Transmitter Circuits - Icom IC-2SET Service Manual

4-1-6
AF
AMP
CIRCUIT
(AF
AND
VR
UNITS)
The
AF
signal
is
applied to
Q1 and
Q2
on the
AF
UNIT.
Q1
is
an
active
filter
that
functions
as a high-pass
filter
to
suppress tone signals for
the
tone squelch operation.
Q2
is
also
an
active
filter
that
functions
as a low-pass
filter
to
suppress
higher noise
signals.
The
filtered
signal
is
applied
to
the
[VOL] control (R1)
on the
VR
UNIT
via
the
AF
mute
circuit (Q3).
When
the
squelch
is
closed,
Q3
cuts the
AF
signal
as the
AF
mute
switch.
The
AF
signal
is
power-amplified at
the
AF
power
amplifier (ICt) to
drive
the speaker.
The
AF
voltage regulator
(Q4~Q6) supplies
power
to
the
AF
power
amplifier.
The
AFS
signal
from
the
MAIN
UNIT
controls
Q6 and mutes
AF
output while receiving
no
signal or
no specified
tone/DTMF
signal.
4-1-7
SQUELCH
CIRCUIT (DET
UNIT)
Some
of
the noise
components
in
the
AF
signal
from
IC1
pin 11
are applied to IC1 pin
13
via
C11, R8,
Cl
3
and
C14.
The [SQL]
control (R2)
on
the
VR
UNIT
adjusts the
pin
13
input level.
The
active
filter
section
in
IC1 amplifies
noise
components
of
frequencies
of
20
kHz and above,
and
outputs the
resulting
signals
from
pin 14.
Output signals
are
rec-
tified
by D1 and are converted
to
DC
voltage.
The
rectified
voltage triggers
the
squelch switch
(Q1).
The
coliector
of
Q1 outputs the squelch
signal.
The
signal
is
applied to
the
CPU
(IC1, pin
27)
on
the
LOGIC
UNIT
through the
BUSY
signal line.
The
CPU
outputs the
RMUTE
and
BUSY
LED
signals.
The
RMUTE
signal,
decoded
at
the output
expander
(IC1)
on
the 10
UNIT, activates the
AF
mute
circuit
(03)
on
the
AF
UNIT
to
cut the
AF
signal.
The
BUSY LED
signal
is
applied
to
01 on
the
LOGIC
UNIT,
turning
OFF
the receive indicator.
4-2
TRANSMITTER CIRCUITS
4-2-1
MICROPHONE AMPLIFIER (MIC UNIT)
AF
signals
from the
built-in
condenser microphone
or
from
the [MIC] jack are applied
to
IC1 pin
3,
and are
pre-emphasized
to
-F6
dB/octave through
C6
and R4
connected to pin
2.
IC1
functions
as the
microphone
amplifier
and the
limiter.
The
output signals
from
IC1 pin
1
pass through
the
splatter
filter
circuit (IC1
pins 5
and
6)
where
signals
of
3
kHz and above
are attenuated. IC1
pin
7
then
outputs the signals.
The
signals
are applied
to
the
modulation
circuit
(PLL UNIT, D2)
in
the
VCO
to
produce
an
FM
signal.
The
VCO
circuit (02,
L2,
D2)
on
the
PLL UNIT
oscillates
the transmit
frequency
with
AF
signal
modulation.
4-2-2
DRIVE AMPLIFIER (MAIN
UNIT)
The
VCO
output, buffer-amplified
at
05
on
the
PLL
UNIT,
is
applied
to
the the transmit/receive switching
circuit
(D14)
on
the
MAIN
UNIT.
The
VCO
output
is
then
amplified at
the predrive amplifier (07)
and the drive
amplifier (06).
The
voltage controlled
by the
APC
circuit
is
applied
to
the
collector
of
06
and
07
to
protect the
RF
power module
from
damage
by an antenna mismatch.
4-2-3
RF
POWER
AMPLIFIER (MAIN
UNIT)
IC1
is
a
power module which provides stable 5
W
output
power.
An RF
signal
from the drive amplifier (06)
is
applied
to
IC1 pin
1.
The
amplified signal
is
output
from
pin
4,
and
applied to
the
antenna connector
through the diode
switching
and low-pass
filter
circuits.
TRANSMITTER
CIRCUIT
BLOCK DIAGRAM
Fig.
2
4
—
2