4- Uhf Receiver Circuits; Vhf Transmitter Circuits - Icom IC-24AT Service Manual

3-2
UHF RECEIVER
CIRCUITS
3-2-1
ANTENNA
SWITCHING
CIRCUIT
(U-SW
UNIT)
The
antenna
switching
circuit
is
mainly the
same
as
the
VHP
band. Refer
to
Section 3-1
-1 for details.
The
following parts
are
different
from the
VHP
band:
The
UHP
signals
pass through the high-pass
filter (LI,
L2,
Cl ~C5
on the
HPP
UNIT) and
the
antenna
switching
circuit
(L1. L2,
C1~C3,
D1 on
the
U-SW
UNIT).
3-2-2
RF
AND
1ST MIXER CIRCUITS
(U-RF UNIT)
The
RP
and
1st
mixer
circuits
are located
on
the
U-RP UNIT
where
the
signals are amplified
at
Q1
and
Q2
,
and
are
then
converted
to
a
1st
IP
signal
at
Q3.
A
frequency
of
the
1st IP
signal
is
30.875
MHz.
Q3
converts the
RP
signal
using the
U-PLL
output
coming
from the
"HLO"
line.
The
UHP RP
circuit
adopts 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
VHP
receiver
circuit.
Refer
to
Section 3-1-3~3-1-5
for
details.
3-3
VHF TRANSMITTER
CIRCUITS
3-3-1
MIC
AMP
CIRCUIT (MIC UNIT)
The
mic
amplifier circuit
amplifies
audio signals with
6
dB/oct.
pre-emphasis from
the
microphone
to
a
level
needed
for
the
modulation
circuit.
The MIC UNIT
has a low
level
amplifier with
pre-emphasis
(IC3a)
and a
limiter
amplifier (IC3b).
3-3-2
MODULATION
CIRCUIT
(V.PLL UNIT)
The
modulation
circuit
modulates the
VCO
oscillating signal
(RP
signal)
using the
microphone
audio
signal.
The
audio
signal
from the
MIC UNIT
(VMOD
signal)
changes
the
reactance
of
a
varactor
diode (02)
to
modulate the
oscillated signal at
the
TX
VCO
(Q5).
The
oscillated signal
is
buffer-amplified
at
Q6
and Q8, and
is
then applied
to
the
drive amplifier
circuit
on
the
MAIN UNIT
via
the
"VLO"
line.
3-3-3
DRIVE
AND POWER
AMPLIFIER
CIRCUITS (MAIN UNIT)
The
drive amplifiers (Q8,
Q9) and power
amplifier (IC1)
amplify the
VCO
oscillating signal to
an
output
level.
Collector currents
of
the drive amplifiers
are
controlled
by
the
APC
circuit (refer to
Section 3-3-4
for details).
3-3-4
APC
CIRCUIT
(APC
AND HPF
UNITS)
The
APC
circuit
selects high or low output
power and
protects the
power
amplifier
from
antenna mismatch
impedance.
D1 on
the
LPP UNIT
detects progressive
wave and D3 on
the
HPP
UNIT
detects
reflecting
wave.
When
the
antenna
is
matched
at
50
£i,
the
combined
voltages
of
progressive
and
reflective
detection
(POET
voltages) are
minimum
values.
The
PDET
signal
is
applied to the
APC
UNIT.
The
APC
UNIT
contains the
APC
control
circuit (Q2,
Q4, Q6,
Q7) and
the
UHP
transmitter
RP
circuit
(Q1, Q5).
Q4
compares
the
PDET
voltages
and
reference voltages.
When
the
PDET
voltages are higher than reference
voltages,
Q4
reduces
collector
currents
of
Q2
on
the
U-YGR
UNIT. These
currents are applied
to
the drive amplifiers
(Q8,
Q9
on
the
MAIN
UNIT)
via the
"APD"
line to
reduces
the
RP
output power.
HIGH
or
LOW
output
power
is
determined
with the reference
voltages
of
the
APC
circuit. IC1
(
pins
6
,
7
)
on
the lO
UNIT
are
used
as the
power
selector ports
and
R4
and
Q3
on
the
U-YGR UNIT
are
used as
reference voltage adjustment.
APDC
PDET
Po
Vcc
3-2