SECTION 4
CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUIT
S
4-1-1 RF CIRCUIT (MAIN-R UNIT)
The RF circuit amplifi es signals within the range of frequen-
cy coverage and fi lters out-of-band signals.
The received signals from the receive antenna connec-
tor (CASE; W6) are applied to the MAIN-R unit via J1
(CHASSIS-R). The applied signals are passed through
the low-pass filter which contains strip-line, L1162, L1193,
C1189, C1198, C9202 and C9203. The fi ltered signals are
through the high-pass fi lter (L1–L4, C2–C5) and then applied
to the RF amplifi er (Q1). The amplifi ed signals are passed
through the bandpass fi lter (FI1) and then applied to another
RF amplifi er (Q2). The amplifi ed signals are passed through
another bandpass fi lter (FI2) to suppress unwanted signals.
The fi ltered signals are applied to the 1st mixer circuit.
4-1-2 1ST MIXER AND 1ST IF CIRCUITS
(MAIN-R UNIT)
The 1st mixer circuit converts the received signals into
fi xed frequency of the 1st IF signal with the 1st LO signal.
By changing the 1st LO signal, only the desired frequency
passes through the bandpass fi lter at the next stage of the
1st mixer circuit.
The RF signals from the bandpass filter (FI2) are mixed
with the 1st LO signal, where come from the 1st VCO circuit
(Q471, Q472, D471), at the 1st mixer circuit (IC71) to pro-
duce a 243.95 MHz 1st IF signal. The 1st IF signal is passed
through the bandpass fi lter (FI71) to suppress unwanted sig-
nals and pass only the desired signals.
The fi ltered signal is applied to the 2nd IF circuit.
4-1-3 2ND MIXER AND 2ND IF CIRCUITS
(MAIN-R UNIT)
The 2nd mixer circuit converts the 1st IF signal into the 2nd
IF signal with the 2nd LO signal.
The fi ltered 1st IF signal from the bandpass fi lter (FI71) is
mixed with the 2nd LO signal (275.00 MHz), where come
from the 2nd VCO circuit (Q631, D630), at the 2nd mixer cir-
cuit (Q131) to produce a 31.05 MHz 2nd IF signal. The 2nd
IF signal is passed through the MCF (FI191). The filtered
signal is applied to the IF amplifi er (Q191) and then applied
to the 3rd mixer circuit in the demodulator IC (IC191).
4-1-4 DEMODULATOR CIRCUIT (MAIN-R UNIT)
The demodulator IC (IC191) contains the 3rd mixer, limiter
amplifi er, quadrature detector, active fi lter and noise ampli-
fi er, etc.
The amplified 2nd IF signal from the IF amplifier (Q191)
is applied to the 3rd mixer section of the demodulator IC
(IC191, pin 16) and is then mixed with the 3rd LO signal to
be converted into a 450 kHz 3rd IF signal. The 3rd IF signal
from the 3rd mixer section (IC191, pin 3) passes through the
ceramic fi lter (FI193) to remove unwanted heterodyned fre-
quencies. The fi ltered signal is amplifi ed at the limiter ampli-
fi er section (IC191, pin 5) and then applied to the quadrature
detector section (IC191, pins 10, 11) to demodulate the digi-
tal audio or low speed data signals.
The 3rd LO signal (30.6 MHz) is produced at the 1st PLL cir-
cuit by doubling it’s reference frequency (X400: 15.3 MHz) at
the doubler circuit (Q550).
E - 4 - 1
FI2
FI1
Q1
Q2
1240–1300 MHz
1st mixer
IC71
3rd LO: 30.6 MHz
LOGIC-R unit
1st LO:
996.05–1056.05 MHz
2nd IF: 31.05 MHz
Fl71
BPF
AMP
AMP
1st IF:
243.95 MHz
2nd mixer
Q131
2nd LO:
275.00 MHz
HPF
BPF
BPF
FI191
D131
IC191
Q191
AMP
BPF
Demodulator
IC
• RF AND IF CIRCUITS