Section 4 Circuit Description - Icom ic-775 Service Manual

SECTION
4
CIRCUIT
DESCRIPTION
4-1
RECEIVER
CIRCUITS
4-1-1
RF SWITCHING
CIRCUIT
(CTRL
AND
RF
UNITS)
The
RF
switching
circuit
leads receive signals
to
bandpass
filters
from an antenna connector
while receiving.
While
transmitting, this circuit
leads the signals
from
the
RF
power
amplifier to the
antenna
connector.
RF
signals
from
an antenna
connector, pass through the
antenna selecting relay (RL3), transmit/receive switching
relays
(RL1,
RL2, RL4, RL12) and low-pass
filter
(LI
5,
Li
6,
C48-C50), and are then applied
to
the
RF
unit via
J10 (RF
unit J7-1).
The
signals from the
CTRL
unit
are either
bypassed
or are
applied to the
6
dB
(R132,
RL2)
and/or
12
dB
attenuators
(R131, RL1).
By
selecting the attenuators, 0 (bypass),
6,
12 and 18
dB
attenuations are obtained.
The
signals
are
then applied
to
RF
filters.
4-1-2
RF
BANDPASS
FILTER CIRCUIT
(RF UNIT)
RF
bandpass
filters
pass only the desired
band
signals
and
suppress any undesired band
signals.
The
RF
circuit
has 9
bandpass
filters
and 2 low-pass
filters.
(1)0.1-0.5
MHz
The
signals
pass through a low-pass
filter
(L84-L86,
C141-
C1
45),
bypass the preamplifiers
and
are then applied
to
the
RF
amplifiers
(Q7,
Q14).
(3)
1.6-30.0
MHz
The
signals
pass through
a
high-pass
filter
(L134,
LI 35,
Li 37,
C226-C229)
to
suppress
strong signals
below
1.6
MHz. The
filtered
signals
are applied
to
one
of
9
bandpass
filters
as
below,
a PIN
attenuator (D42),
and
then applied
to
or
bypassing
the preamplifier
circuit.
Used
RF
filter
Band
Control
signal
Input
diode
Band
Control
signal
Input
diode
0.1-0.5
MHz
B1
019(1/2)
6-8
MHz
0.5-1.
6
MHz
B2
019(1/2)
8-1
1
MHz
KM
1.6-2
MHz
B3 11-15
MHz
027(1/2)
2-3
MHz
B4 15-22
MHz
3-4
MHz
B5
023(1/2)
22-30
MHz
4-6
MHz
B6
025(1/2)
4-1-3
PREAMPLIFIER CIRCUITS
(RF UNIT)
(1
.6
MHz
and
above)
The
IC-775/DSP has 2
gain levels of preamplifier
circuits.
One
has 10
dB
gain over
a wideband
frequency range
and
the other
has
1
6
dB
gain
for
21
-28
MHz
Jbands.
When
the
[PREAMP/ATT]
switch
is
set to
[PRE1]
or [PRE2],
the
signals
above
1
.6
MHz
are applied to the preamplifier
1
(Q22,
Q23) or preamplifier
2 (Q24,
Q25)
circuit,
respec-
tively.
Amplified or
bypassed
signals are applied
to
the
1
st
mixer
circuits.
4-1-4
RF AMPLIFIER
AND
1st
MIXER
CIRCUITS
(RF UNIT)
(2)
0.5-1
.6
MHz
The
signals pass through
a 10
dB
attenuator (R117)
to
prevent
distortion
caused by
very strong signals
and are
then applied
to
a low-pass
filter
(L88, L89,
C148-C150).
The
filtered
signals
bypass
the preamplifiers
and
are then
applied
to
the
RF
amplifiers (Q7,
Q14).
•
RECEIVER
CONSTRUCTION
The
1st mixer
circuit
mixes
the receive signals with the 1st
LO
signal to
convert the receive
signal
frequencies
to
a 69
MHz
1st
IF.
The
IC-775/DSP has two
1st
mixer
circuits for
the dualwatch
function.
The
signals from the
preamplifier
circuit,
or signals which
bypass
the preamplifiers, are divided
at
L52.
Each
signal
is
applied to
a 30
MHz
cut-off
low-pass
filter,
RF
amplifier (Q7
or
Q1
4)
and then
to
a
1
st
mixer (Q5/Q6
or
Q1
2/Ql
3).
RF
unit
FI1a/FMb
,
Crystal
i
\
fiitef
/
89.0115
MHz
1
st
mixer
A
Q12,
Q13
IstLO
A:
69.1115-99.0015
MHz
IF unit
2nd mixer FI2 or 3rd mixer
IC34
FI3
IC1S FM-FI7
9.0115
MHz
L
Ceramic it
To
FM
detector (IC1 5)
4th mixer
IC13
Product
detector/
XI 5th mixer IC14
455 kHz
ToAF
selector
switch
2nd
LO: 60.0
MHz
3rd
LO: 9.4685
MHz
4th LO: 11.15
MHz BFO
:
10.7
MHz
(DSP
receive
OFF)
5th
LO
:
10.715
MHz
(DSP
receive
ON)
4-1