Circuit Description; Receive Circuits - Icom ic-751a Instruction Manual

SECTION
11
CIRCUIT DESCRIPTION
11-1
RECEIVE CIRCUITS
(1)
RF CIRCUITS
•
RF UNIT
(2)
IF
CIRCUITS
• RF UNIT
•MAIN
UNIT
The
receive signal
from the antenna connector
is
fed to
J10
of the
RF
unit
in
the receiver circuit
when D4
is
turned
OFF
and
RL1
on
the
CONNECTOR
unit
is
turned
ON.
The
incoming
signal
to the
RF
unit passes
through an
"L"
attenuator,
consisting of
R92 and R93
on
the
RF
unit,
for
20dB
of attenuation
when
the
PREAMP/ ATT
SWITCH
is
set
to the
ATT
position.
The
BPF
switching voltage
is
obtained with
IC1
and IC2 by decoding
the B1
~
B-1 1
band
signals
from
the
LOGIC
unit.
The
ON/OFF
switching voltage
is
provided by
IC2.
R13V
is
pro-
vided
from
the
OR
gate consisting
of
D5
and D6.
The
attack time of
this control voltage
is
determined by
R9
and RIO. The immediate
release
is
provided by D1 and
D2 when
switching to the transmit
mode.
09
and 01
0 comprise
a
double-balanced mixer
using low-noise
2SK125
J-FETs.
The
mixer, driven with 13.8V, provides an
excellent noise figure
and converts the incoming
signal
to the
70.4515MHz
first
IF
signal.
The
first
LO
output
signal
from
the
PLL
unit
is
fed
through
a
high-pass
filter,
is
amplified
by 02,
is
filtered
by
a
low-pass
filter,
and
then
is
applied to the
first
mixer
as
its
local oscillator signal
(70.5515
~
100.4515MHz).
R18,
L13
and Cl 4
are for
feedback
to
improve
the
frequency
characteristics of Q2.
The
first
IF signal
is
filtered
by
the FI1 monolithic
crystal filter
(±7.5kHz/-3dB) and
is
then amplified by
the
08
dual-gate
FET.
The
second gate of
08
is
controlled by the
AGC
voltage.
The
signal
is
fed through the
D19 T/R
switching
diode and
a
high-pass
filter
to the second IF
mixer
of the
IC3 double-balanced
mixer where
the
signal
is
converted to the
9.0115MHz
second
IF
signal.
The
signal
is
then
filtered
by
a
low-pass
filter
to
remove
the
local oscillator
components and
is
fed to the
MAIN
unit
through
P3.
The
second
LO
signal
(61.44MHz) from
the
PLL
unit
is
fed to IC3
as
the
local oscillator signal for
the second mixer.
The
IF signal
from
the
RF
unit passes through
a noise
blanker
circuit,
and
is
supplied to a
9MHz
filter
through the
033
amplifier.
The
signals
always
are
passed through the noise blanker
which
is
con-
trolled
by
a
noise blanker gate consisting of
D68
~
D71
called a
double-balanced diode
switch.
If
the noise blanker detects pulse
noise, the blanker puts
out
a
control voltage via
D72
which
momen-
tarily cuts off
the
signal.
The
9MHz
filter
circuit consists
of FI2,
FIS and
a
bypass
circuit.
The
signal
from
the
filter
circuit
is
supplied to the
IC10 mixer
through the
084
buffer amplifier,
and
is
converted to
a
455kHz
signal.
The
signal
is
then
fed
through
039
to
a
455kHz
filter
circuit
consisting
of
FI3, FI4, FIB,
FI6 and an
optional
filter.
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