Receiver Circuits - Icom IC-M25 Service Manual

SECTION 5

5-1 RECEIVER CIRCUITS

RF CIRCUITS
The RF circuits consist of RF filters, antenna switch, RF
amplifier (RF AMP) and so on. The received signal is extract
ed and amplified in the RF circuits.
• ANTENNA SWITCHING CIRCUIT
The RF signal from the antenna is passed through the LPF
(L81, L82, C83–C86 and C89), antenna SW (D52, D90, L90,
C90 and C91) and 2-staged tuned BPF (D92, D93, L92,
L93, C95, C97–C100 and C117), and then applied to the RF
AMP (Q90).
• RF AMPLIFIER
The filtered RX signal is amplified by the RF AMP (Q90)
and passed through another 2-staged BPF (D130, D131,
L96, L97, C109, C110 and C112–L115) to remove unwant-
ed signals. The filtered signal is applied to the 1st mixer
(Q150).
• RF CIRCUITS
D131 D130
From RF AMP To 1st mixer
BPF
1ST IF CIRCUITS
The RX signal is mixed with the 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,
Q151
To IF IC
IF
AMP
CIRCUIT DESCRIPTION
D52 D90
ANT
TX CIRCUITS
SW
D 93 D 92
Q90
RF
BPF BPF BPF
AMP
1st LO signals
from VCO
FI151
21.7MHz From RF AMP
BPF
1st IF signal
Q150
2ND IF AND DEMODULATOR CIRCUITS
IC170 is an IF 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 9) is applied to the IF IC (IC170, pin 2) as the
2nd LO signal, and it is mixed with 21.7 MHz 1st IF signal
from the 1st IF AMP (Q151), resulting in the 450 kHz 2nd IF
signal. The 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) which extracts the 450 kHz signal, and then applied
to the internal quadrature detector.
The demodulated AF signal is output from the pin 9.
The quadrature detector is a frequency demodulator which
uses a discriminator (X170) as a phase delayer.
SQUELCH CIRCUIT
A portion of AF signal from the IF IC is adjusted in level by
ANTENNA
the D/A converter (IC190), and then applied to the noise
amplifier (IC170, R174–R176, C179 and C180).
LPF
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,
which represents the noise level. The voltage is output from
the pin 14 of IC170, and then applied to the A/D port of the
T1CON
CPU (IC360, pin 33).
The CPU (IC360) compares the applied voltage and squelch
reference level to control the audio signal ON/OFF (emits/
mutes).
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/octave of frequency response, and then
applied to the D/A converter (IC190) to be adjusted in level.
The level-adjusted AF signal is amplified by the AF AMP (IC364)
and AF power amplifier (IC362), and then applied to the speak-
er (CHASSIS: SP1).
D131 D130
Q90
To 1st mixer
BPF BPF
• 2ND IF, DEMODULATOR AND RX AF CIRCUITS
IC365
LPF
BEEP
Int. SP
Ext. SP
5-1
D52 D90
ANT
TX CIRCUITS
LPF
SW
D 93 D 92
RF
BPF BPF
AMP
T1CON
IC1
PLL IC
21.25 MHz
2ND LO signal
X1
21.25MHz
IC260D
DET
AF
MUT E
DETMS
R286, C285 IC190
VOLIN D/A IC
LPF
SP1 De-emphasis
J250
IC362
SPO
AF
AF OUT DE -
AM P ENPHA
ANTENNA
FI170
450KHz
BPF
I C170
From 1st IF AMP
IF IC
X170
RSSI
NOISE LEVEL
450KHz
Q1
To IF IC