Icom IC-W2A Service Manual page 8

3-2
UHF RECEIVER
CIRCUITS
3-2-1
ANTENNA
SWITCHING
CIRCUIT
(CONNECTOR AND UHF
RF
UNITS)
The
received signals enter the
CONNECTOR
UNIT
and
are
divided
into
VHP RF
signals
and
UHF
RF
signals.
The
UHF
RF
signals
pass
through the high-pass
filter
(L1
,
L2,
C2-C6)
and are applied
to
the
bandpass
filter
(L11~L13,
L20,
C42-C49). These
UHF
RF
signals
are then applied
to
the
antenna
switching
circuit.
While
receiving,
D19
turns
OFF, and the receive signals are
applied
to
the
URF A
UNIT through a low-pass
filter
(L9,
L10,
C39, C41, D21, D22).
3-2-2
RF
AND
1st
MIXER CIRCUITS
(URF
A
AND
UHF RF
UNITS)
The
UHF
RF
signals are amplified
at
02
and 01
,
and then are
converted
to
a
1st IF signal at
02.
The
frequency
of the 1st IF
signal
is
35.800
MHz.
02
on
the
UHF RF
UNIT
converts the
RF
signal
using the
U
PLL
output
coming from 09.
The
UHF
RF
circuit
employs a
helical coil
bandpass
filter
circuit (LI
,
L2)
to
suppress out-of-band
signals.
3-2-3
IF,
AF
AND SQUELCH
CIRCUITS
These
are the
same
as the
ones
commonly
used
with
the
VHF
receiver
circuit.
Refer
to
SECTION 1-1-3-1-1-5
for
de-
tails.
3-3
VHF TRANSMITTER
CIRCUITS
3-3-1
MIC
CONTROL
CIRCUIT (LOGIC
UNIT)
The
mic
control
circuit
controls the external or internal audio,
DTMF
tone
and
TONE
CALL
signals.
An
audio signal
from
the external
microphone
is
applied
to
the
microphone
control
circuit
(IC17, IC18, 01, 02).
The
audio
signal
passes
through the attenuator
(R19—R21) and
is
con-
nected
to
the
internal
microphone.
The
audio
signal amplified
at
IC20
is
applied
to
the
VHF RF
UNIT.
When
the
DTMF
tone
or
TONE
CALL
signal outputs
to
the
mic
amp
circuit,
05
turns
OFF
the
audio
signal to
prevent
malfunctioning.
3-3-2
MIC
AMP
CIRCUIT (VHF
RF
UNIT)
The
mic
amplifier circuit
amplifies audio signals with 6 dB/oct.
of
pre-emphasis from
the
microphone
to
a
level
needed
for
the
modulation
circuit.
The
mic
amplifier
circuit
has a low
level
amplifier with pre-
emphasis
(IC9a)
and
a
limiter
amplifier (IC9b).
3-3-3
MODULATION
CIRCUIT (VHF
RF
UNIT)
The
modulation
circuit
modulates the
VCO
oscillating signal
(RF
signal) using the
microphone audio
signal.
The
audio
signal
from IC9
(pin
1)
changes
the reactance
of
a
varactor
diode (DIO)
to
modulate
the
oscillated signal at
the
TX
VCO
(Q23, Q24).
The
oscillated signal
is
buffer-amplified
at
Q25
and
Q26
and
is
then applied
to
the
drive amplifier
circuit.
3-3-4
DRIVE
AND POWER
AMPLIFIER CIRCUITS
(VHF
RF
UNIT)
IC1
1
is
a power module which
provides
a
stable
5
W
(DC13.5
V)
of
output power.
The
drive amplifier (Q41)
and power
amplifier (IC1
1)
amplify
the
VCO
oscillating
signal to
an
output
level.
The
output
signal
passes through
the
ARC
circuit
(D17-19) and
bandpass
circuit
and
is
applied
to
the
antenna connector.
Collector voltage
for
the drive amplifier
is
controlled
by the
ARC
circuit.
3-3-5
APC
CIRCUIT (VHF
RF
UNIT)
An
ARC
circuit
protects the
power module
(IC11) from
a
mismatched
output ioad
and
selects
either
HIGH
or
LOW
output power.
The
ARC
circuit
(D17-19,
L19,
R119, R121, C138, C139)
detects the output
signal
from
the
power module
(IC1
1
,
pin
5).
035
compares
the
ARC
detected voltages
and
reference
voltages.
When
the
ARC
detected voltages are higher than
the reference voltages,
035
reduces
the collector
voltage
of
Q36
and 037.
This voitage
is
applied
to
the
drive amplifier
(041)
to
reduce the
RF
output power.
R123 L20
VHF
APC
CIRCUIT
3
—
2