Receive Circuits - Icom IC-M91D Service Manual Addendum

SECTION 4.

4-1 RECEIVE CIRCUITS

RF CIRCUIT (RF UNIT)
The RX signal from the antenna is passed through a two-
staged LPF (L81, L82, C83–C86 and C89), ANT SW (D90),
and two tuned BPFs (D92, D93, L92, L93, C95, C97–C100
and C117), and then applied to the RF AMP (Q90).
The amplifi ed signal is applied to the 1st IF circuit, through
two tuned BPFs (D130, D131, L96, L97, C109, C110 and
C112–C115).
• RF CIRCUITS
From the TX AMP circuit
D131 D130
BPF
To the 1st IF circuit
1ST IF CIRCUIT (RF UNIT)
The RX signal from the RF circuits is applied to the 1st IF
mixer (Q150), and mixed with the 1st LO signal from the VCO
(Q21, Q22, and D20–D22), resulting in a 21.7 MHz 1st IF
signal.
The 1st IF signal is fi ltered by the 1st IF fi lters (FI150 and
FI151), and then applied to the 1st IF AMP (Q151). The am-
plifi ed signal is applied to the 2nd IF circuits.
• 1ST IF CIRCUITS
RF UNIT
Q151
To the IF IC
IF
AMP
CIRCUIT DESCRIPTION
RF UNIT
D52
ANT
SW
Q90
D92
D93
BPF BPF
BPF
RF
AMP
1st LO signal
from the VCO
FI150
FI151
21.7MHz
From RF AMP
BPF
Q150
2ND IF CIRCUIT (RF UNIT)
The 1st IF signal from the 1st IF circuits is applied to the IF IC
(IC170, pin 16). The IF IC contains the 2nd IF mixer, 2nd IF
AMP, detector, and so on, in its package.
The 1st IF signal is mixed with the 21.25 MHz 2nd LO signal
from the reference frequency oscillator (TCXO: X1), resulting
in a 450 kHz 2nd IF signal. The 2nd IF signal is passed through
the 2nd IF fi lter (FI170) to remove sideband noise. The fi ltered
signal is amplifi ed by the 2nd IF AMP, and demodulated by
the quadrature detector with discriminator (X170).
The demodulated AF signal is applied to the RX AF circuits
on the MAIN UNIT.
• 2ND IF CIRCUITS
LPF
To the RX AF circuit
RX AF CIRCUIT (MAIN UNIT)
The demodulated AF signal from the IF IC (RF UNIT: IC170)
is amplifi ed by the AF AMP (IC363A), and then applied to
the noise canceller IC (IC382, pin 36), through the AF SW
(IC380, pins 7, 1).
The processed AF signal is passed through the AF mute SW
(IC260D, pins 11, 10), LPF (IC363C, pins 9, 8) and de-em-
phasis circuit (R279 and C266), and then applied to the D/A
converter (IC190, pin 1) to adjust the level.
The level-adjusted AF signal is amplifi ed by the AF AMP
(IC362), and then output to the internal speaker (CHASSIS:
SP1).
When an external speaker is connected, the AF signal from
the D/A converter (IC190, pin 2) is amplifi ed by the AF AMP
(IC362), and then output to the external speaker, through
J290.
• RX AF CIRCUITS
MIC signal
SP1
SP
4 - 1
IC1
PLL IC
REF
FI170
X1
2ND LO
450KHz
21.25MHz
BPF
DET
IF IC
From the 1st IF circuit
IC170
X170
450KHz
IC380
IC363A
AF
From the demodulator circuit
AMP
SW
IC382
NOISE CANCEL IC
VOLIN
AF DE-
LPF
MUTE
ENPHA
IC363C
IC260D
VOL
AF
AMP
IC362
RF UNIT
MAIN UNIT
IC190
D/A