Icom IC-R8500 Service Manual page 10

Then
the
RF
signals
from
the switching diode (D17) are
applied
to
the
1
st
mixer
circuit
•
Bandpass
filters
Receive
freq.
{MHz]
SW
diode
BPF
lelect
signal
Components
0.1- 0.49999
D3
B1
L7.L8.
C15-C17
0.5- 1.59999
D3
B2
LIO.
L11.C22-C25
1.6-
1.99999
D7
B3
L23, L24,
C42-C44
2.0- 3.99999
D8
B4
L18-L20.
C34-C39
4.0- 7.99999
D8
B5
L26-L29,
C46-C51
8.0-10.99999
D9
Be L31-U3. C54-C59
11.0-14.99999
D9
B7
L36-L38.
C62-Ce7
15.0-21.99999
DIO
B8 L41-L43, C70-C75
22.0-29.99999 DIO B9
L46-L49.
C79-C84
4-1-8
HF 1ST MIXER CIRCUIT
(RF-A UNIT)
The
1st
mixer
circuit
converts the
RF
signals
to
a
desired
48.8
MHz
1st IF
signal.
In
this
way,
the
VCO-A
circuit
output frequencies are used
for
1 st
LO
signals
after
being
amplified at
the
LO
amplifier (IC2).
The
received
RF
signals from the
bandpass
filter
circuit
are
mixed
with 1st
LO
signals
at
the
1st
mixer
circuit
(06,
Q7)
after passing
through
the
low-pass
filter
circuit
(LS9,
C98-C100).
The
mixed
1st IF signal
is
passed through the
crystal
filter
(FI1) to
suppress out-of-band
signals then
amplified
at
the
IF amplifier
circuit
(Q8).
The
amplified 1st IF signal
is
applied
to
the 2nd mixer
circuit.
•
1st
LO
frequency
and
1st IF
frequency
4-1-10
IF
CIRCUIT (MAIN
UNIT)
The
10.7
MHz
2nd
IF signal
from
the
MIX
unit
Is
applied
to
the
mode
switch (D3)
after
being
amplified
at
the
IF ampli-
fier circuit
(Q2).
However,
the 10.7
MHz
2nd
IF signal from
the
RF-A
unit
is
applied
to
the
mode
switch (D3)
directly.
The
IF signal
from
the
mode
switch
is
then applied to
a
different
circuit
depending on
the receiving
mode.
(1)
WFM
mode
The 2nd
IF signal
from
the
mode
switch
(D3)
is
applied
to
the
IF amplifier circuit (Q4).
The
amplified IF signal
is
passed
through
the
bandpass
fillers
(Fll, FI2) to
suppress
out-of-band
signals.
The
filtered IF signal
is
then applied
to
the
WFM
demodulator
circuit.
(2)
Other
modes
The 2nd
IF sign
2
U
from
the
mode
switch (D3)
is
passed
through the
bandpass
filter
(FI3) to
suppress
out-of-band
signals,
and
then
applied
to
the
IF amplifier
circuit (Q15).
The
amplified
IF signal
Is
then applied
to
the 3rd mixer
circuit.
4-1-11
3RD
MIXER
CIRCUIT
(MAIN
UNIT)
The
3rd mixer
circuit
mixes
the
2nd
IF signal
and
3rd
LO
signal to
produce
a
455 kHz
3rd
IF signal.
The
3rd
LO
signal
is
generated
at the
DOS
circuit
(PLL
unit, IC1
3)
and
is
applied
to
the 3rd mixer
circuit
via
the
buffer-amplifier
(Q28).
The 2nd
IF signal
from
the IF amplifier
circuit (Q15) is
applied
to
the 3rd mixer
circuit
(IC6, pin
6),
and
also the 3rd
LO
signal (10.245
MHz)
Is
applied
to pin
8
of
the
1C.
The
mixed
3rd IF signal
is
output from
pin
3,
and
is
passed
through
different
bandpass
fillers
depending on
the
receiv-
ing
mode
and passband
width.
The
filtered
3rd IF signal
is
applied
to
the
matched
demodulator
circuit with
the
receiv-
ing
mode
via
the
IF amplifier
circuit (016).
Receive
freq.
[MHz] 1st
LO
freq.
[MHz]
1st IF freq.
[MHz]
0.1-29.99999 48.9-78.79999
46.8
4-1-9
HF 2ND MIXER
CIRCUIT
(RF-A UNIT)
The
2nd
mixer
circuit
converts the
1st IF signal to the
desired 10.7
MHz
2nd
IF frequency.
The
2nd LO
signal
is
used
in
the
mixer
circuit to
produce a 2nd
IF frequency,
and
the signal
is
generated
at
the
VCO-B
circuit
(VCO-B
board,
Q1
.
Q3)
after being amplified
at
the
LO
amplifier (IC1
).
•
Bandpass
filter
selection
Mode(s)
Bandpass
fitters)
Passband
width
FM.
AM-W
FI7.
R6
12 kHz
FM-N,
AM
FIS.
R6
5.5
kHz
SSB, CW. AM-N
FI4
2.2
kHz
CW-N
Optional
FL-52A
500 Hz
4-1-12
DEMODULATOR
CIRCUIT (MAIN
UNIT)
The
1st IF signal
from
the
IF amplifier
(Q8)
is
applied
to
the
The
demodulator
circuit
converts the 3rd
IF signal
into
AF
signals. 4 separate demodulator
circuits
are
employed
for
4-3