Icom IC-04A Service Manual page 10

r+5
4-2-3
POWER
AMPLIFIER CIRCUIT
Amplified signals at
Q212
are
power-amplified
at
IC202.
IC202
is
a small-sized
power module
giving stable out-
put
power
of
more than
5W
with
a driving
power
of
150mW.
The output
power from IC202
is
passed
through the
APC
detector
circuit,
the
antenna swit-
ching
circuit,
a
low-pass
filter,
and then
applied to
the
ANTENNA CONNECTOR.
While transmitting the
antenna
switching
circuit
con-
sisting
of
Q218,
D213
and
D214
is
turned
ON
and
L221 and
C284 become
parallel
resonance
circuits
to
prevent
signals
being
applied to the receiver circuits.
Q210
controls
the
bias
voltage of
Q211,
0212
and
IC202
to
prevent
unwanted
emissions
when
switching
from
receive to transmit
mode,
or
when
the
PLL
circuits
are
unlocked.
4-2-4
APC
AND POWER
SET CIRCUITS
(PLL
UNIT)
The
APC
detector
circuit
consists of
L219,
D21
1
and
D212.
When
antenna
impedance
is
matched
at
50Q,
voltage detected
at
D211 and
D212
has
a
minimum
value.
However,
when antenna impedance
is
in
a
mismatched
condition,
the detected voltage
becomes
higher
than
it
is
in
the
matched
condition.
0216
and
0217
form
the
differential
amplifier
circuit.
The
base bias of
021
7
(reference voltage)
is
determin-
ed
by R263,
R264 and R265.
The
voltage detected at
021
1
and D21
2
is
combined by
R245 and R246, and fed to the base of
0216.
When
the
antenna
is
mismatched with the transceiver
the
base
voltage of
0216
is
higher
than the base
voltage of
021
7.
The
0216
collector current
and
021
5
base current are
then reduced, decreasing the
0214
col-
lector current.
This
decreases the output
power
of
0211
and
0212
until
the
base voltage of
0216
becomes
the
same
as the base voltage of
0217.
L219
In
a
matched
condition,
RF
POWER
SWITCH
SI
05
is in
the
"HIGH"
position
and
RF
output
power
can be
ad-
justed
by R264.
In
the
"LOW"
position a series
combina-
tion of
R267
and
R266
is
connected
in
parallel
with
R265. RF
output
power can then be
adjusted
by R267.
The output voltage detected
at
D21
1
and D21
2 passes
through
R272
and
is
applied to
meter comparator
IC105A
on
the
MAIN
UNIT.
4-3
PLL
CIRCUITS
The PLL
circuits
adopt
a
dual
modulus
prescaler
system
.
The
circuits
generate the
desired
frequency
directly
in
the
VCO
circuit.
The PLL
circuits
are
composed
of prescaler
IC203 and
PLL
1C
IC204.
N
data
is
the
number
of
times desired frequency
is
divid-
ed by the reference frequency.
The
desired
frequency
is
transmit
frequency
in
transmit
mode
and
the
1
st
LO
fre-
quency
in
receive
mode.
Desired
frequency
N-data
=
Reference frequency
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4-3