
modulation and CTCSS (or DCS) modulation are
applied to this synthesizer. Transceiver functions,
such as TX/RX control, PLL synthesizer settings, and
channel programming, are controlled 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 L1001, L1002, L1003, L1004 and
L1005, capacitors C1004, C1008, C1009, C1011, and
C1014, 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
bandpass filter consisting of L1009, L10014 before
RF amplification by Q1012 (2SK4227).
The amplified RF is then band-pass filtered again
by varactor-tuned resonators L1022, L1026 to ensure
pure in-band input to 1st mixer Q1025 (2SK228).
Buffered output from the VCO Unit is amplified
by Q1021 (2SC5107) and low-pass filtered by L1030
/ L1031 and C1178 / C1180 / C1182, to provide a pure
1st local signal between 112.3 and 152.3 MHz to the
1st mixer.
The 43.95 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 crystal
X1001 (14.500 MHz), and the 1st IF is converted to
450 kHz by the 2nd mixer and stripped of unwant-
ed components by ceramic filter CF1001 or CF1002.
VX-4000 UHF Service Manual

Circuit Description

After passing through a limiter amplifier, the signal
is demodulated 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 14.500 MHz reference signal from
c r y s t a l X 1 0 0 1 . P L L s y n t h e s i z e r I C Q 1 0 2 4
(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 14.500 MHz refer-
ence signal by 725 using the reference counter (20.0
kHz comparison frequency). The phase detector
comparison frequency to be eight times the channel
spacing (2.5kHz). The VCO output is divided by the
prescaler, swallow counter and programmable
counter. These two signals are compared by the
phase comparator and input to the charge pump. A
voltage proportional to their phase difference is de-
livered to the low-pass filter circuit, then fed back
to the VCO as a voltage with phase error, control-
ling and stabilizing the oscillating frequency. This
synthesizer also operates as a modulator during
transmit.
The RX-VCO is comprised of Q1015 (2SK508)
and D1017, D1018 (1SV276x2), and oscillates be-
tween 356.050 MHz and 468.050 MHz according to
the programmed receiving frequency. And the TX-
VCO is comprised of Q1014 (2SC4226) and D1015,
D1016, D1019 (1SV276x2, 1SV230), and oscillates
between 400.000MHz and 512.000MHz according to
the programmed transmit frequency. The VCO out-
put passes through buffer amplifier Q1018
(2SC5107), and a portion is fed to the buffer ampli-
fier Q1019 (2SC5107) of the PLL IC, and at the same
time amplified by Q1021 (2SC5107) to obtain stable
output. The VCO DC supply is regulated by Q1008
(2SC4154E). Synthesizer output is fed to the 1st mix-
er by diode switch D1024(1SS321) during receive,
and to drive amplifier Q1020 (2SC5107) / Q1022
(2SC5415E) / Q1031 (2SC2954) for transmit. The
reference oscillator feeds the PLL synthesizer, and
is composed of crystal X1001 (14.500 MHz), the tem-
perature compensation circuit which includes D1033
(MC2850) and thermostats TH1003 and TH1002, and
transmit (DCS) modulation circuit D1029 (1SV230).
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