Vertex Standard VX-2000V, Service Manual, Page: 15 / 59

15
Circuit Description
Receive Signal Path
Incoming RF energy 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 switching diodes D1002 (
UM9957F
) and
D1006 (
RLS135
), then delivered to the receiver
front end.
The RF energy then enters a varactor-tuned
bandpass filter, consisting of coils L1008, L1010,
L1011, L1013, L1014, T1001, and T1002, capaci-
tors C1045, C1052, C1054, C1057, C1058, C1069,
C1072, C1073, C1091, C1092, C1093, C1094,
C1096, C1097, C1100, C1101, C1105, C1106,
C1109, and C1110, plus diodes D1007/1008 (both
KV1470
) and D1011~1014 (all
HVU350
). After
bandpass filtering, the in-band RF signals are
amplified by RF preamplifier Q1012 (
2SC4226
),
then delivered to the first mixer stage.
Buffered output from the VCO is amplified by
Q1038 (
2SC5107
), providing a pure local signal
between 130.3 and 156.3 MHz for injection to the
first mixer, Q1021 (
SGM2016
). The resulting 17.7
MHz first IF then passes through monolithic crys-
tal filter XF1001, which strips away all but the
desired signal, and the signal is then amplified
by Q1026 (
2SC4215Y
). The amplified first IF sig-
nal is then applied to FM subsytem IC Q1029
(
BA4116FV
), which contains the second mixer,
a limiter amplifier, and a noise amplifier.
The second local oscillator signal is generated
by PLL reference/2nd LO transistor Q1047
(
2SC4116GR
), using the 17.25 MHz oscillator
crystal X1002 as a reference. This signal is mixed
with the 17.7 MHz local at Q1029, producing a
450 kHz second IF. The second IF signal then
passes through ceramic filter CF1001
(
CFWM450G
) or CF1002 (
CFWM450E
) to strip
away unwanted mixer products, and then is ap-
plied to the limiter amplifier in Q1029, which re-
moves any amplitude variations in the 450 kHz IF.
Speech detection by CD1001 (
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 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 audio filter in Q1009.
The processed signal then passes through the
audio mute gate Q1008 (
DTC323TK
) and the
volume control, then enters the audio power
amplifier Q1003 (
TDA2003H
), which provides up
to 2 Watts of audio power to the external speak-
er jack.
Squelch Control
The squelch circuitry consists of a noise am-
plifier, bandpass filter, and noise detector with-
in Q1029, plus control circuitry within micropro-
cessor Q1039 (
MB89677
).
When no carrier is received, noise at the out-
put of the detector stage in Q1029 is amplified,
bandpass filtered, and detected by Q1029. The
resulting DC squelch control voltage is passed
to pin 33 of microprocessor Q1039. With no car-
rier being received, pin 33 remains low, signal-
ing pin 5 of Q1039 to keep the green "B
USY
" LED
off, and simultaneously signaling pin 19 of Q1039
to command audio mute gate Q1008 to block re-
ceived audio.