Icom IC-2100H Service Manual page 6

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 transmitting.
The
circuit
does
not allow transmit signals to
enter receiver
cir-
cuits.
Received
signals enter the
antenna connector and pass
through the
low-pass
filter
(L17-L20,
C55-C64).
The
filtered
signals
are
passed
through
the A74 type
antenna
switching
circuit
(D10, D11, L22, L23)
and
are then applied
to
the
RF
amplifier
(Q20).
3-1-2
SQUELCH ATTENUATOR
The
attenuator
circuit
attenuates the
signal
strength
to
a
maximum
of
10
dB
to protect
the
RF
amplifier
from
distortion
when
excessively strong signals are received.
The
current flow
of
the
antenna
switching circuit
(DIO,
Dll)
is
controlled
by the [SQL] control via
the attenuator controller
(IC7).
When
the
[SQL]
control
is
rotated
clockwise
deeper
than
12
o'clock, the current of
DIO
and Dll
is
increased.
In
this
case,
DIO
and Dll
act
as
an
attenuator.
3-1-3
RF
CIRCUIT (MAIN
unit)
The
RF
circuit
amplifies signals within the range
of
frequen-
cy
coverage and
filters
out-of-band
signals.
The
signals
from the
antenna
switching
circuit
pass through
the tunable
bandpass
filter
(D13).
The
filtered
signals
are
amplified
at
the
RF
amplifier
(Q20)
and then enter another
three-stage
bandpass
filters
(D14--D16) to
suppress unwant-
ed
signals.
The
filtered
signals are applied to
the
1st
mixer
circuit (Q21).
The
tunable
bandpass
filters
(D13-D16) employ
varactor
diodes
to
tune the center frequency
of
the
RF
passband
for
wide bandwidth
receiving
and good image response
rejec-
tion.
These
diodes are controlled
by the
PLL
lock voltage via
the
tune
control circuit (IC2,
D4).
3-1-4
1ST MIXER
AND
1ST
IF
CIRCUITS
(MAIN
unit)
The
1st
mixer
circuit
converts the received signals
to
a
fixed
frequency
of
the
1st IF
signal with the
PLL
output frequency.
By changing
the
PLL
frequency, only the desired
frequency
will
pass
through
a
pair of crystal
filters
at
the next
stage
of
the
1
St
mixer.
The
RF
signals
from the
bandpass
filter
are applied to
the
1st
mixer
circuit
(Q21).
The
applied signals are
mixed
with
the
1st
LO
signal
coming from
the
RX-VCO
circuit (Q33,
D23)
to
produce a 15.65
MHz
1st IF signal.
The
1st IF signal
passes
through a
pair of crystal
filters
(Flla/b) to
suppress
out-of-
band
signals.
The
filtered
signal
is
amplified at the 1st IF
amplifier (Q22)
and
applied to the
2nd
IF
circuit.
3-1-5
2ND
IF
AND DEMODULATOR
CIRCUITS
(MAIN
unit)
The
2nd mixer
circuit
converts the
1st IF
signal
to
a 2nd
IF
signal.
A
double-conversion
superheterodyne
system
improves the
image
rejection ratio
and obtains stable
receiv-
er gain.
The
1st IF signal
from
the
IF amplifier (Q22)
is
applied
to
the
2nd
mixer section
of
the
FM
IF
1C (IC6, pin 16)
and
is
then
mixed
with
the
2nd
LO
signal for
conversion
to
a 450 kHz 2nd
IF signal.
IC6 contains the
2nd
mixer, limiter
amplifier,
quadrature
detector,
S-meter
detector, active
filter
and noise
amplifier
circuits, etc.
A
frequency from
the
PLL
reference oscillator
is
used
for
the
2nd
LO
signal (15.2
MHz).
The 2nd
IF
signal
from
the
2nd mixer
(IC6, pin 3)
passes
through the ceramic
filter
(F12)
(during
wide channel spacing
selection or
passes through
FI3 during
narrow channel spac-
ing selection;
[EUR], [ITA] only).
It
is
then amplified
at
the
lim-
iter
amplifier section (1C6, pin 5)
and applied
to
the
quadra-
ture
detector section
(IC6,
pins 10, 11
and X2)
to
demodulate
the
2nd
IF signal into
AF
signals.
The
AF
signals
are output
from
pin
9
(ICS)
and
are then
applied to
the
AF
amplifier
circuit.
•2nd
IF
AND DEMODULATOR
CIRCUITS
3-1-6
AF
CIRCUIT (MAIN
unit)
The
AF
amplifier circuit
amplifies the
demodulated
AF
signals
to drive
a speaker.
The AF
signals
from IC6
(pin 9)
are
amplified
at
the active
fil-
ters
(Q23 HPF,
Q24
LPF) and pass through the detector
mute
switch (Q25),
and are
level
adjusted
with the
volume
control
on
the
LOGIC
unit.
The
AF
amplifier
ICS
amplifies the signals to
a
sufficient level
to
drive
the speaker.
The
AF
mute
switch
(Q28)
turns
ON
to
cut the signal to
be
input to
the
AF
amplifier (ICS) during
transmission.
3-
1
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