Circuit Descripiton; Receiver Circuits - Icom IC-M400BB Service Manual

SECTION 4.
4-1 RECEIVE CIRCUITS
RF CIRCUIT (MAIN UNIT)
The RX signal from the antenna is passed through the LPF
(L21, L22, and C114–C116), ANT SW (D16), attenuator (D19,
R96) and BPF (L201, C201 and C202), and then applied to
the RF AMP (Q201).
The amplifi ed signal is divided and applied to the 1st IF cir-
cuits for MAIN CH or CH70, through another BPF (FI201: for
MAIN CH; L301–L303 and C302–C308: for CH70).
• RF CIRCUIT
FI201 Q201
To the 1st IF circuits
RF
BPF
AMP
1ST IF CIRCUIT FOR MAIN CHANNELS (MAIN UNIT)
The RX signal from the RF circuits is applied to the 1st IF
mixer (Q202), and mixed with the 1st LO signal from the VCO
(Q3, Q4 and D1–D3), resulting in a 21.7 MHz 1st IF signal.
The 1st IF signal is fi ltered by the 1st IF fi lter (FI202 and
FI203), and then applied to the 1st IF AMP (Q203). The am-
plifi ed signal is applied to the 2nd IF circuits.
• 1ST IF CIRCUIT (For main channels)
Q 203
FI202,FI203
To the 2nd IF circuits
(MAIN CH)
IF
AMP
21.7MHz
2ND IF CIRCUIT FOR MAIN CHANNELS (MAIN UNIT)
The 1st IF signal from the 1st IF circuits is applied to the IF IC
(IC201, 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: X101), result-
ing in a 450 kHz 2nd IF signal. The 2nd IF signal is passed
through the 2nd IF fi lter (FI204) to remove sideband noise.
The fi ltered signal is amplifi ed by the 2nd IF AMP, and de-
modulated by the quadrature detector with discriminator
(X201).
The demodulated AF signal is applied to the RX AF circuits,
through the AF SW (IC504B, pins 4, 3).
• 2ND IF CIRCUIT (For main channels)
IC504B
IC506B/C
To the RX
MAINDET
AF circuits
AF
BPF
SW
IC201
CIRCUIT DESCRIPTION
D14,D16
D18
D19
ANT
LPF
BPF ATT
SW
Transmit circuits
From the RF circuits
Q202
(MAIN CH)
XTAL
BP F
1st IF mixer
1st LO signal
FI204
CERAMIC
X201
BPF
From the
1st IF circuits
(MAIN CH)
2N D
FM 2ND IF
MIX
DET AMP
2ND LO
BUFF
21.25 MHz
1ST IF CIRCUIT FOR CH70 (MAIN UNIT)
The RX signal from the RF circuits is applied to the 1st IF
mixer (Q302), and mixed with the 125.649 MHz 1st LO signal
which is generated by the TCXO (X401) and tripled by the
BPF (L402, L403 and C412–C416), resulting in a 30.875 MHz
1st IF signal.
The 1st IF signal is fi ltered by the 1st IF fi lters (FI302 and
FI303), and then applied to the 1st IF AMP (Q303). The am-
plifi ed signal is applied to the 2nd IF circuits.
• 1ST IF CIRCUIT (For CH70)
ANT
To the 2nd IF circuits
(CH70)
2ND IF CIRCUIT FOR CH70 (MAIN and LOGIC UNITS)
The 1st IF signal from the 1st IF circuits is applied to the IF IC
(IC301, 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 30.425 MHz 2nd LO signal
which is generated by 2nd LO oscillator (X302), resulting in
a 450 kHz 2nd IF signal. The 2nd IF signal is passed through
the 2nd IF fi lter (FI304) to remove sideband noise. The fi ltered
signal is amplifi ed by the 2nd IF AMP, and demodulated by
the quadrature detector with discriminator (X301).
The demodulated AF (FSK; Frequency Shift Keying) signal is
applied to the DSC decoder (LOGIC UNIT: IC11, pin 2) which
converts the sub audible tone signal into serial data, through
the LPF (Q702) and DSC SW (LOGIC UNIT: IC3, pins 7, 1).
The serial data is applied to the CPU (LOGIC UNIT: IC1,
pin 103) to control the transceiver (emergency alarm, DSC
indication and so on.).
• 2ND IF CIRCUIT (For CH70)
IC1 IC11 IC3
DSC
DSC
CPU
DECODER
SW
LOGIC UNIT
4-1
Q303 FI302,FI303
From the 1st IF circuits
Q302
(CH70)
IF XTAL
AMP BPF
1st IF mixer
30.875MHz
1st LO signal
FI304
X301
CERAMIC
BPF
Q702
CH70DET
FM 2ND IF 2N D
LPF
DET AMP MIX
MAIN
2ND LO
BUFF
UNIT
IC301
From the
1st IF circuits
(CH70)
X302
30.425MHz