Transmitter Circuits - Icom IC-2000 Service Manual

The
AF
OUT
signals
from
the
VOL1
line
are
also amplified
at
the
AF
amplifier
(LOGIC
unit 02) to
output
AF
signal
from the
microphone connector.
3-1-5
SQUELCH
CIRCUIT
(MAIN and
LOGIC
UNITS)
A
squelch
circuit
cuts out
AF
signals
when
no
RF
signals
are being received.
By
detecting noise
components
in
the
AF
signals,
the squelch
circuit
turns the
AF mute
switch
OFF.
A
portion of
the
AF
signals from the
FM
IF
1C (IC4 pin
9)
are applied
to
the active
filter
(104 pin 10)
through the
noise
filter
(0145
—
0147,
R128).
The
active
filter
section
in
104 amplifies noise
components
of
frequency
of
20
kHz
and
above,
and are
rectified at
the noise detector (D26)
and
then triggers the noise switch
(Q45).
3-2-3
DRIVE/POWER AMPLIFIER CIRCUITS
(MAIN
UNIT)
The
signal
from
the
modulation
circuit
is
passed through
the transmit/receive switching
circuit
(D2)
and
amplified
at
the pre-driver (Oil), driver (012),
and
the
power module
(102) in
sequence
to
obtain 50
W*
(at
13.8
V
DC)
of
RF
power.
*
10
W
for
the IC-2000 Thailand
version.
The
amplified signal
is
passed through the
antenna
switching circuit (D7),
ARC
detector
circuit (L10,
D8,
D9),
and
low-pass
filter
(L11, LI
2,
061, 062)
and
is
then
applied
to
the
antenna connector.
The
collector current of the driver
(012) are controlled
by the
ARC
circuit to
protect the
power module from a
mismatched
condition
as
well
as
to stabilize
the output
power.
The noise switch (045) converts the
rectified signals to
a "High"
or
"Low"
signal
and
applied
this to
the
CRU
(LOGIC
unit 102 pin
5)
as
the
busy
signal.
When
the
CRU
receives "High," the
CRU
outputs the
mute
signal
through the
Data expander
1C
(MAIN
unit IC9,
pin 11)
to
cut the
AF
signals at
the detector
mute
switch
(037).
3-2-4
APC
CIRCUIT (MAIN
UNIT)
The
ARC
circuit
protects the
power module
(IC2)
from
a
mismatched
output load
and
selects High or
Low
output
power.
Even
when
the squelch
is
closed, the
AF mute
switch
(040)
opens
at
the
moment
of emitting
been
tones.
3-2
TRANSMITTER
CIRCUITS
3-2-1
MICROPHONE
AMPLIFIER
(LOGIC
UNIT)
The
ARC
detector circuit (L10,
D8, D9) detects forward
signals
and
rectified signals at
D8 and
D9
respectively.
The combined
voltage
is
at
a
minimum
level
when
the
antenna
is
matched
at
50
O
and
is
increased
when
it
is
mismatched.
The
detected voltage
is
applied
to
the inverting amplifier
(IC3a)
to
control the
base
of
016 and
input
current
of
IC2
(pin 2)
and
012
using
013.
The microphone
amplifier circuit
amplifies audio signals
with -F6
dB/octave pre-emphasis characteristics
from
the
microphone
to
a
level
needed
for the
modulation
circuit.
The
AF
signals
from
the
microphone
through the
mic mute
switch
(04),
and
are amplified
at
the
microphone
amplifier
(05)
and
the limiter amplifier (IC1 a)
which has a negative
feedback
circuit for -1-6
dB/octave
pre-emphasis.
The
amplified signals
are applied
to
the low-pass
filter
(ICIb)
to
filter
out
RF components and
then applied
to
the
MAIN
unit
as
the
"MOD"
signal.
3-2-2
MODULATION
CIRCUIT (MAIN
UNIT)
The modulation
circuit
modulates
the
VCO
oscillating
signal
(RF
signal)
using the
microphone audio
signals.
The
audio
signals
(MOD) change
the
reactance
of
D19
on
the
VCO
circuit (022,
023, D19)
to
modulate
the oscillated
signal at
the
VCO
(022, 023).
The
oscillated signal
is
amplified
at
the buffer amplifier
(024)
and LO
amplifier
(025, 026), then applied
to
the drive amplifiers.
When
the
antenna
impedance
is
mismatched,
negative
input
voltage of the
inverting amplifier (IC3a) increases,
and
the
base voltage
of
016
and
the collector current of
013
decreases
to
reduce the output
power.
•
APC
CIRCUIT
3-2