Icom IC-M412 Service Manual page 10

2ND IF AND DEMODULATOR CIRCUITS
(CH70-RX)
The 2nd IF circuits consist of 2nd mixer, 2nd IF filter and
2nd IF amplifier, convert the 1st IF signal into the 2nd IF
signal, then filter to fi lter 2nd IF signal only and amplify. And
the demodulator circuit converts the 2nd IF signal to AF
signals.
The amplified 1st IF signal is applied to the IF IC (IC14, pin
16), and converted into the 450 kHz 2nd IF signal at the
internal 2nd mixer, by being mixed with the 2nd LO signal
generated by the internal reference frequency oscillator
(X5). The converted 2nd IF signal is output from pin 3,
and filtered by the external 2nd IF filter (FI6). The filtered
2nd IF signal is backed to the IF IC (pin 5), and saturation-
amplified by the internal 2nd IF AMP. The amplified 2nd IF
signal is FM-demodulated by the quadrature detector with a
discriminator (X4).
The demodulated DSC signals are applied to the DSC
decoding circuits.
DSC DECODING CIRCUITS (CH70-RX only)
The demodulated DSC siganls from the IF IC (IC14) are
filtered by LPF to remove audible signals. The filtered sub
audible tone signals are applied to the DSC decoder.
The IC15 is the DSC decode IC which converts the sub
audible tone signals into the serial data (logic "H/L" status).
DSC DECODING CIRCUITS
Q65
LPF KTC4075
R571 R573 R574 R575
18K 10K 10K 10K
C571 C574
0.1 0.022
DSC decoder
IC15
1
R577
V+
C581
2
IN
3
10K 0.01 LOCK
4
GND
5
Q
6
Q
7
DATA
NJM2211M
• 2nd IF AND DEMODULATOR CIRCUITS (CH70-RX)
LTM450EW
FI6
I O
2nd IF filter
The decoder detects it then outputs serial data to the CPU
(LG; IC1, pin 41) to control the transceiver (emergency
alarm, DSC indication, etc.).
R583
47
C586
0.033
14
CAP
13
CAP
12
R
11
LOOP
10
REF
9
NC
8
FSK
R584
100K
4 - 4
R541
390
2nd LO OSC.
Discriminator
R542
CR-845
X5
1.5K
X4
C566
JTB450C24
22P
IC14
1 16
OSCI N MIXIN
2 15
OSCOUT GND
3
14
MIXOUT N-RE C
4 13
VCC N-DET
5 12
IFIN RSSI
6
11
DEC I FOUT
7 10
FILOUT QUAD
8 9
FILIN AFOUT
TA31136FN G
R544
DETDSC
470
C541
100P
CP
CP15