Icom IC-775

4-2-4
FM
AND AM
MODULATION CIRCUITS
(IF
UNIT)
(1)
While in FM
mode
The
microphone
signals from
ICfOa are
amplified at IClOb
and
pass
through the
limiter
amplifier (ICIfa)
and the
splatter
filter
(ICIIb).
The
subaudible tone
signal (67.0-
254.1
Hz) from
the PLL
unit is also
applied to ICIla
pin 2
for
repeater
use.
The
resulting
signals
are applied to
the
modulation circuit
(D65)
via R292.
The
modulation circuit
changes the
reac-
tance of
the
local
oscillator
(Q44,
X3) to
obtain FM
modu-
lation. The
modulated
signal is applied to
the
transmitter IF
amplifier
(Q78)
via
IC1
2.
(2)
While in AM
mode
The
microphone
signals from
ICIIb are
applied to the
balanced
modulator (IC12)
and upset the
balance to
obtain
AM
modulation. The
local
oscillator
(Q44,
X3)
oscillates a
9.0115 MHz
signal
\with no
FM modulation.
4-2-5
RF
SPEECH
COMPRESSOR
CIRCUIT
(IF
UNIT)
The
RF
speech
compressor
compresses the
transmitter IF
signal to
increase the
average talk power.
When
the [COMP]
switch is ON,
the IF
signal from
a
455
kHz
filter is
buffer-amplified at
033
and is
then
applied to
the
RF
compressor
circuit
(035,
D31).
The
resulting
signal
is
applied to
the IF
amplifier
(036).
The 2nd
gate of
036
is
controlled by
the
[DRIVE]
control (VR-A unit R22).
When
the
[COMP]
switch
is OFF, the
IF signal
bypasses the
compressor
circuit
via
D27
and D29.
The
resulting
signal is
mixed
with 9.4665 MHz LO
signal at
IC19
to obtain
a
9
MHz
IF signal.
4-2-6
IF
AMPLIFIER
(IF AND RF
UNITS)
The
9
MHz IF
signal from
IC19
passes through the
9
MHz
IF
filter (FI2)
in
CW/RTTY modes or
in SSB mode
with
[COMP]
ON.
The IF signal
bypasses the IF
filter in SSB
mode
with [COMP]
OFF
and in
RTTY DATA mode.
The
signal
is
amplified at
078,
passes
through FI9 to
suppress
the
unwanted
signal and is
applied to a
transmit IF
ampli-
fiers
(045).
In FM
or AM mode,
the
9
MHz IF
signal from
IC12
passes
through
FI9 and is then
applied to the
transmit
IF
amplifier
(045).
The
amplified
signal from Q45
is mixed with
the 2nd LO
signal
(60
MHz)
and
converted to a
69
MHz IF
signal at
IC34.
IC34
is used
in
receiving and transmitting.
The
69
MHz IF
signal
enters the RF
unit, is
amplified at the IF
amplifier
(016)
and is then
converted to the
displayed
frequency at
the
balanced mixer
(017, 018)
with the
1st
LO
signal.
The
gates
of the IF
amplifiers (IF unit
045,
RF unit
016)
are
controlled
by ALC
bias
voltage from the ALC
circuit. R23
adjusts
the total
transmitter gain.
4-2-7
RF
CIRCUIT
(RF AND PA
UNITS)
The RF
circuit amplifies the
displayed
frequency signal to
obtain 200 W
of RF output
power.
The signals
from the
balanced mixer
(017, 018)
is ampli-
fied at
01 9
and
then pass
through one of
9
bandpass
filters.
(Refer to
p.
4-1
for used RF
filter.) The filtered
signal
is
amplified
at
032,
and
is then applied to the
PA unit.
The signal
which enters
the PA unit is
amplified at the
predrive
amplifier
(01),
drive
amplifier
(02, 03)
and power
amplifier (04a, 04b)
in sequence to
obtain a
stable
200
W
of RF
output power.
The signal from
the power amplifier is
applied to
one of the
low-pass filters in the
FILTER unit.
4-2-8
LOW-PASS
FILTER
CIRCUIT
(FILTER
UNIT)
The
low-pass filter
circuit consists of 7
Chebyschev
low-
pass
filters to
suppress the
higher harmonic components.
The
signal from the
power amplifier
is applied to one of
the
low-pass
filters..
According to the
operating frequency, the
I/O
expander (IC1)
selects a
low-pass filter using IC2.
The
filtered signal is
then applied to
one of 2 antenna
connectors
via the
antenna tuner circuit.
4-2-9
ALC CIRCUIT
(IF
UNIT)
The ALC
(Automatic Level
Control) circuit
controls the gain
of IF
amplifiers in order
for the
transceiver to output a
constant RF
power set by
the [PWR]
control even when the
supplied
voltage shifts, etc.
The RF
power signal level is
detected at D2 (CTRL
unit)
and applied to
the IF unit as
the “FOR” voltage.
The “FOR”
voltage from
the CTRL unit is
applied to IC3a
(pin
2).
The “POC”
voltage,
controlled by the
[PWR] control
(VR-A
unit R40), is
applied to IC3a (pin
3)
as the reference
voltage.
When
the “FOR” voltage
exceeds the “POC”
voltage, ALC
bias voltage
from IC3a (pin
1)
controls the IF
amplifiers
(045,
RF unit
016)
using 03.
This adjusts the output
power to the
determined
level
by the
[PWR] control until the
“FOR” and
“POC” voltages
are equalized.
In AM mode, IC3a
operates as an
averaging ALC amplifier
with
Q5
and
C223.
07
turns ON and
the “POC” voltage is
shifted for 50
W AM output
power
(maximum) through
R362.
The
ALC bias
voltage from
Q3
is
also applied to the
inversion-amplifier (IC3b
pin
6)
to obtain
an ALC meter
signal. It passes
through the meter
switching circuit (IC6
pin
12)
and is then applied to
the S/RF meter. IC6
pins
3
aknd 12
are shorted while
transmitting and
when the ALC
meter is selected with
the [METER] switch.
4-6