Receiver Circuits - Icom IC-2SET Service Manual

SECTION
4
CIRCUIT
DESCRIPTION
4-1
RECEIVER
CIRCUITS
4-1-1
ANTENNA
SWITCHING
CIRCUIT
(MAIN
AND APC
UNITS)
Received
signals enter the
antenna connector and pass
through a iow-pass
filter
(L2~L4,
C21~C25). The
signals
are applied
to
the antenna switching
circuit
(D7, D9,
L5,
L6,
and
C26~C28) and
then
to the
RF UNIT
via
the
RFIN
signal
line.
The
antenna switching
circuit
employs a
two-stage A/4-type diode switching system.
The antenna
switching
circuit
functions
as
a
low-pass
filter
while
in
receiving
and
becomes
very
high
impedance
while
in
transmitting.
4-1-2
RF CIRCUIT
(RF UNIT)
The
signals
from
the
antenna switching
circuit
pass
through a
bandpass
fiiter
(LI,
D2),
and
are applied to
the
RF
amplifier (Q1, Q2).
Amplified signais are reappiied
to
the other
bandpass
filter
(L2, L3,
D4,
D5)
to
suppress
unwanted
signals.
The
signals are applied
to
the
1st
mixer
circuit
(MAIN
UNITQ1).
D2,
D4 and D5
are varactor diodes
that
track the
band-
pass
filters
and
are controlled by the
PLL
lock voltage.
These diodes tune the center frequency
of
the
bandpass
filters for
wide bandwidth reception
and good image
response
rejection.
4-1-3
1ST
MIXER CIRCUIT (MAIN
UNIT)
The
signals
from
the
RF
circuit
are
mixed with the 1st
LO
signal
from the
PLL UNIT
to
produce a 30.875
MHz
1st
IF signal.
4-1-4
1
ST
IF
CIRCUIT (MAIN
AND
DET
UNITS)
After
passing through the matching
circuit (LI),
the
1st IF
signal
is
applied
to
a
pair of crystai filters (FI1) to
suppress
out-of-band
signals.
The
1st IF signal
enters the
DET
UNIT and
is
amplified
at
the
IF amplifier (Q2)
and
then
applied
to
the
2nd
mixer
circuit.
4-1-5
2ND
IF
AND DEMODULATOR
CIRCUITS (DET
UNIT)
The
1st
IF
signal
from
Q2
is
appiied to the
2nd
mixer
section
of IC1,
and
is
mixed
with the
2nd
LO
signal
to
be
converted
to
a
455 kHz 2nd
IF signal.
IC1
contains the
2nd
mixer,
local oscillator, limiter
ampiifier
and
quadrature detector
circuits.
The
local
oscillator
section
and X2
generate 30.42
MHz
for
the
2nd LO
signal.
The 2nd
IF signal
from the
2nd
mixer
(101, pin 4)
passes
through
the
ceramic
filter,
FI1,
where unwanted
signals
are suppressed.
It
is
then
amplified
at
the limiter amplifier
section
(Id,
pin 6)
and
applied to the
quadrature detector
section
(101,
pin 10
and ceramic discriminator XI)
to
demodulate
the
2nd
IF
signal
into
an
AF
signal.
AF
signal output
from
Id
pin
1 1
is
applied
to
the
squelch
circuit
and de-emphasis
circuit
(R7, 024,
025).
This de-
emphasis
circuit
is
an
integrated
circuit
with
frequency
characteristics
of
-6
dB/octave.
The
resulting signal
is
applied to the
AF
amp,
optional
tone squelch and
optional
DTMF
decoder
circuits.
RECEIVER
CIRCUIT
BLOCK DIAGRAM
4—1