Section 4 Circuit Description; Receiver Circuits - Icom IC-R8500 Service Manual

SECTION
4
CIRCUIT
DESCRIPTION
4-1
RECEIVER
CIRCUITS
4-1-1
RF
CONVERTOR
CIRCUIT
(CONV
UNIT)
The
RF
convertor
circuit
converts
1025-1999.99999
MHz
RF
signals
to
200-989.99999
MHz RF
signals.
(1)
30.00000-1024.99999
MHz
signals
RF
signals
from the
antenna connector
(J1)
pass through
the switching relays (RL1, RL2)
to
bypass
the
RF
convertor
circuit.
The
bypassed
RF
signals either
bypass
or are
passed through the
RF
attenuator
circuit
(R17-R19.
R25-R27)
then applied
to
the
RF-B
unit
via J3.
(2)
1025.00000-1999.99999
MHz
signals
RF
signals
from
the
antenna connector
(J1)
are applied
to
the
RF
amplifier
circuit
(ICI, IC3) via the switching relay
(RL1) and tunable high-pass
filter
(D3-D5, D20, D21.
L2-L4).
The
amplified signals are
mixed
with convertor
LO
signals
at
the
RF
convertor
circuit
(1C2)
to
produce
200.00000-989.99999
MHz RF
signals.
The
converted
RF
signals are applied
to
the
RF-B
unit
via
J3
after
bypassing
or
passing through the attenuator
circuit.
The
convertor
PLL
circuit
(IC6;
VCO,
IC7:
PLL
1C)
gener-
ates
a 1000
or
1010
MHz
LO
signal
and
applies
them
to
the
convertor
circuit directly
or divides
them by 2
at 108.
4-1-3
VHF/UHF RF
CIRCUIT
(RF-B UNIT)
The RF
circuit
amplifies the received signals within the
range
of
frequency coverage and
fitters
out-of-band
signals.
The
RF
circuit
consists
of
4 bandpass
After circuits
with
an
RF
amplifier for
each.
The
received signals
from the
CONV
unit
are passed
through
to
the tunable
bandpass
filter
via the switching relay
(RL1),
then ampliAed
at
the
RF
ampllAer
circuit.
The
amplified
RF
signals
are again passed through another
bandpass
Alter to
suppress out-of-band
signals.
The
Altered
signals are ampliAed
at
the other
RF
amplifier
circuit
(IC3),
then applied
to
the
1st
mixer
circuit after
passing through
the low-pass
or
bandpass
with
tuned notch
circuit.
The
tunable
bandpass
Alters
employ
varactor
diodes
to
tune
the center
frequency
of the
RF
passband
for
wide band-
width receiving
and good image response
rejection.
These
diodes are
controlled by the
CPU
(MAIN
unit.
IC33)
via the
voltage
amplifier
circuit (IC2).
A
Ga-As
FET
is
used
for the
RF
ampliAers (Q1-Q4)
to
provide high
sensitivity within
wide-band
coverage,
and
also
to
provide 10
dB
amplifying gain.
•
Convertor
LO
signals
Receive frequency Convertor
LO
frequency
1025-1199 99999
MKz
500
MHz
1200-1989 99999
MHZ
1000
MHz
1990-1999 99999
MHz 1010MHz
4-1-2
RF
ATTENUATOR
CIRCUIT
(CONV
UNIT)
The
attenuator circuit
attenuates the signal strength
up
to
30
dB
to
protect the
RF
ampliAer from
distortion
when
excessively strong signals are
received.
The
RF
attenuator
circuit consists of
2 separate
attenuator
circuits
connected
in
series.
The
1st
stage
of
the
RF
attenuator
circuit
(R17-R19)
provides 20
dB
attenuation;
the
2nd
stage
(R25-R27)
provides 10
dB
attenuation via
a
"n"type
attenuator.
•
Tunable
bandpass
filter
and
RF
amplifier
Receive frequency
(MHz)
BPF
select
signal
Varactor diodes
RF
amp.
30.0- 89.99999
BPF1
D3.
D4 Q1
90.0- 242.09999 BPF2
D9-D12 Q2
242.1- 499.99999 BPF3
D15, Die, D33,
D34
Q3
500.0-1024.99999 BPF4 D19-D23
Q4
4-1-4
TUNED NOTCH
CIRCUIT
(RF-B UNIT)
The
tuned notch
circuit
activates while
RF
signals higher
than
500
MHz
are received.
The
circuit
prevents the
1
st
LO
signal
from
entering the
antenna
connector and
also re-
duces
IF disturbance.
The
tuned notch
circuit
{D29-D32, L51-L54, L74)
is
de-
signed
between
the high-pass
(L82,
C166-C168) and
low-
pass
Alter
(L55, L56,
C96-C98, Cl
00).
The
high-pass
filter
reduces
IF disturbance (266.7
MHz), and
the tuned notch
and low-pass
Alter circuits
prevent the
1st
LO
signal
from
entering
the antenna
connector.
•
VHF/UHF RF
and
mixer
circuits
30.0-1
999.9999a
MHz
CONV
unit
30.0-989.99999
MHz
—
10
de
ATT
RF-B
unit
^
~
^BPF2^
-
^BPF3^
¥
BPF4/
Convertor
LO
signal
IC3
r^h
Bu^
-6<^
1st
LO
signal
2nd
LO
signal
•
MIX
unit
7
78.7
MH
z
iC2
266.7
MHz
-<8>
10.7
MHz
to
MAIN
unit
4-1