Vertex Standard VX-4000V, Руководство по обслуживанию

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4-1
Circuit Description
Reception and transmission are switched by "RX" and
"TX" lines from the microprocessor unit (MPU).
Main Receiver Signal Path
The receiver uses double-conversion superheterodyne
circuitry, with a 43.95MHz 1st IF and 450 kHz 2nd IF. The
1st LO, produced by a PLL synthesizer, yields the
43.95MHz 1st IF.
The 2nd LO uses a 43.5 MHz (43.95 MHz-450 kHz) sig-
nal 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 demodulated 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 synthe-
sizer. Transceiver functions, such as TX/RX control, PLL
synthesizer 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
L1001, L1002, L1003 and L1006, capacitors C1001,C1006,
C1009, C1013, and C1023, and antenna switching diodes
D1006,D1007 and D1008 for delivery to the receiver front
end.
Signals within the frequency range of the transceiver
are then passed through a varactor-tuned bandpass filter
consisting of L1008, L1009 / L1024, L1025 before RF am-
plification by Q1012 (
3SK228
).
The amplified RF is then band-pass filtered again by
varactor-tuned resonators L1018, L1019 / L1038, L1039 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 L1042 / L1046
and C1132 / C1139 / C1142, to provide a pure 1st local
signal between 112.3 and 152.3 MHz to the 1st mixer.
The 43.95MHz 1st mixer product then passes through
dual monolithic crystal filters XF1001 and XF1002 (7.5 kHz
BW), and is amplified by Q1029 (
2SC4215Y
) and deliv-
ered to the input of the FM IF subsystem IC Q1026
(
TA31136FN
).
This IC contains the 2nd mixer, 2nd local oscillator,
limiter amplifier, FM detector, noise amplifier, and squelch
gates.
The 2nd LO in the IF-IC is produced from crystal X1001
(14.500MHz), and the 1st IF is converted to 450kHz by the
2nd mixer and stripped of unwanted components by ce-
ramic filter CF1001 or CF1002. After passing through a
limiter amplifier, the signal is demodulated by the FM
detector.
Demodulated receive audio from the IF-IC is ampli-
fied by Q1031 (
2SA1602A
) / Q2014 (
CXA1846N
). After
volume adjustment by the AF power amplifier Q2029
(
TDA7240AV
), the audio signal is passed to the optional
headphone jack or 4-ohm loudspeaker.
PLL synthesizer
The 1st LO maintains stability from the PLL synthesiz-
er by using a 14.500 MHz reference signal from crystal
X1001. PLL synthesizer IC Q1024 (
SA7025DK
) consists of
a prescaler, reference counter, swallow counter, program-
mable 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 reference sig-
nal by 725 using the reference counter (20.0 kHz compar-
ison frequency). The phase detector comparison frequen-
cy 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 deliv-
ered to the low-pass filter circuit, then fed back to the VCO
as a voltage with phase error, controlling and stabilizing
the oscillating frequency. This synthesizer also operates
as a modulator during transmit.
The RX-VCO is comprised of Q1015 (
2SK520
) and
D1017, D1018, D1035, D1036 (
HVU356x4
), and oscillates
between 177.950MHz and 217.950MHz according to the
programmed receiving frequency. And the TX-VCO is
comprised of Q1014 (
2SC5107
) and D1015, D1016, D1019
(
1SV276x3
), and oscillates between 134.000MHz and
174.000MHz according to the programmed transmit fre-
quency. The VCO output passes through buffer amplifier
Q1018 (
2SC5107
), and a portion is fed to the buffer am-
plifier Q1019 (
2SC5107
) of the PLL IC, and at the same
time amplified by Q1021 (
2SC5107
) to obtain stable out-
put. The VCO DC supply is regulated by Q1008
(
2SC4154E
). Synthesizer output is fed to the 1st mixer by
diode switch D1024 (
1SS321
) during receive, and to drive
amplifier Q1020/Q1022 (
2SC5415Ex2
) for transmit. The
reference oscillator feeds the PLL synthesizer, and is com-