Icom IC-R10 Service Manual page 9

(4)
SSB/CW
The
signal
from the 3rd mixer
passes through the ceramic
filter
(FI101)
and
is
amplified
at
the 3rd
IF
amplifiers
(Q151,
Q161) and
buffer amplifier (Q201).
The
amplified signals
are applied
to
the
SSB/CW
detector
(D201
,
D202) and
mixed
with
BFO
signals
from the
BFO
circuit
(LOGIC
unit;
X231)
via
the
BFO
amplifier (0951) to
demodulate
AF
signals.
The
demodulated
AF
signals are applied to
the
AF
switch (IC301b,
pin 11) after
being
pre-amplified at
the
AF
pre-amplifier (0202).
•
Bandpass
filter
selection
Mode(s)
Bandpass
filter(s)
Passband
width
NFM
/
AM
FI111
15 kHz
SSB
/
CW
FI101
4 kHz
WFM
—
—
4-1-6
SUB-BAND RECEIVER CIRCUIT (MAIN
UNIT)
The
sub-band
receiver
circuit is
employed
by the
band
scope and SIGNAVI
functions.
A
portion of
the 1st IF signals
from the
RF
unit
is
mixed
with
the
sub 2nd
LO
signals
at
the
2nd
mixer
circuit
(0411) to
be
converted
into
455
kHz sub 2nd
IF signal.
The 2nd
IF
signal
Is
applied to the IF
1C (IC471
,
pin
5)
via
the
IF
filter
(FI411)
and 2nd
IF
amplifier (0421).
The
IF 1C
(IC471) outputs
signals to
lead the
CPU
(LOGIC
unit;
IC1)
under
conditions
of
RSSI
or
noise squelch.
4-1-8
SQUELCH
CIRCUIT (MAIN
UNIT)
A
noise squelch
circuit
cuts out
AF
signals
when
no
RF
signal
is
received.
By detecting noise
components
in
the
AF
signals,
the
CPU
switches the
AF
mute
switch.
Some
of
the noise
components
in
the
AF
signals
from IC71
(pin
9)
are
passed through the
active
filter
(IC71
,
pins
8, 7),
and
then applied
to
the noise detector section.
The
squelch
control pot
(RF
unit;
R11) adjusts the input
level of
the
active
filter,
and the
level
is
used
for
the
squelch threshold
reference.
The
detected noise signals
are applied
to
the
CPU
(LOGIC
unit;
IC1, pin 65) via
the
"SQL"
line.
The
CPU
then analyzes the noise
condition
and
controls the
"MUTE"
port
to
cut
off
the
AF
signal using the
AF
mute
switch
(LOGIC
unit;
IC950).
4-1-9
VSC CIRCUIT (LOGIC
UNIT)
The
VSC
(Voice
Scan Control) detects
AF
signals
and
mutes undesired signals
such as unmodulated, beat
and
noise
component
signals.
When
the
VSC
function
is
ON
and
an unmodulated
signal
is
received,
squelch functions
the
same
as closed (no signal condition)
even
when
it's
open, or the
scan
function
resumes
for
a
short period
on
any scan
setting
during scanning.
A
portion of the
AF
signals
from the low-pass
filter
(Q950)
is
applied
to
the
VSC
control circuit
(IC201) to
detect
demodulated signals while the
VSC
function
is
ON.
When
audio
component
signals
are included
in
the
AF
signals,
the
VSC
1C (IC201
)
outputs
a low
level
signal
from
pin
13
to
the
CPU
(IC1
,
pin
1
)
to
release the
mute
switch (IC950).
4-1-7
AF AMPLIFIER CIRCUIT
(MAIN UNIT)
The
AF
amplifier circuit
amplifies the
demodulated
AF
signals to drive
a speaker.
The
AF
signals
from the
AF
switch (IC301) are applied
to
the
LOGIC
unit
through a
"DETOM"
line.
The
AF
signals
are
amplified at
the
AF
amplifier
(LOGIC
unit;
0950),
and
are
then applied
to
the
AF
mute
switch
(LOGIC
unit;
IC950).
The
output signals from the
AF
mute switch
(LOGIC
unit;
IC950) are applied to the
[VOL] control
(RF
unit;
R11),
and
are then power-amplified
at
the
AF
amplifier
(MAIN
unit;
IC361
,
pin 4) to drive
a
speaker.
4-1-10
BFO
CIRCUIT (LOGIC UNIT)
The
BFO
(Beat
Frequency
Oscillator) circuit
consists
of
X231, IC231 and D231. The
oscillator
provides a beat
frequency
signal
(USB; 456.5 kHz, LSB; 453.5 kHz)
to
the
SSB/CW
detector
circuit
(MAIN
unit;
D201, D202)
for
demodulating the 3rd
IF signal into
AF
signals.
The
oscillating
frequency
for
each
mode (LSB/USB/CW)
is
switched by changing the capacitance
of
a varactor diode
(D231).
When USB mode
is
selected,
the
CPU
(IC1, pin 3)
outputs
"LOW"
level
voltage
to
D231
to
change
the
BFO
frequency
to
456.5 kHz.
•
Squelch
and
AF
amplifier circuits
4-3