Icom IC-M412 Service Manual page 7

SECTION 4
4-1 RECEIVER CIRCUITS
ANTENNA SWITCH (MAIN-RX/CH70-RX)
The received signals (RX signals) from the antenna are
passed through the LPF (harmonic fi lter) and ANT SW (as
an LPF in RX).
The ANT SW toggles RX line and TX line. While receiving,
the TX line and antenna is disconnected to prevent RX
signals entering. The RX line is disconnected from the GND
simultaneously, and an LPF which guides RX signals to the
RX circuits is composed.
While transmitting, serial-connected PIN diodes (D14, 21)
are forward biased and the TX line and the antenna is
connected by D14. Simultaneously, the RX line is connected
to the GND by D21 to prevent transmit signal entering.
The RX signals from the ANT SW (=LPF) are applied to the
RF circuits via the trap (BEF; Band Eliminate Filter).
RF CIRCUITS (MAIN-RX)
RF circuits consist of RF fi lters and RF amplifi er (RF AMP)
that fi lter and amplify the signals of frequency which desired
to receive.
The fi ltered RX signals from the trap (BEF; Band Eliminate
Filter) are applied to the tuned RF AMP. The amplified RX
signals are passed through the tuned 2-pole BPF.
• RF CIRCUITS (MAIN-RX)
2-pole Tuned BPF
L39
C176
C183
5P LS-440
0.75P
R173
10K
UNIT ABBREVIATIONS:
FR= FRONT-A UNIT, LG= LOGIC-A UNIT, MA= MAIN-A
CIRCUIT DESCRIPTION
C162
L38
C175
0.5P
LS-440
Tuned RF AMP
• ANT SW (MAIN-RX/CH70-RX)
ANT SW (TX)
D14 C126
0.001
L308CC B
L31
LA-243
TX power detector
R127
The tuned RF AMP and 2-pole BPF are adjusted so that
it responds to receiving frequency and rejects all others,
by the variable capacitor whose capacitance is varied by
applied tuning voltage "BPFV."
The fi ltered RX signals are then applied to the 1st IF circuits.
R8V
LS-440
L36
9P
C164
2.5P
Q21
3SK131
R149
100K
4 - 1
LPF (harmonic filter)
L21 L22
LA-243 LA-243
C128
4P
C150
0.001
ANT SW
TX: BPF
RX: LPF
C153
220P
CP15
To ANTENNA
CP20
C151
22P
Trap
(BEF)