Icom IC-751A - 6-2 PLL Adjustment

The amplified signal
is
fed
through
a
double-tuned filter composed
of
L32, C77
and L33 to
eliminate
spurious
components,
and
then
fed
to
the Q12 and
Q13 transmit
mixer.
The
frequency of the
local
oscillator ranges from 70.5515
~
100.4515MHz
which converts
the
IF
signal to
the desired
frequency of 0.1
~
30MHz.
In the transmit
mode, D45
is turned
ON
and
the
signal is
amplified
about 20dB by
the
Q14 wideband amplifier
and is
output to
the
PA
unit
through J8.
Cl
71 and R86
are provided to
taylor the
fre-
quency
characteristics.
(2)
PA CIRCUITS
The RF signal
input from the
RF unit
through
PI
is
amplified by
the
Q1
class
A
amplifier.
The output
from Q1 is
converted to a
balanced output by
LI
and
is
amplified by
the Q2
and Q3 class
AB
push-pull
amplifier. The
negative
feedback
circuits inserted
between
the collector
and the base
of Q2 and
Q3
provide wide
frequency
characteristics. The
idling
current of Q2 and
Q3 is
controlled by
the
junction
voltage of D1.
The current is set
at
about 100mA by
R27,
R30
prevents the set
value
from deviating
due
to
variations
of D1
characteristics.
The output
of Q2 and
Q3 is
fed
to
the
L4
impedance
matching
section and is
amplified by
the Q4 and Q5 class
AB push-pull
am-
plifier
to
provide 100
watts
output power.
11-3
PLL CIRCUITS
The
PLL
unit outputs
two
oscillator signals for
the RF unit, i.e.,
the
variable first local
oscillator output (1st LO)
of 70.55
~
100.45MHz
necessary
for the. first
mixer, and the
fixed
local
oscillator output
(2nd
LO)
of
61.44MHz
necessary
for the second
mixer. In
addition,
the marker
signal
is
also
generated in this unit and
is sent to
the
RF
unit.
•
PLL 1C
CHIP
IC1
(M54929P)
is
a
multi-function
1C
containing a
phase comparator,
a
programmable
divider, a
reference
frequency
oscillator circuit, a
divider and
a
swallow
counter
controller. By
using this
1C
with
IC2 (M54466L, a
swallow
counter), it
can
perform pulse
swallow
dividing.
[MAIN
LOOP]
Reference
Frequency
(10kHz)
(3960
~
6950)
1st LO
[MARKER
GENERATOR]
MARKER
MARKER
OUTPUT
10kHz X n
-51
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