Standard Horizon QUEST-X GX1500S Service Manual page 8

Circuit Description
Transmitter
Voice audio from the microphone is delivered to the
MAIN Unit. After passing through microphone amplifier
Q1002 (NJM12902V), a pre-emphasis network, limiter
(IDC: instantaneous deviation control), and low-pass fil-
ter network Q1052 (LM2902PW), the audio is adjusted
for optimum deviation level and delivered to the next
stage.
Voice or DSC (Digital Selective Calling) encode signal in-
puts from the low-pass filter network Q1052 (LM2902PW)
is by the TX & SUB VCO Q1025 (2SK520-K4) and varac-
tor diode D1010, D1011, D1012, & D1013 (all 1SV286B),
which oscillates the fundamental transmit frequency with
direct frequency-modulation using varactor diode D1009
(1SV325). The modulated signal passes through buffer
amplifier Q1024 and Q1020 (both 2SC5006), diode switch
D1007 (DAN235U) to drive amplifiers Q1014 (2SC5006)
and RF power amplifier module Q1005 (S-AV37).
The RF energy then passes through antenna switch D1002
(XB15A30B) and low-pass filter (LPF) consisting of coils
L1001 & L1002 and capacitors C1002, C1004, & C1007, and
finally to the antenna connector.
RF output power from the RF power amplifier module
Q1012 (S-AV37) is sampled by C1005 and C1006 and is
rectified by D1001 (1SS321). The resulting DC is fed
t h r o u g h A u t o m a t i c P o w e r C o n t r o l l e r s Q 1 0 0 2
(LM2904PW) to RF power amplifier module Q1012 (S-
AV37), thus providing positive control of the power out-
put.
Generation of spurious products by the transmitter is min-
imized by the fundamental carrier frequency being equal
to the final transmitting frequency, modulated directly in
the transmit VCO. Additional harmonic suppression is
provided by a low-pass filter consisting of coils and ca-
pacitors, resulting in more than 70 dB of harmonic sup-
pression prior to delivery of the RF energy to the antenna.
DSC Encoder/ Decoder
Encoder
The DCS (Digital Selective Calling) encode signal which
D / A c o n v e r t e d i n t h e 1 6 - b i t M P U I C Q 2 0 0 3
(M30624FGPGP) on the CNTL Unit is fed through the
low-pass filter Q1052 (LM2902PW) on the MAIN Unit to
the TX & SUB VCO Q1025 (2SK520-K41).
Decoder
Incoming the receiving DSC code on the CH70 from the
antenna connector are passed through a SUB-RX RF am-
plifier Q1013 (3SK294) to the bandpass filter consisting
of coils L1014, L1015, & L1016 and capacitors C1066,
C1068, C1099, C1100, C1101, C1102, C1103, & C1104, then
the filtered signal deliver to SUB-RX 1st mixer Q1023
(3SK131).
8
Buffered output from the TX & SUB VCO Q1025 (2SK520)
is amplified by Q1024 and Q1020 (both 2SC5006) and
bandpass filtered by coils L1021 & L1022 and capacitors
C1106, C1107, C1121, C1122, & C1123, to provide a pure
1st local signal (134.225 MHz) for delivery to the SUB-RX
1st mixer Q1023 (3SK131).
The 22.3 MHz 1st mixer product then passes through
monolithic crystal filters XF1001 and XF1002 (±6.5 kHz
BW) and buffer amplifier Q1028 (2SC4400-3), then deli-
vere to the SUB-RX FM IF subsystem IC Q1045
(TA31136FN). This IC contains the 2nd mixer, 2nd local
oscillator, limiter amplifier, FM detector, noise amplifier,
and squelch gates.
The 2nd local is produced by crystal X1002 (21.850 MHz)
then deliver to the 2nd mixer in the SUB-RX FM IF sub-
system IC Q1045 (TA31136FN). The 1st IF is converted to
450 kHz by the 2nd mixer and stripped of unwanted com-
ponents by ceramic filter CF1002.
Filtered signal from the ceramic filter CF1002 is applied
to the limiter amplifier in the SUB-RX FM IF subsystem
IC Q1045 (TA31136FN), and then demodulate by the FM
detector in the SUB-RX FM IF subsystem IC Q1045
(TA31136FN). Demodulated signal from the SUB-RX FM
IF subsystem IC Q1045 (TA31136FN) is passes through
the low-pass filter Q1044 (2SC4154E) to the DSC Decod-
er IC Q1045 (NJM2211M) which the receiving DCS code
is decoded. The decoded DCS signal delivered to the 16-
bit MPU IC Q2004 (M30624FGAGP).
The SUB-RX 1st Local signal maintains stability from the
PLL synthesizer by using a 29.950 MHz reference signal
from crystal X1001. PLL synthesizer IC Q1035
(TB31202FNG) consists of a prescaler, reference counter,
swallow counter, programmable counter, a serial data in-
put port to set these counters based on the external data, a
phase comparator, and a charge pump.
The PLL synthesizer IC divides the 29.950 MHz reference
signal by 2396 using the reference counter (12.5 kHz com-
parison frequency). The reference oscillator feeds to the
PLL synthesizer IC Q1035 (TB31202FNG). for the 2nd lo-
cal signal.
The VCO output is divided by the prescaler, swallow
counter and programmable counter. These two signals are
compared by the phase comparator, and applied 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.
The TX & SUB VCO is consisted of Q1025 (2SK520-K4)
and varactor diode D1010, D1011, D1012, and D1013 (all
1SV286B), which oscillates at 134.225 MHz. The TX & SUB