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LOGIC
PCB
ASSEMBLY
4.12
The
oscillator signal
is
fed via amplifiers
Q4
and
Q6
to
the
mixer
Q7
and
Q8.
Oscillator
a.g.c.
and
output
limiting
is
provided by
Q5
and
associated
components.
Q1
1
regulates the collector supply
voltage
to
the oscillator section.
When
the
Fine
Tune
control
switch
is
set to
OFF,
the positive supply via
R58
is
disconnected,
thus
disabling the Fine
Tune
reference
circuit.
and
mixer.
4. 13
The
mixer
Q7/Q8
is
fed with the variable
5.3MHz
described
above,
and
a fixed
5MHz
reference from the frequency standard.
The
resultant (tunable)
300kHz
'difference'
frequency
is
fed via the shaper/amplifier
Q9
to
the
gate IClc.
Tuning
(Spin
Wheel)
Input
4.
14
Rotation of the Fine
Tune
spin
wheel encoder
(described
in
para
3.
12)
applies a
train
of pulses
in
phase quadrature
to
pins
1
1
and
12 of the
Logic board.
The
pulses at pin
1 1
lead those
on
pin 12
by 90
for
one
direction of rotation
and
lag
by 90°
for
the opposite
rotation.
These
input pulses are
shaped by
the
Schmitt
triggers
IC3
and
IC4,
and
fed via
IC5a
to
the pulse multiplier
circuit.
Pulse Multiplier
and
Up/Down
Detector
4.15
The
pulse multiplier
IC5a
and
IC5d
receives the tune pulse
train
from
the
Schmitt
triggers
IC3
and
IC4,
and
IC4,
and by
forming a pulse from
each edge
effectively multiples the
spin
wheel
signal
by
four, thus
increasing
its
sensitivity.
4. 16
A
delayed
version of the spin
wheel
signal
is
fed
to
IC8a/5 and
is
clocked through
by
the
x4
pulse
train
applied
to
IC8a/3.
This
enables IC8a
to
detect
the direction
of rotation of the
spin
wheel.
When
turned
in
one
direction such
that
IC5a/l
leads
IC5a/2
by 90,
then the 'Q' output of IC8a
stays
high,
conversely
the opposite rotation reverses the
phase
relationship
and
the 'Q'
output then goes low.
The
state
of
IC8a/l
is
fed through
IC8b (except
when
operating
the step switch)
to
the
Up/Down
line.
Step
Switch
4. 17
When
the
biased
step
switch
(Fig.
20,
S3)
is
operated
it
applies
an
earth
to
the
appropriate
Set or Reset pin of
IC8b
thus forcing the
Up/Down
detector
to
the
required
state,
according
to
the step direction required.
Pulse
Divider
4.18
The
pulse divider
IC13
receives information from the
Range
switch, the
Tune
Control switch
(Fast,
Slow, Hold)
and
the
tuning
spin
wheel.
Its
function
is
to
provide
different 'gearing'
for
the
fast
and
slow
tuning
modes,
as follows:-
(1)
In
fast
tune
mode
the tuning
rate
is
such
that
each frequency
range
is
covered
in
30
turns
of the
spin
wheel.
By
acting
as
a variable divider
IC13
maintains
this
30
turn relationship
despite the
widely
differing
frequency
coverages
of the various ranges.
9081
&
9082 Vol
.
2
4-3
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