Uhf Transmitter Circuits; Pll Circuits - Icom IC-24AT Service Manual

3-4
UHF TRANSMITTER
CIRCUITS
3-4-1
MIC
AMP
CIRCUIT
(MIC UNIT)
This
is
the
same
as
the
one commonly
used
with
the
VHP
transmitter
circuit.
See
Section
3-3-1
for details.
3-4-2
MODULATION
CIRCUIT
(U*PLL
UNIT)
The
audio
signal
from the
MIC UNIT (UMOD)
changes
the
reactance
of
a diode(03)to modulate
the oscillated signal
at
the
VCO
(Q11).
The
oscillated signal
is
amplified
at
the
buffer amplifiers
(Q2,
Q3
on
the
U«PLL UNIT and
Q7
on
the
MAIN
UNIT) and
is
then applied
to the drive amplifier circuit
on
the
APC
UNIT.
3-4-3
DRIVE
AND POWER
AMPLIFIER
CIRCUITS
(APC
AND
U*YGR
UNITS)
The
drive amplifiers (Q1 on
the
APC
UNIT
and
Q1
on
the
U*YGR
UNIT) and power
amplifier (IC1
on
the
U*YGR
UNIT)
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).
3-4-4
APC
CIRCUIT
(APC
AND
HPF
UNITS)
The
APC
circuit
of the
UHF
band
is
mainly the
same
as
the
VHP
band. Refer
to
Section 3-3-4
for details.
The
following
parts are
different
from the
VHP
band:
Reflective detector
:
D4
on
the
U*YGR
UNIT
Drive
amplifiers
:
Q1 on
the
APC
UNIT,
Q1
on
the
U*YGR
UNIT
Reference
voltage adjustment:
R3
and
Q4
on
the
U*YGR
UNIT
3-5
PLL CIRCUITS
3-5-1
GENERAL
DESCRIPTION
A
PLL
circuit
provides stable oscillation of
the transmit
frequency and the receive
LO
frequency.
The PLL
circuit
compares phases
of
the divided
VCO
frequency
and
reference frequency.
The PLL
output frequency
is
controlled
from a
crystal oscillator
and
the divided
ratio
(N-
data) of
a
programmed
divider.
3-5-2
VHF
VCO
CIRCUIT
(V*PLL UNIT)
The
VCO
circuit
oscillates
the
PLL
output
frequency
controlling
a varactor diode.
The V*PLL UNIT
separately
has
receiver
VCO
(02)
and
transmitter
VCO
(05)
for
wide range
oscillation
and
clear
oscillation
respectively.
01
switches the
VCOs.
3-5-3
UHF
VCO
CIRCUIT (U*VCO
UNIT)
Q1
1
oscillates
both the receive
LO
and
transmit frequency.
When
receiving, IC1
(pinlO
)
is
grounded and
turns
D2
ON
to
shift
the
VCO
oscillating
frequency.
3-5-4
PHASE DETECTOR
CIRCUIT
(V-PLL
AND
U*PLL
UNITS)
The
phase
detector
circuit
detects the
off
phase com-
ponents
of
the
VCO
frequency using
a
stable reference
frequency.
Both ICIs on
the
V-PLL and U*PLL
UNITS
are the
500
MHz
one-chip
PLL
ICs that
contain
a two-modulous
prescare,
swallow
counter,
programmable
divider
and phase
detector.
The
VCO
oscillated signal
is
amplified at
07
(VHP
band; on
the
V*PLL
UNIT)
or
Q1 (UHF
band;
on
the
U-PLL
UNIT) and
is
then
applied to
the
PLL
1C (pini
).
In
IC1,
the entered signal
is
divided
at
a prescaler
and
programmable
divider sections.
A
reference frequency
is
also divided
at
a
divider section.
Both
of
the divided
signals
are
compared
at
a phase-detector section
to
output
their
differential signal.
3-3