Circuit Description; Receiver Circuits - Icom IC-725 Service Manual

SECTION
4
CIRCUIT
DESCRIPTION
4-1
RECEIVER CIRCUITS
4-1-1
RF SWITCHING
CIRCUIT
(PA
AND
MAIN
UNITS)
RF
signals
from the
antenna connector pass through the
transmit/receive switching relay (RL13)
and
a low-pass
filter,
and
are applied to
the
MAIN UNIT
via
P2 (MAIN
UNIT:
J12).
The
signals
from the
PA UNIT
either
bypass
or
are
attenuated
at
20
dB
attenuator (R102,
R103,
RL1).
There
are no
non-linear
components between
the antenna
connector and
attenuator to prevent distortion
caused
by strong
signals.
The
signals
are then applied
to
RF
filters.
4-1-2
RF
BANDPASS
FILTER CIRCUIT
(MAIN
UNIT)
The
RF
UNIT has 7
RF
bandpass
filters
(BPF)
for
signals
above
1.6
MHz
and
1
low-pass
filter
(LPF) for
signals
below
1.6
MHz. The
signals
pass through
one
of the
bandpass
or
low-pass
filters
depending on the receive frequency
range.
(1)
0.5~1.6
MHz
A
diode
is
not
used
at the
low-pass
filter
entrance
removing
diode distortion
from very strong
signals.
Signals
bypass a
preamplifier
by
the
bypass
switch (Q12).
(2)
1.6
MHz
AND ABOVE
Signals are applied
to
a high-pass
filter
consisting of L42,
L43,
C143~C146.
This
filter
suppresses strong signals
below
1.6
MHz
such as a broadcasting
station.
The
filtered
signals
are applied
to
one
of
7 bandpass
filters
depending on
the frequency
of
the signals
and
then
to
the
preamplifier
circuit (Q8, Q9).
(3)
FILTER
SWITCHING
CIRCUIT
An RF
bandpass
filter is
selected with
BPF
switching
voltage
(B0~B7) from
the
CPU
via
IC16
current amplifier.
The
switching voltage
of
the
BPF
entrance
is
higher than
the
BPF
exit
to
improve
multi-signal
and
strong
signal
characteristics.
4-1-3
PREAMPLIFIER CIRCUIT (MAIN
UNIT)
The
preamplifier circuit
uses low-noise junction
FETs
(2SK125X2)
to
provide
10
dB
gain over
a wideband
frequency
range.
When
the [PRE] switch
is
turned
ON,
the signals
from
the
RF
filter
are amplified
by
the preamplifier circuit
(Q8,
Q9).
When
the
[PRE] switch
is
turned
OFF,
the signals
bypass the preamplifier through
D30
and D32.
When
the
operating
frequency
is
below
1.6
MHz, 012
turns
ON
and
the signals
bypass the preamplifier regardless
of
the
[PRE] switch.
Amplified or
bypassed
signals
are applied to the
1
st
mixer
circuit via
the low-pass
filter.
The
low-pass
filter
cuts
off
at
35
MHz
to
suppress
image frequency
at
the
1st
mixer
circuit
(013, 014).
PREAMP
CIRCUIT
Q10
4-1-4
1ST MIXER CIRCUITS (MAIN
UNIT)
The
signals
from the low-pass
filter
enter the
1st
mixer
circuit (013,
014) to
be converted
to
a 70.45
MHz
1st IF
signal.
EXACTNESS
1ST
IF
FREQUENCY
MODE FREQUENCY
(MHz)
SSB
70.4515
CW
70.4506
AM,
FM
70.4500
The
1
St
mixer
circuit
employs a balanced mixer using
low-
noise
junction
FETs (2SK125x2)
to
expand
the
dynamic
range.
The
1st
LO
signal (70.951
5~1
00.451 5
MHz)
enters the
MAIN
UNIT from
the
PLL
UNIT
via J5.
The
signal
is
amplified
at
04,
filtered
by a low-pass
filter,
and
then
applied
to
the 1st
mixer
circuit (Q13, 014).
The
low-pass
filter
employs a
ring
core
inductor to
prevent
1
st
LO
leakage
signals.
The
output
level
from
Q4
is
approx.
25
dBm.
The
1st IF signal
is
applied
to
an
MCF
(Monolithic Crystal
Filter; FI1) to
suppress
out-of-band signals.
The
signal
is
amplified at the 1st
IF
amplifier (015),
and
then
applied
to
the
2nd
mixer
(101).