Transmitter Circuits - Icom IC-2GA Service Manual

4-2
TRANSMITTER
CIRCUITS
(1)
MICROPHONE
AMPLIFIER (MAIN
UNIT)
AF
signals
from the
built-in
condenser microphone
or
from
the external
microphone jack are amplified
at a limiter
amplifier (IC205),
and
are
pre-emphasized
to
6 dB/octave
(300 kHz).
Pin
1
outputs
a
square
wave
which
con-
tains
many
high
harmonic components. The
signals
pass
through the
splatter
filter in
IC205
where
signals of 3
kHz
and above
are attenuated.
The
signals
from
pin 7 are
applied to the
VCO
UNIT
for
frequency modulation
(FM).
(2)
DRIVE AMPLIFIER
(RF UNIT)
The
VCO
output
is
buffer amplified
at
Q12. Passing
through D16, the signals are then amplified
at
the drive
amplifier (Q10).
(3)
RF
POWER
AMPLIFIER
(RF
UNIT)
Q10
output
Is
amplified
at
IC3 to 7
W
(HIGH) or
1
W
(LOW).
When
transmitting, the
RF signals amplified
at
IC3
pass
through
the
antenna
switching
diode
(D15)
and
the
low
pass
filter
where harmonic
signals are
rejected.
The
signal
is
then applied
to
the
antenna
connector.
(4)
APC
CIRCUIT
(Automatic
Power
Control)
(RF UNIT)
IC3
output passes through the
APC
detector circuit (D13,
D14,
L15,
etc.)
where
its
RF output
level
is
detected.
The
detected output,
compared
at
the
differential
amplifier
(Q13) to
the reference voltage
output
of
the
power
set
circuit,
is
applied to
the base
of
015.
When
the
antenna
is
matched
at
50 O, the detected output
is
at a
minimum.
When
mismatched, the detected output
increases.
The
current
at
the
collectors of
Oil and
Q15, as
well as the gain of the driver (Q10),
decreases
until
the
detected output
and the reference voltage
become
equal.
In
this
way, the
power module
(IC3)
is
protected
from
damage.
017
acts
as
a
switch for
the reference voltage
used
in
the
APC
circuit. In
this
way,
HIGH/LOW
transmit output
power
switching
is
accomplished.
APC
CIRCUIT
RX
1st
mixer
current
is
controlled
4-3
PLL CIRCUITS
(1)
GENERAL
The PLL
circuit,
using
a
one-chip
PLL
1C (IC2), directly
generates the desired frequency. IC2 sets the dividing
ratio
based on
serial
data
from
the
CPU
(IC501),
and compares
the phases
of
the
VCO
signal
and
the reference
oscillator
frequency.
It
detects the out-of-step
phase and outputs
it.
(2)
REFERENCE OSCILLATOR
CIRCUIT
(RF
UNIT)
X3
oscillates a
10
MHz
signal.
IC2 divides
this
signal with
N-data
from
the
CPU.
In
this
way, one
of
six
tuning steps
(5,
10, 12.5, 15, 20,
25 kHz) can be
selected.
(3)
CHARGE
PUMP AND
LOOP
FILTER
CIRCUITS
(RF UNIT)
Phase-detected signals
from
pins 5
and 12 are converted
to
DC
voltage
by
the charge
pump
(Q6'^Q8)
and
the lag-lead
loop
filter
(R48-R50, C81, Cl 12
and
Cl
13).
The frequency
at
which
the
VCO
oscillates
is
controlled
by
varactor diodes (D703, D704).
DC
voltage (PLL lock voltage)
is
provided through the loop
filter.
The
output
of
the loop
filter
passes through Q9, and
is
used
to control
the
bandpass
filter
(D3,
D5, D6)
in
the receiver
RF
circuit.
When
the
power
save
function
is
on, the
charge
pump
control
circuit
(020—022, D18)
cuts off
the charge
pump
and
the
VCO.