Icom IC-M25, Руководство по обслуживанию

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5-1
SECTION 5 CIRCUIT DESCRIPTION
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).
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
the D/A converter (IC190), and then applied to the noise
amplifier (IC170, R174–R176, C179 and 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,
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
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).
• RF CIRCUITS
• 2ND IF, DEMODULATOR AND RX AF CIRCUITS
• 1ST IF CIRCUITS
LPF
ANT
SW
ANTENNA
BPF
RF
AMP
BPF BPF
IF
AMP
X170
450KHz
BPF
LPF
LPF
AF
AM P
X1
21.25MHz
BPF BPF
AF
MUT E
21.7MHz
1st IF signal
FI150,
Q151
To IF IC From 1st IF AMP
To 1st mixer From RF AMP
1st LO signals
from VCO
D131 D
TX CIRCUITS
92
D52
Q90
IC1
PLL IC
DE -
ENPHA
IF IC
D/A IC
SPO
AF OUT
J250
Q150
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
FI151 D 93
D90
D130
T1CON
IC260D
DETMS
SP1
LPF
ANT
SW
ANTENNA
BPF
RF
AMP
BPF BPF
IF
AMP
X170
450KHz
BPF
LPF
LPF
AF
AM P
X1
21.25MHz
BPF BPF
AF
MUT E
21.7MHz
1st IF signal
FI150,
Q151
To IF IC From 1st IF AMP
To 1st mixer From RF AMP
1st LO signals
from VCO
D131 D
TX CIRCUITS
92
D52
Q90
IC1
PLL IC
DE -
ENPHA
IF IC
D/A IC
SPO
AF OUT
J250
Q150
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
FI151 D 93
D90
D130
T1CON
IC260D
DETMS
SP1
LPF
ANT
SW
ANTENNA
BPF
RF
AMP
BPF BPF
IF
AMP
X170
450KHz
BPF
LPF
LPF
AF
AM P
X1
21.25MHz
BPF BPF
AF
MUT E
21.7MHz
1st IF signal
FI150,
Q151
To IF IC From 1st IF AMP To 1st mixer From RF AMP
1st LO signals
from VCO
D131 D
TX CIRCUITS
92
D52
Q90
IC1
PLL IC
DE -
ENPHA
IF IC
D/A IC
SPO
AF OUT
J250
Q150
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
FI151 D 93
D90
D130
T1CON
IC260D
DETMS
SP1
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).