Vertex Standard VX-6000U, Serivce Manual

Page: 19 / 80

4-1
Circuit Description
Transceiver functions, such as PLL synthesizer
settings and channel programming, are controlled
via the microprocessor unit (MPU). Reception and
transmission are switched by "RX" and "TX" lines,
also controlled from the MPU.
Main Receiver Signal Path
The receiver uses double-conversion superhet-
erodyne circuitry, with a 43.95 MHz 1st IF and 450
kHz 2nd IF. The 1st LO, produced by a PLL synthe-
sizer, yields the 43.95 MHz 1st IF. The 2nd LO uses
a 43.5 MHz (43.95 MHz - 450 kHz) signal generated
by a crystal oscillator. The 2nd mixer and other cir-
cuits use a custom IC to convert and amplify the 2nd
IF, and detect FM to obtain demodulated signals.
Incoming RF signals from the antenna connector
are delivered to the PA Unit, and pass through a
low-pass filter (LPF) network consisting of coils
L6003, L6004, and L6005, capacitors C6023, C6024,
and C6025, and antenna switching relay RL6001 for
delivery to the receiver front end in the MAIN Unit.
Signals within the frequency range of the trans-
ceiver are then passed through a varactor-tuned
band-pass filter consisting of L1006/L1009 before RF
amplification by Q1018 (
2SC3356
).
The amplified RF is then band-pass filtered again
by varactor-tuned resonators L1016/L1020 before
further RF amplification by Q1034 (
2SC4227
). The
amplified RF is then delivered to the 1st mixer D1023
(
GN2011
).
Buffered output from the VCO Unit is amplified
by Q1031 (
2SC4227
) and low-pass filtered by L1030/
L1004 and C1214/C1215/C1217 to provide a pure 1st
local signal between 406.05 and 436.05 MHz for in-
put to the 1st mixer.
The 43.95MHz 1st mixer product then passes
through dual monolithic crystal filters XF1001 and
XF1002 (±7.5 kHz BW) for wide band or XF1003 and
XF1004 (±3.75 kHz BW) for narrow band, and is am-
plified by Q1041 (
2SC4227
) and delivered to the
input of the FM IF subsystem IC Q1036
(
TA31136FN
). This IC contains the 2nd mixer, 2nd
local oscillator, limiter amplifier, FM detector, noise
amplifier, and squelch gate.
The 2nd LO in the IF-IC is produced from crystal
X1001 (14.500MHz), 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.
After passing through a limiter amplifier, the signal
is demodulated by the FM detector.
Demodulated receive audio from the IF-IC is
amplified by Q2018 (
CXA1846N
). After volume ad-
justment by the AF power amplifier Q2019
(
TDA7240AV
), the audio signal is passed to the
speaker jack or the internal 4-Ohm loudspeaker.
PLL Synthesizer
The 1st LO, a PLL synthesizer, maintains stabili-
ty using a 14.500 MHz reference signal from TCXO
X1001. PLL synthesizer IC Q1033 (
SA7025DK
) con-
sists 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 reference signal by 725 us-
ing the reference counter (20.0 kHz comparison fre-
quency). The phase detector comparison frequency
is designed to be eight times the channel spacing
(2.5 kHz). The VCO output is divided by the pres-
caler, swallow counter and programmable counter.
These two signals are compared by the phase com-
parator and sent to the charge pump. A voltage pro-
portional 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 stabi-
lizing the oscillating frequency. This synthesizer also
operates as a modulator during transmit.
The RX-VCO is composed of Q1015 (
2SK508
) and
D1008/D1009 (
1SV282
X2), and oscillates be-tween
406.050 MHz and 436.050 MHz according to the pro-
grammed receiving frequency. And the TX-VCO is
comprised of Q1019 (
2SC4226
) and D1013, D1015
(
1SV276
X2), D1014 (
1SV229
), and oscillates be-
tween 450.000MHz and 480.000MHz according to
the programmed transmit frequency. The VCO out-
put passes through buffer amplifier Q1025
(
2SC5107
), and a portion is fed to the buffer ampli-
fier Q1026 (
2SC5107
) of the PLL IC, and at the same
time amplified by Q1031 (
2SC4227
) 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 D1022 (
1SS321
) during receive,
and to Tx drive amplifier Q1035 (
2SC5107
) for trans-
mit.