4 - 1
SECTION 4
CIRCUIT DESCRIPTION
4-1 RECEIVER CIRCUITS
RF CIRCUITS
The RF circuits consist of RF filters, antenna switch (ANT
SW), RF amplifi er (RF AMP), etc., and extract and amplify
the signal of frequency which is desired to receive.
• ANTENNA SWITCHING CIRCUIT
The RF signal from the antenna is passed through the LPF
(L81, L82, C83–C86, C89) and antenna SW (D52, D90,
L90, C90, C91), and then applied to the RF AMP (Q90)
through the 2-staged tuned BPF (D92, D93, L92, L93, C95,
C97–C100, C117).
• RF AMPLIFIER
The filtered RX signal is amplified by the RF AMP (Q90)
and passed through another 2-staged BPF (D130, D131,
L96, L97, L110, L112–L115) to remove unwanted signals.
The filtered signal is applied to the 1st mixer (Q150).
• 2ND IF AND DEMODULATOR CIRCUITS
IC170 is an IF demodulate IC which contains the 2nd local
oscillator, 2nd mixer, limiter and quadrature detector in its
package.
The 21.25 MHz reference frequency signal from the PLL IC
(IC1, pin 2) is applied to the IF demodulate IC (IC170, pin 2)
as the 2nd LO signal, and then mixed with 21.7 MHz 1st IF
signal from the 1st AMP (Q151). The resulting signal of 450
kHz 2nd IF signal is output from the pin 3 of IC170.
The 2nd IF signal is filtered by the external ceramic filter
(FI170) to extract 450 kHz signal only, and then applied
to the internal quadrature detector to be frequency-
demodulated. The demodulated AF signals are output from
the pin 9.
The quadrature detector is a frequency demodulator which
uses a discriminator (X170) as a phase delayer, and
provides demodulation without any adjustment.
SQUELCH CIRCUIT
A portion of AF signal from the IF demodulate IC is adjusted
in level by the D/A converter (IC190), and then applied to
the noise amplifer (IC170, R174–R176, C179, C180).
Only noise components (approximately 30 kHz signal) are
amplified by the noise amplifier, and then rectified by the
internal noise detector to be converted into DC voltage
corresponding to noise level; the squelch voltage. The
squelch voltage is output from the pin 14 of IC170, and then
applied to the A/D port of the CPU (IC360, pin 33).
The CPU (IC360) compares input voltage and preset
squelch level to control audio signals ON/OFF (emit/mute).
RX AF CIRCUITS
The demodulated AF signal from the IF demodulate IC (IC170,
pin 9) is passed through the AF mute SW (IC260D) and LPF
(IC365). The filtered AF signal is de-emphasized by R286 and
C285 to obtain –6 dB/oct. of frequency response, and then
applied to the D/A converter (IC190). The level-adjusted AF
signal is amplified by the AF power amplifier (IC362), and then
applied to the speaker (CHASSIS: SP1).
• RF CIRCUITS
• 2ND IF, DEMODULATOR AND RX AF CIRCUITS
• 1ST IF CIRCUITS
LPF
ANT
SW
ANTENNA
BPF
RF
AMP
BPF BPFBPF
To 1st mixer
D131
D
TX CIRCUITS
92
D52
Q90
D93
D90
D130
T1CON
BPF
IF
AMP
21.7MHz
1st IF signal
FI150,
Q151
To IF IC
From RF AMP
1st LO signals
from VCO
Q150
FI151
X170
450KHz
BPF
LPF
LPF
AF
AM P
X1
21.25MHz
AF
MUT E
From 1st IF AMP
IC1
PLL IC
DE -
ENPHA
IF IC
D/A IC
SPO
AF OUT
J250
IC170
FI170
450KHz
IC190
IC362
IC365
VOLIN
VOL
BEEP
De-emphasis
R286, C285
SQLO
SQLI
RSSI
NOISE LEVEL
21.25 MHz
2ND LO signal
DET
Int. SP
Ext. SP
IC260D
DETMS
SP1
1ST IF, 2ND IF AND DEMODULATOR CIRCUITS
• 1ST IF CIRCUITS
The RX signal is mixed with 1st LO signal at the 1st
mixer (Q150), resulting in the 21.7 MHz 1st IF signal. The
converted 1st IF signal is filtered by the crystal filters (FI150
and F151) to remove unwanted out-of-band signals.
The filtered 1st IF signal is amplified by the 1st IF AMP
(Q151), and then applied to the IF demodulate IC (IC170,
pin 16).
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