Icom IC-04A Service Manual page 11

4-3-1
REFERENCE FREQUENCY
CIRCUIT
(PLL UNIT)
IC204
incorporates a
swallow
counter of 6 binary
bits,
a
programmable
counter of
1 1
binary
bits,
a
phase
com-
parator, a
charge
pump
and
a
frequency divider for the
reference frequency.
A
5.12MHz
(#04
6.4MHz)
signal
is
oscillated
at
reference
oscillator
Q213
and
X203, and
is
fed to
pin
17
of
IC204.
IC204
divides
the frequency by 1/1024
and
a reference
frequency
of
5kHz
(#04 6.25kHz)
is
ob-
tained.
The
reference
frequency
is
fed to
pin
8 of
IC203.
4-3-2
DUAL
MODULUS
PRESCALER
Signals
from
the
VCO
UNIT
are buffer amplified at
Q220
and
divided
N
times
at
IC203 and IC204.
Signals
are
then phase detected
at
IC204 and
the detected
signals are output
from
pin
1 1
.
IC202
is
a prescaler that divides signals generated
by
the
VCO
UNIT by
either
1/64
or 1/65.
4-3-3
LOOP
FILTER,
VCO,
MODULATION
CIRCUITS
(PLL
AND VCO
UNITS)
Output from
pin
1 1
of
IC204 determines the
character-
istics
of the
PLL
circuits
through
a lag lead
type loop
filter
consisting of
R248,
R249
and C294. This
out-
put controls
D701
in
the
VCO
UNIT.
The
VCO
(0702) employs
a Colpitts oscillator
circuit.
The
VCO
free
run
frequency
is
shifted
by
inductive
capacitance with
0701 and D702.
In
receive
mode,
0701
turns
OFF
then the
free run
fre-
quency
is
determined
by L703,
C703
and
C702.
In
transmit
mode, 0701
turns
ON
then
D702
is
activated.
Thus
C706
and
C705
are parallel
connected
with
C702
and C703.
As
a
result
the free
run frequency
is
shifted
lower than receive frequency.
Stable oscillation
is
controlled
by varactor diode
D701
and
is
achieved over
a
wide frequency
range.
While the
transceiver
is
transmitting,
modulation
signals are applied to the
anode
of
D
702
,
d^anging
its
capacitance performing frequency modulation.
4-3-4
UNLOCK
CIRCUIT
(PLL UNIT)
When
the
PLL
cirucit
is
unlocked, pin
10
of
IC204
is
"LOW"
and
a
"LOW"
signal
is
fed to pin
8
of
IC102A
as
unlock
signals
through
a
time constant
circuit
con-
sisting of
R251 and C296.
In
transmit
mode
pin
9 of
IC102A
is
"LOW"
thus unlock
signals are applied to the
MPU
from
pin
10
of
IC102A.
MPU
PLL
IC
w
/C407 JC20U
10
PRESCALER
PLL
IC
IC203
K20i