Circuit Description; Receiver Circuit; Pll Circuit - Icom IC-04A Service Manual

SECTION
4 CIRCUIT
DESCRIPTION
4-1
RECEIVER CIRCUITS
4-1-1
ANTENNA
SWITCHING
CIRCUIT
(PLL
UNIT)
Receive
signals
enter the
PLL UNIT from
ANTENNA
CONNECTOR
J201 and pass through
a
Chebyschev
low-pass
filter
consisting of
L223, L224, and
C289~C291
.
The
antenna switching
circuit
employs
a 4/2-type diode switching
system which does
not
allow current to
flow while
receiving.
4-1-2 RF CIRCUIT
(PLL
UNIT)
The
receive signals
from
the
antenna
switching
circuit
pass through
a
bandpass
filter
consisting of
C202,
C203
and L201, and
are amplified at
RF
amplifiers
0201
and
0202.
Bandpass
filters
are
designed
for
the
after
stage
of
each RF
amplifier circuit to further sup-
press out-of-band
signals.
After passing
through the
bandpass
filter,
signals are
fed to
1
St
mixer
0203
for
conversion
to
2
1
.8MHz
1
st
IF
signals with
LO
signals
from the PLL
circuit.
4-1-3
IF
CIRCUIT
(PLL
UNIT)
1st
IF
signals
from
0203
pass through
a
pair
of
crystal
filters
(FI201) to
suppress out-of-band
signals
and
un-
wanted heterodyned frequency
signals.
After passing
through the
filter,
the
1st
IF
signals
are amplified
at
IF
amplifier
0204, pass through matching
coil
L208, and
are fed to
IC201
IC201 contains the
2nd LO
circuit,
2nd
mixer
circuit,
limiter
amplifier circuit
and
quadrature detector
circuit.
The 2nd
LO
circuit
and
X201
generate 21
.345MHz
2nd
LO
signals
which
are
used
at
the
2nd
mixer
section of
IC201.
1
St IF
signals
from
L208
are fed to pin
16
of
IC201
,
and
are
mixed
with
2nd
LO
signals for
converting the 1st
IF
signals to
455kHz
2nd
IF
signals.
L22t
The 2nd
IF
signals are
output from
pin
3
and pass
through
high-quality
ceramic
filter
FI202
to
suppress
un-
wanted
heterodyned frequency
signals.
They
are
then
amplified
at
the
limiter
amplifier section
(pin
5 of
IC201
and
applied to a
quadrature detector
circuit (the
quadrature detection section of
IC201) and ceramic
resonator
X202
to
demodulate 2nd
IF
signals to
AF
signals.
S-Meter
Amp
S-M#ter
Amp 455kHz
21.345MHz
1st
IF
Amp
Q20S Q20S
FI202
X207
02M
4-1