Icom IC-R8500 Service Manual page 9

Varactor diodes are
employed
at
the
tuned notch
filter
circuit to
control the center
frequency
of
the IF
signal,
and
are
controlled
by
the
CPU
(MAIN
unit,
IC33) via the current
amplifier
circuit (IC7a).
•
Tuned
notch
filter
characteristics
4-1-5
VHF/UHF 1ST MIXER CIRCUIT
(RF-B UNIT)
The
1st
mixer
circuit
converts the received
RF
signals to
a
fixed
frequency
of
the
1st IF signal with
a
PLL
output
frequency. By changing the
PLL
frequency, only the de-
sired
frequency
will
pass through
the
bandpass
filters
at the
next stage
of the
1
st
mixer.
4-1-6
VHF/UHF 1ST IFAND 2ND MIXER
CIRCUITS
(MIX UNIT)
The 2nd
mixer
circuit
converts the
1st IF signal
to
a 2nd
IF
signal.
The
1st IF signal
from the
RF-B
unit
is
passed
through the
bandpass
filter
to
suppress unwanted
out-of-band
signals.
The
778.7
MHz
1st IF signal
is
passed
through the
dielectric
notch
filter
(FI2)
to
obtain
good image response
rejection for
the 21.4
MHz
lower frequency from
the receiving
frequency
after
passing through the
dielectric
filter
(FH
).
And, the 266.7
MHz
1st IF signal
is
passed
through the
helical
notch
filter
(L15)
to
obtain
a
good image
response
rejection
for
the 21
.4
MHz
lower frequency from
the
receiv-
ing
frequency
via the
helical
bandpass
filter
(Li
4).
The
filtered 1st IF signal
is
mixed
with
2nd
LO
signals
at
the
2nd
mixer
circuit
(ICI) to
produce a
10.7
MHz
2nd
IF signal.
The 2nd
IF signal
is
applied
to
the
MAIN
unit via
J4
after
suppressing
unwanted
higher
harmonic components
at
the
low-pass
filter (L9,
L22. C25, C27),
The
2nd
LO
signals are
generated
at
the
VCO-B
circuit
(VCO-B
board.
Q1
,
Q3),
and
are applied
to the
2nd
mixer
circuit after
being amplified
at
the
LO
amplifier
circuit
()C2).
The
filtered
RF
signals
are
mixed
with
1
st
LO
signals
at
the
1st
mixer
circuit (IC4, pin 3) to
produce a 778.7
or 266.7
MHz
1st IF signal.
The
1st IF signal
is
output from
pin
5,
and passed through
the
low-pass
filter
circuit
(L75-L77,
Cl 41 -Cl
45) to
suppress unwanted harmonic components.
The
filtered 1st IF
signal
Is
applied
to
the
MIX
unit via J2.
The
1st
LO
signals are generated
at the
VCO-A
circuit
(VCO-A
board,
Q1,
Q3),
and
are applied
to
the
1st
mixer
(IC4, pin 6) after
being amplified
at
the
LO
amplifier
circuit
(IC6) via the
VI
LO
line.
• 1st
LO
frequency and
1st
IF
frequency
Receive
freq.
[MHz]
1st
LO
freq.
[MHz]
let IF freq.
(MHzj
30.0-
89.99999
808.7- 866.69999
778.7
90.0- 242.09999 868.7-1020.79999
778.7
242.1- 499.89999 1020.8-1278 69999
778.7
500.0-1024.99999 766.7-1291,69999
2B6.7
4-1-7
HF RF
FILTER CIRCUIT
(RF-A UNIT)
'
The
RF
filter circuit
consists
of
an antenna
switching
circuit,
9 bandpass
filters
and an
RF
amplifier
circuit.
IC-R8500 has
2
antenna connectors
for the
HF
bands
(ANTI and ANT2); ANTI
employs an SO-239
connector
with
50
Q
impedance and
ANT2
employs a phono/RCA
connector
with
500
Q
impedance.
The
RF
signals from
ANTI
(J2)
are applied
to
the
bandpass
filter to
suppress
out-of-band signals via the
antenna
switching relay (RLI).
However,
the
RF
signals
from
ANT2
(J1)
are
passed through the step-down transformer
(LI)
to
be
converted
into
50
Q
impedance
when ANT2
is
selected.
Then
the
RF
signals
are applied
to
the
bandpass
filter.
The
filtered
RF
signals
are bypassed
or applied
to
the
RF
amplifier
circuit (Q1, Q2),
depending on
the syatus of the 10
dB
attenuator,
The
RF
amplifier
circuit
(Q1,
Q2)
provides
10
dB
gain, therefore,
the
bypassed
RF
signal strength
seems
attenuated
when
the 10
dB
attenuator
is
turned
ON.
•
HF RF
and
mixer
circuits
4-2