Section Circuit Description; Receiver Circuits - Icom IC-2000 Service Manual

SECTION
3
CIRCUIT
DESCRIPTION
3-1
RECEIVER CIRCUITS
3-1-1
ANTENNA
SWITCHING
CIRCUIT
(MAIN
UNIT)
The antenna
switching
circuit
functions
as a low-pass
filter
while receiving
and
a resonator
circuit
while trans-
mitting.
The
circiut
does not allow transmit signals
to
enter receiver
circuits.
Received
signals enter the
antenna connector and
then
pass through the low-pass
filter
(L9-L12, C54-C62),
the
X/4 type antenna switching
circuit (D7,
D16, D17, L19,
L20,
C107, Cl
08)
and
are then applied
to the
RF
ampli-
fier (Q20).
3-1-2
RF
AND
1st
MIXER
CIRCUITS
(MAIN
UNIT)
The
1st
mixer
circuit
converts the received signal to
a
fixed
frequency
of
the
1st IF
signal with a
PLL
output
frequency.
By changing
the
PLL
frequency, only the
desired frequency
will
be passed
through a
pair of crystal
filters at
the next stage of
the
1
st
mixer.
The
signals from the
antenna
switching
circuit
are passed
through the tunable band-pass
filter
(D14,
LI
8)
and
amplified at
the
RF
amplifier (Q20).
The
amplified signals
are again passed
through the tunable band-pass
filter
(DIO, D13, D31, L16, L17, L26)
and
applied to the 1st
mixer
(Q19).
The
signals are then
mixed
with a 1st
LO
signal
coming from
the
VCO
circuit to
produce
a 17.2
MHz
1st IF signal.
The
1st IF
signal
is
passed through a
pair
of crystal
filters
(FI1)
and
is
then applied
to
the
IF
ampli-
fier
(Q1
8).
•
FM DETECTOR
and
SQUELCH
CIRCUITS
3-1-3
2nd
IF
AND DEMODULATOR
CIRCUITS
(MAIN
UNIT)
The 2nd
mixer
circuit
converts the
1st IF signal to
a
2nd
IF signal.
A
double
superheterodyne system (which
converts receive signal twice)
improves
the
image
rejec-
tion ratio
and
obtains
stable receiver gain.
The
1st IF signal
from
the crystal
filter
(FI1)
is
amplified
at
Q18
and applied
to
a
2nd
mixer section
of
IC4
(pin 16).
The
signal
is
then mixed
with
a
2nd
LO
signal for
conver-
sion
to
a
455 kHz 2nd
IF signal.
IC4 contains the
2nd
mixer, local oscillator, limiter ampli-
fier,
quadrature
detector,
S-meter detector
and
active
filter.
The
local oscillator
section
generates 16.745
MHz
using X2.
The 2nd
IF signal
from
the
2nd
mixer
(IC4
pin
3)
passes through ceramic
filters
(FI2) to
remove
unwanted heterodyned frequency and
fix
a
passband
width.
It
Is
then amplified
at
the
limiter
amplifier (IC4,
pin
5)
and
applied
to
the
quadrature detector
(IC4,
pins
7,
8
and
X3)
to
demodulate
the
2nd
IF
signal
into
AF
signal.
The
AF
signal (detector signal)
is
output
from
pin 9.
3-1-4
AF
CIRCUIT (MAIN
UNIT)
The
AF
signals
from
IC4
(pin
9)
pass through
the
AF
switch (Q30)
and
are amplified
at
the active
filters
(Q32
HPF;
Q33
LPF).
Those
signals
pass through the detector
mute
switch
(Q37),
and
are
ievel
adjusted with
the
volume
control
on
the
LOGIC
unit.
The
AF
amplifier
IC8 amplifies the
signals to
a
sufficient
level to drive the
speaker.
The
AF
mute
switch (Q40)
turns
ON
to
cut the signai to
be
input to
the
AF
amplifier
(108)
during transmission.
Fig. 1
3-1