Icom ic-775 Service Manual page 17

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