Racal Instruments 9081 Maintenance Manual page 44

LOGIC
PCB ASSEMBLY
LOGIC
PCB
ASSEMBLY
19-0842
(9081)
AND
19-0948
(9082)
FIG.
5
Introduction
4.7
The
Logic
Assembly
receives information from the following sources
and
processes
it
to
provide appropriate
commands
to
the succeeding
circuits:-
Divider
and
Phase Detector Assembly
.
Channel Spacing
Switch
S4.
Range
Switch
S2.
Tune
Control Switch
S6.
Single Step Switch
Up/Down
information.
Fine
Tune
Control Switch.
Fine
Tune
potentiometer/and
external
fine tune.
Tuning
spin
wheel encoder.
Reference frequency.
Diagrams
for
reference are
Figs. 5,
20 and 22
at the
back
of the
book and
Fig. 3. 2 at the
end
of
Chapter
3.
300kHz
Reference Frequency
4.8
Referring to Fig. 5, a fixed 300kHz
reference frequency (derived
in
19-0849
from
the
frequency
standard)
is
fed
in
via socket
SKI and
shaper
Q10
to
the gate
ICld/13.
This
gate
is
enabled by
a
'0'
on ICld/12 when
the Fine Tune
Control switch
(S7,
Fig.
20)
is
at
OFF,
thus
allowing
the fixed
300kHz
reference
to
pass via ICla
to
the divide-
by-eight package
IC2
which
feeds
37.5kHz
as a channel spacing reference
to
the channel
spacing
dividers
in
the Divider and
Phase Detector Assembly.
4.9
When
the
FINE
TUNE
switch S7
is
set to
INT
or
EXT, gate ICld
is
disabled
by
+12V
from
S7,
and
gate IClc
is
enabled
via IClb.
At
the
same
time
+12V
is
applied, via R58,
to
the Fine Tune
oscillator
Q3
to
Q6
and mixer
Q7/Q8
and
Q9
which
supply a tunable
300kHz
reference
via
IClc and ICla
to
IC2.
Fine
Tune
Reference
4. 10 For fine tuning purposes a variable
300kHz
reference
is
produced
via the mixer
Q8/Q9,
as follows.
4.
11
Transistor
Q3
is
a
crystal oscillator
operating
in
a Colpitts-type
circuit,
the
frequency being determined by
the
5.3MHz
crystal
XL1 and
voltage variable
capacitance
(varactor)
diode D3.
The
oscillator
frequency can be
'pulled'
over
a limited
range by
the
front
panel
FINE
TUNE
potentiometer, or by an
external
slow-sweep
input,
which
applies a positive d.c. voltage
to
the junction of XL1 and D3.
4-2
9081
&
9082 Vol.2