Vertex Standard VX-2000U, Service Manual

Page: 15 / 65

15
Circuit Description
Receive Signal Path
Incoming RF from the antenna jack is delivered
to the M
AIN
Unit, and passes through a low-pass
filter consisting of coils L1001, L1002, L1004, and
L1006, capacitors C1002, C1006, C1009, C1017,
C1019, C1022, C1041, and C1046, and antenna
switching diodes D1002 (
UM9957F
) and D1005
(
RLS135
), then passed to the receiver front end.
Signals within the frequency range of the trans-
ceiver then enter a varactor-tuned band-pass fil-
ter consisting of coils L1008, L1010, L1011, L1013,
L1014, L1015, L1016, L1017, L1018, L1019, L1020,
L1023, and L1024, capacitors C1045, C1052, C1054,
C1056, C1058, C1068, C1072, C1073, C1086, C1090,
C1091, C1093, C1094, C1095, C1097, C1100, C1101,
C1105, C1106, C1110, and C1111, plus diodes
D1007, D1008, D1011, D1012, D1013, and D1014
(all
HVU350
). After bandpass filtering, the in-
band RF signals are amplified by RF preamplifi-
er Q1012 (
2SC4227
).
Buffered output from the VCO is amplified by
Q1038 (
2SC5107
) to provide a pure 1st local sig-
nal between 406.05 and 436.05 MHz for injection
to the 1st mixer Q1021 (
SGM2016
). The result-
ing 43.95 MHz 1st IF then passes through mono-
lithic crystal filter XF1001, which strips away all
but the desired signal, and the signal is then
amplified by Q1026 (
2SC4215Y
). The amplified
1st IF signal is then applied to FM IF subsystem
IC Q1029 (
BA4116FV
), which contains the 2nd
mixer, 2nd local oscillator, a limiter amplifier, and
a noise amplifier.
A 2nd local signal is generated by PLL refer-
ence/2nd local oscillator Q1046 (
2SC4116GR
),
using the 14.5 MHz crystal X1002 in a 3rd-over-
tone mode as a reference. This signal is mixed
with the 43.95 MHz IF at Q1029 to produce the
450 kHz 2nd IF when mixed with the 1st IF sig-
nal within Q1029. The 2nd IF then passes through
the ceramic filter CF1001 (
CFWM450G
) or
CF1002 (
CFWM450E
) to strip away unwanted
mixer products, and then is applied to the limit-
er amplifier in Q1029, which removes amplitude
variations in the 450 kHz IF. Speech detection by
t h e c e r a m i c d i s c r i m i n a t o r C D 1 0 0 1
(
CDBM450C24T
) is then performed, converting
the second IF into an audio signal.
Detected audio from Q1029 is amplified by
Q1017-1 (
NJM2904V
) and then applied to the de-
emphasis network, consisting of capacitors
C1084/C1089, resistors R1038/R1049, and Q1007-
4 (
NJM2902V
). The de-emphasized audio sig-
nal is then applied to CTCSS subsystem IC Q1009
(
MX165BDW
), which contains the T
X
/R
X
audio
filter, CTCSS decoder, and CTCSS encoder; if a
CTCSS tone is present on the incoming signal, it
is removed by the high-pass filter in Q1009. The
processed signal passes through the audio mute
gate Q1008 (
DTC124EK
) and the volume con-
trol, then enters the audio power amplifier Q1003
(
TDA2003H
), which provides up to 2 Watts to
the external speaker jack or internal speaker.
Squelch Control
The squelch circuitry consists of a noise amplifi-
er, band-pass filter, and noise detector within Q1029,
plus control circuitry within Q1039 (
MB89677
).
When no carrier is received, noise at the out-
put of the detector stage in Q1029 is amplified,
band-pass filtered and detected by Q1029. The
resulting DC squelch control voltage is passed
to pin 33 of microprocessor Q1039. When no car-
rier is being received, pin 33 of Q1039 remains
low, signaling pin 5 of Q1039 to keep the green
"B
USY
" LED off, and simultaneously signaling pin
19 of Q1039 to command AF mute gate Q1008
(
DTC124EK
) to block receiver audio.