Icom IC-R10 Service Manual page 8

D152, D153, D172, D173, D213,
D214 and D238 employ
varactor diodes that are controlled
by
the
CPU
(LOGIC
unit;
IC1
)
to track the
bandpass
filter.
These
varactor diodes tune
the center frequency
of
an
RF
passband
for
wide
width
receiving
and good image response
rejection.
4-1-3
1ST
MIXER
AND
1ST
IF
CIRCUITS
{RF UNIT)
The
1st
mixer
circuit
converts the received
RF
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 the
band
pass
filters
at
the next
stage
of
the
1st
mixer.
The
amplified signals are
mixed
with 1st
LO
signals
from the
1st
VCO
unit at
a
1st
mixer (IC271)
to
produce
1st IF
signals
(266.7
MHz;
340.0
MHz-999.9999 MHz,
429.1
MHz;
0.5
MHz-339.9999
MHz
and
1
.0
GHz-1
.3
GHz).
The
1st IF
signals are applied
to
each
IF
filter
(FI401
for
266.7
MHz
IF
signal, FI411
for
429.1
MHz
IF signal) to
suppress out-of-band
signals.
The
1st
IF
signals are
amplified
at
the 1st IF amplifier (Q421)
and
then applied
to
the
2nd
mixer
on
the
MAIN
unit.
•
1st
LO
frequency and
1st
IF
frequency
Receive frequency
[MHz]
1st
LO
frequency
[MHz]
1st IF
frequency
[MHz]
0.5-
339.9999 429.6
-
769.0999
429.1
340.0-
609.9999 606.7
-
876.6999
266.7
610.0-
999.9999 343.3
-
733.2999 266.7
1000.0-
1300.0000 733.3
-
870.9000 429.1
4-1-4
2ND
MIXER
AND
2ND
IF
CIRCUIT
(MAIN UNIT)
The 2nd
mixer
circuit
converts the
1st IF
signal
to
a 2nd
IF
signal.
The
1st IF
signals (266.7
MHz
or
429.1
MHz) from
the
RF
unit
are applied to
the
2nd
mixer
circuit
(IC11)
for
mixing
with the
2nd
LO
signals
to
be converted
into
a 10.7
MHz
2nd
IF
signal.
The 2nd
IF
signal
is
applied
to
the
IF
filter
(Fill) to
suppress out-of-band
signals,
and
is
then
amplified
at
the
2nd
IF amplifier (Q21
).
4-1-5
3RD
IF
AND DEMODULATOR
CIRCUIT
(MAIN
UNIT)
The
3rd mixer
circuit
converts the
2nd
IF signal to
a 3rd
IF
signal.
IC71 contains the 3rd
mixer, limiter amplifier,
quadrature
detector,
s-meter detector
and noise detector
circuits.
The 2nd
IF
signal (10.7
MHz)
from the
2nd
IF
amplifier
(Q21)
is
applied
to
the 3rd mixer section
of IF
1C (IC71, pin
16)
and
is
then
mixed
with
a 10.245
MHz
3rd
LO
signal
generated by X501 on the
RF
unit for
conversion
to
a
455
kHz
3rd
IF signal.
The
3rd
IF
signal
is
demodulated
at
each
demodulator
circuit.
(1)
FM
The
3rd
IF signal
from the 3rd
mixer
(1071,
pin
3)
passes
through
the
ceramic
filter
(FI111) to
suppress
unwanted
heterodyned frequency
signals.
It
is
fed
back
to 1071,
then
amplified
at
the
limiter
amplifier section (pin 5)
and
applied
to
the quadrature detector section
(pin 10) to
demodulate
the 3rd
IF
signal into
AF
signals.
The
AF
signals are output
from
pin 9
and passed
through the
de-emphasis
circuit
(R79, 086) with frequency characteristics
of
-6
dB/octave,
and then applied
to
the
AF
switch (10301
c,
pin
4).
A
portion
of
the
AF
signals
from
pin 9
(1071)
passes
through
the low-pass
filter
(R78,
084) and
is
applied
to
the
OPU
(LOGIO
unit; 101)
as
AFO
control signals.
(2)
WFM
The
signal
from the 3rd mixer
is
applied
to
the
limiter
amplifier section (1071, pin
5).
The
amplified signals are
demodulated
at
the quadrature detector section (1071, pin
5).
The AF
signals are output from
pin 9 (1071)
and
are
applied
to
the
AF
switch (10301c, pin 4)
the
same
as
in
FM
mode.
By connecting R81
to
R72
in
parallel,
the output
characterstics
of pin 12,
"RSSI",
change
gradually.
Therefore, the
FM
IF
10
can
detect
WFM
components.
(3)
AM
The
signal
from the 3rd mixer passes through the
ceramic
filter
(FI111)
and
is
amplified
at
the 3rd IF amplifiers (Q151,
Q161).
The
amplified signal
is
demodulated
at
the
AM
detector (Q171).
The
demodulated
AF
signals
are applied
to
the
AF
switch (10301
d,
pin
8)
via
the
ANL
control
circuit
(Q172).
•
2nd
IF,
3rd
IF
and demodulator
circuit
IC71
to
AF
switch
IC301
4-2