Icom IC-725

4-1-5
IF
CIRCUITS (MAIN UNIT)
The 1st
IF
signal from Q15
is converted to a 9
MHz 2nd
IF
signal at the
2nd mixer
(iCI). IC1 is
a
DBM (Double
Balanced
Mixer).
EXACTNESS
2ND IF
FREQUENCY
MODE
FREQUENCY
(MHz)
SSB
9.0115
CW
9.0106
AM,
FM
9.0100
The 2nd
LO signal (61.44
MHz) from the PLL
UNIT via J4
is applied to
the 2nd
mixer. The converted
2nd iF signal
passes
through D4 (D35
for transmitting) and
is applied
to
the
MCF (FI2) to
suppress unwanted
signals.
The
signal
output from FI2 passes
through the noise
blanker gate
(D5~D8) and is amplified at
the 2nd IF
am-
plifier
(Q21).
The signal
enters
one
of the
three
9
MHz
filters
(FIS,
FI4, optional CW
narrow filter)
or optional AM
•
FM
UNIT via D52.
The filters are selected
with mode
selecting
signals
(SSB
•
CW, AM, CW-N)
and the “T8”
voltage
line.
The signal
from a 9
MHz filter
is
amplified
at
the 2nd
IF
amplifiers (Q27~Q29)
and applied to the
demodulator
circuit.
Dual-gate
FETs are used
on the 1st and 2nd
IF amplifiers
(Q1
5,
Q21
,
Q27).
The
2nd
gates of Q1
5,
Q21
and Q27
are
controlled by
AGC
bias voltage. A rapid
time constant
is used
for Q27 to
prevent
raising the edge
distortion
of
receive signals.
R140,
connected
to the gate of
Q28,
improves the
temperature
characteristics
of the
receiver gain.
R138
adjusts the
receiver gain.
4-1-6
NOISE
BLANKER
CIRCUITS
(MAIN UNIT)
The IC-725
uses a
noise
trigger noise
blanker circuit that
cuts out
pulse-type
noise signals at
the noise
blanker gate
(D5-D8).
A
portion of the
signals from
FI2 is
amplified
at
the noise
amplifiers
(Q16,
IC2)
and detected at
the noise
detector
(D1
2,
D1
3).
The detected
voltage
from
the
noise detector
is
applied to the
noise blanker
switch
(Q1
9).
The
threshold
level of the
noise blanker
switch is set at
0.9 V.
When the detected
voltage exceeds
the threshold
level,
Q20
outputs a
blanking signal
to
activate
the noise
blanker gate
(D5~D8).
A
portion of the
detected
voltage is applied to
the noise
AGC
circuit
(Q18)
and fed
back
to
the noise
amplifier (IC2)
as
noise AGC
voltages. The
time constant
of the
noise
AGC
circuit is
determined by
R43, R47 and C60.
This
AGC circuit does
not
operate
to
detect pulse-type
noise.
When
the
operating
frequency or
mode is changed,
the
“DNB”
signal line
becomes
“LOW,”
turning
Q20
ON. The
noise
blanker gate
prevents PLL
click noise.
4-1-7
BFO
CIRCUIT
(MAIN UNIT)
A 9
MHz signal
oscillated at
the BFO circuit
(031,
XI)
is
buffer-amplified at
Q42
and used at the
balanced
modulator
(IC6) and a
product
detector (ICS).
The BFO
frequency is
shifted
with
a
mode
signal using
D67~D69.
In
USB
mode, the
“USB” signal
line becomes
“HIGH,”
turning ON D69.
The
frequency is then adjusted
with
C294 to
set
the USB
carrier point.
At CW mode
transmitting, the “CW”
signal line
becomes
“HIGH”
and
033
becomes OFF,
turning ON D68.
The
frequency
is then adjusted
with L83 to
set the
CW
transmit
carrier point.
In
LSB
mode, the
“LSB” signal
line becomes
“HIGH,”
turning
ON D67.
The frequency is
then adjusted
with
L82 to set
the LSB
carrier point.
IF
CIRCUIT
062
4
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2