Vertex Standard VX-900 - Manual (Page 17)

17
Circuit Description
Overview
The VX-900 is a UHF FM hand-held transceiver de-
signed to operate in the frequency range of 400 to
512 MHz.
Circuit Configuration by Frequency
The receiver is a double-conversion superhetero-
dyne with a first intermediate frequency (IF) of 44.25
MHz and a second IF of 450 kHz. Incoming signals
from the antenna are mixed with the local signal
from PLL to produce the first IF of 44.25 MHz.
This is then mixed with the 43.8 MHz second local
oscillator (using the 14.6 MHz reference crystal) out-
put to produce the 450 kHz second IF. This is subse-
quently detected to produce the demodulated signal.
The transmit signal frequency is directly generated
by the PLL VCO, and modulated by the signal from
the microphone. It is then amplified and sent to the
antenna.
Receive Signal Path
Front-end RF Amplifier
Incoming RF from the antenna jack is delivered to
the RF Unit and passes through a low-pass filter and
high-pass filter consisting of coils L1002, L1003,
L1005
, L1007, L1001, L1004, L1006, L1008, L1009
and L1011, capacitors C1002, C1005, C1009, C1011,
C1019
, C1028, C1003, C1012, C1015, C1018, C1022,
C1026
, C1030, C1035, C1036, C1039 and C1042, and
antenna switching diode D1016 (HVU131).
Signals within the frequency range of the transceiver
are then amplified by Q1012 (BIC702C) and enter a
varactor-tuned band-pass filter consisting of coils
L1014
, L1015, and L1017, capacitors C1081, C1083,
C1094
, C1097, C1105, C1106, C1115, C1117, C1121,
C1123
, C1134, C1139, and C1145, resisters R1079,
R1106
and R1124, and diodes D1027, D1028, D1030,
D1031
, D1032, and D1033 (all HVC350) before de-
livery of the RF signal to the first mixer.
First Mixer
Buffered output from the VCO is amplified by Q1029
(2SC5226-4/5) to provide a pure first local signal be-
tween 355.75 and 467 MHz for injection to the first
mixer D1037 (GN2011-Q). The 44.25 MHz first mixer
product then passes through monolithic crystal fil-
ters XF1001 and XF1002 (MDF0028, ±5.5 kHz BW)
to strip away all undesired mixer products.
IF Amplifier
The first IF signal is amplified by Q1044 (2SC5226-
4/5
).
The amplified first IF signal is applied to FM IF sub-
system IC Q1049 (TA31136FN), which contains the
second mixer, second local oscillator, limiter ampli-
fier, noise amplifier, and S-meter amplifier.
A second local signal is generated by Q1051
(2SC4617) using the 14.6 MHz crystal X1002 as a
reference, producing a 43.8 MHz signal; this which
yields a 450 kHz second IF when the reference sig-
nal is mixed with the first IF signal within Q1046.
The second IF then passes through the ceramic fil-
ter CF1001 (PBFC450R15D: wide channels),
CF1002
(PBFC450R9DR: narrow channels) to strip
away all but the desired signal, and is applied to the
limiter amplifier in Q1049, which removes ampli-
tude variations in the 450kHz IF, before detection of
the speech by the ceramic discriminator CD1001
(CDBC450CX24).
Audio Amplifier
Detected audio from Q1049 is applied to
Q1027
(AK2345) and the audio low-pass filter, and
then through the volume control (Q1021:M62364FP)
to the audio amplifier Q1007 (TDA2822D;external
speaker) or Q1011 (TDA2822D;internal speaker), pro-
viding up to 0.5 Watt to the optional headphone jack
or 16-ohm loudspeaker.
Attention: Audio output is BTL output. Both sides
of the audio output are above ground, and this line
must not be connected to a speaker line which uses
a grounded shield.
Squelch Control
The squelch circuitry consists of a noise amplifier
and band-pass filter within Q1049, and noise detec-
tor D1058/D1059 (DA221).
When no carrier received, noise at the output of the
detector stage in Q1049 is amplified and band-pass
filtered by the noise amplifier section of Q1049 and
the network between pins 7 and 8, and then recti-
fied by D1058.