Section 4 Circuit Description; Receiver Circuits - Icom IC-M506 Service Manual

SECTION 4.
4-1 RECEIVE CIRCUITS
RF CIRCUITS (MAIN UNIT)
The RX signal from the antenna is passed through the LPF
(L22, L23 and C104–C106), ANT SW (D15), and then ap-
plied to the RF AMP (Q101).
The amplifi ed signal is divided and applied to the 1st IF cir-
cuits for MAIN CH or CH70, through the attenuator (D201,
R201 and C201: for MAIN CH) and BPF (FI201: for MAIN CH:
FI301: for CH70).
• RF CIRCUITS
D201
FI201
BPF
To the 1st IF circuits
ATT
1/3
FI301
BPF
To the 1st IF circuits
1ST IF CIRCUITS FOR MAIN CHANNELS (MAIN UNIT)
The RX signal from the RF circuits is applied to the 1st mixer
(D202), and mixed with the 1st LO signal, resulting in the
30.15 MHz 1st IF signal.
The 1st IF signal is amplifi ed by the 1st IF AMP (Q202, Q203),
fi ltered by the 1st IF fi lter (FI202 and FI203), and then applied
to the another IF AMP (Q205). The amplifi ed signal is applied
to the 2nd IF circuits.
• 1ST IF CIRCUITS (For main channels)
FI202
FI203
Q205
IF XTAL
To the 2nd IF circuits
(MAIN CH) AMP BPF
30.15MHz
2ND IF CIRCUITS 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 29.7 MHz 2nd LO signal
from the 2nd LO oscillator (X202), resulting in the 450 kHz
2nd IF signal. The 2nd IF signal is passed through the 2nd IF
fi lter (FI204) to remove the sideband noise. The fi ltered sig-
nal is amplifi ed by the 2nd IF AMP, and demodulated by the
quadrature detector with discriminator (X201).
The demodulated AF signal is applied to the RX AF circuits,
through the AF SW (IC882, pins 4, 3).
• 2ND IF CIRCUITS (For main channels)
X201
IC201
IC505D: IC882B:
De-
emphasys AF AF FM
MUTE MUTE
DET
NOISE
IC506B/C:
BPF
DET
To the RX
AF circuits
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CIRCUIT DESCRIPTION
MAIN UNIT
Q101
RF
AMP
ANT
D15
ANT
LPF
SW
Transmit circuits
Q202,
Q203
D202
IF From the RF circuits
(MAIN CH)
AMP
FI204
CERAMIC
BPF
From the
1st IF circuits
(MAIN CH)
2ND IF
AMP
29.700MHz
2ND LO
NOISE
AMP
BUFF
X202
1ST IF CIRCUITS FOR CH70 (MAIN UNIT)
The RX signal from the RF circuits is applied to the 1st IF
mixer (Q302), and mixed with the 134.825 MHz 1st LO signal,
which is generated by the VCO (Q3, Q4, D3, and D4), result-
ing in the 21.7 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 (Q305). The am-
plifi ed signal is applied to the 2nd IF circuits.
• 1ST IF CIRCUITS (For CH70)
Q305
To the 2nd IF circuits IF
(CH70)
AMP
2ND IF CIRCUITS FOR CH70 (MAIN UNIT)
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 21.25 MHz 2nd LO sig-
nal, which is generated by the reference frequency oscillator
(X101), resulting in the 450 kHz 2nd IF signal. The 2nd IF
signal is passed through the 2nd IF fi lter (FI304) to remove
the sideband noise. The fi ltered signal is amplifi ed by the 2nd
IF AMP, and demodulated by the quadrature detector with
discriminator (X301).
DSC DECODING CIRCUITS (MAIN UNIT)
The demodulated AF (FSK: Frequency Shift Keying) signal
is applied to the DSC decoder (LOGIC UNIT: IC12, pin 2),
which converts the sub audible tone signal into the serial data,
through the LPF (IC514, pins 10, 8) 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 CIRCUITS (For CH70)
IC506B/C IC505C:
AF
De-
BPF
MUTE
emphasys
IC506B/C:
BPF
To the RX
AF i it
4-1
FI302
FI303
Q302
From the 1st IF circuits
XTAL (CH70)
BPF
21.7MHz
FI304
X301
CERAMIC
BPF
IC301
From the
1st IF circuits
(CH70)
FM 2ND IF
DET AMP
NOISE
2ND LO
NOISE
DET
AMP BUFF