Pll Circuits - Icom IC-T7A Service Manual

An
RF
signal
from the
band
switch
(D351,
D352)
is
buffer-amplified
at
Q201
(for
VHF)
or
Q701
(for
UHF) and
then applied
to
the
drive amplifier
(PA board Q402)
via
the
other
band
switch
(D201, D701).
The
applied
RF
signal
from
the
band
switch
is
amplified at
a
drive amplifier (Q402)
and then
amplified
again
at
the
power
amplifier (Q403).
The
amplified
RF
signal
is
passed though
the
low-pass
filter
(VHF)
or
high-pass
filter
(UHF), and then applied
to
the
antenna connector
via
the transmit/receive switching circuit
(D51,
D52,
D202
for
VHF.
D551, D552, D723,
D724
for
UHF).
4-2-4
APC
CIRCUIT
(IF
AND
2F UNITS)
The
APC
circuit stabilizes
transmit output
power and
selects
HIGH and
LOW
output power.
The
APC
circuit
consists of
APC
sensor,
APC
control
(IF
unit)
and
APC
set
(2F
unit)
circuits.
The
APC
sensor
circuit
(IF
unit
R250)
detects
a
driving
current
from a
drive
voltage
at
the
PA
board.
The
detected
current
is
applied
to
the ope-amplifier 1C
(1
F
unit
IC250
pin
2)
in
the
APC
control
circuit,
and compared
with
a
"PSET"
voltage
which
is
supplied
from the
APC
set
circuit
(2F
unit
IC301
).
The
output voltage
from
pin
1
of
IC250
is
applied
to
the
APC
control circuit
(IF unit
Q255
base)
to
control
"VGGC"
voltage.
The "VGGC"
APC
control
signal
is
separated
for
VHF
(VGG1) and
UHF
(VGG2) by
resistors.
The
VGG1
line
is
for
the
APC
control signal for
the
drive amplifier
and
the
VGG2
line
is
for
the
power
amplifier.
Low
output
power
is
obtained
by changing
the
"PSET"
voltage
coming from
pin
1
of
IC301
on
the
2F
unit.
The
"PSET" voltage
is
controlled
by
power
set pots
(R302
for
VHF, R304
for
UHF)
and an "H/L" signal via the
CPU
(LOGIC
unit pin
56).
A
thermistor (R266) controls
APC
reference voltage
("PREF"
voltage)
to
reduce the output
power
when
the
temperature
is
increased.
4-2-5
ANTENNA SWITCHING
CIRCUIT
(1FUNIT)
The antenna
switching
circuit
applies receive signals to
the
receiver circuit
and transmit signais
to
the
antenna connec-
tor.
VHF ANTENNA SWITCHING
CIRCUIT
When
transmitting,
D51,
D52 and D202
are turned
ON.
The
signal
passes though
the
low-pass
filter
(L11, L12,
C16-C22) and
is
then applied to the
antenna
connector.
The
low-pass
filter
suppresses high
harmonic components.
UHF ANTENNA
SWITCHING CIRCUIT
When
transmitting,
D723, D724,
D551 and
D552
are turned
ON. The
signal
passes though
the
low-pass
(L8, L9,
C10-C14) and
high-pass (L2, L3,
C1-C5)
filters
and
is
then
applied
to
the
antenna connector.
The
high-pass
filter
suppresses low harmonic components.
4-3
PLL
CIRCUITS
4-3-1
VHF
PLL CIRCUIT
(1
F UNIT)
The
oscillated signal at
the
VCO
circuit
(DUAL
VCO
board
0304, 0305, D303)
is
ampiified at
a
buffer-ampilfier (0306)
and
is
again amplified
at
another
buffer-amplifier (0352).
The
amplified signal
is
applied to the
PLL
1C (IC851
pin
2),
and
then divided
by
serial
data from the
CPU
and
phase-detected
with
the divided reference frequency.
The
phase
difference
is
output
as
pulses.
The
output
signals
from IC851
(pin 8)
are converted
to
DC
voltages
(lock
voltage)
by
the loop
filter
(R366,
C362) and
are then fed
back
to
the
VHF
VCO
circuit
to stabiiize
the
VCO
frequency.
The
DC
voltage
is
also applied
to
the receiver tuned
bandpass
filters
as a
"VTUNE"
signal.
PLL CIRCUIT
VHF-VCO
Q304, Q305. D303
4-3