Circuit Descripiton; Receiver Circuits - Icom IC-M23 Service Manual

SECTION 4.

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).
• RF CIRCUITS
D131 D130
To 1st mixer
BPF BPF
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).
• 1ST IF CIRCUITS
FI150, FI151
Q151
To IF IC
IF
BPF
AMP
1st IF signal
CIRCUIT DESCRIPTION
D52 D90
ANT
TX CIRCUITS
LPF
SW
D 93 D 92
Q90
RF
BPF BPF
AMP
T1CON
1st LO signals
from VCO
21.7MHz From RF AMP
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.
ANTENNA
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).
• 2ND IF, DEMODULATOR AND RX AF CIRCUITS
IC365 IC260D
LPF
BEEP DETMS
VOLIN
SP1
Int. SP
J250
SPO
Ext. SP
AF OUT
4 - 1
IC1
PLL IC
21.25 MHz
2ND LO signal
X1
I C170
21.25MHz
DET
AF IF IC
MUT E
X170
450KHz
R286, C285 IC190
D/A IC
LPF
De-emphasis
IC362
AF DE -
AM P
ENPHA
FI170
450KHz
BPF
From 1st IF AMP
RSSI
NOISE LEVEL