Icom IC-2100H

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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