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LOGIC
PC
B
ASSEMBLY
(2)
In
slow tune
mode
IC13
acts as a fixed
divider
because
the tuning
rate
has a
constant
number
of
channel changes per
revolution of the
spin
wheel
.
4.19
The
pulse divider functions
as follows.
Tuning
signals
from the
spin
wheel
are fed
into
103
via inverter IC9a.
With
the
Tune
Control switch
S6b
set to
FAST
the
line
to
pin
24
is
open
circuit
which
enables IC9b
and
allows
the
'carry'
output
at
IC13/7
to
be
fed
into the 'preset
enable'
at
IC13/1,
thus
converting IC13
to
a
programmable
divider,
with an output
fed via 1C
1
Ob/6
to
IC7a/3.
4.20
Division
ratio
commands
are applied
to
the
A,B,C
and
D
lines
of
IC13 from
the
four
upper bands of
the
Range
switch S2.
Range
5,
for
example,
commands
division
by one whereas Range
2
commands
division
by
eight.
4.21
When
the
Tune
Control switch S6
is
set to
HOLD
or
SLOW,
a
'1'
is
applied via
pin
24
to
IC9b/6.
This disables
IC9b,
which
cuts off
the carry
output loop
and
converts
IC13
to
a fixed division
by
16.
Thus a constant
number
of channels
is
obtained
for
each
revolution of the
spin
wheel.
4.22
In
the
HOLD
position of the
Tune
Control switch
an
open
circuit
is
applied from
switch
S6a
which
turns
on
Q1
and clamps IC10b/6
to
+12V, which
inhibits
the
output of 1C
1
3, thus
electronically
dis-engaging
the spin
wheel.
IC10b/5
receives
an
input
from
the biased Step
Up/Down
switch
when
this
is
operated
for
single step
channel
shifting.
The
tuning information received
by IClOb
is
fed
to
IC7a/3 where
it
is
processed
by
the
range
and
channel spacing
control circuitry
described
below.
Principles of
Channel Spacing
Control
4.23
The
function of the pulse multiplier
is
to
detect
the direction of rotation of the
Fine
Tune encoder and
from
this
information
control the direction of
counting
(up
or
down)
in
the
variable divider
in
the
main
oscillator
loop
(Divider
and
Phase Detector
Assembly
19-0846).
At
the
same
time
it
allows the
magnitude
of the
-r
N
number
set into
that
divider
to
be changed,
to
achieve
the required
channel spacing.
The
system operates
as
follows.
4.24
The
channel spacing
is
determined by
gating a
certain
number
of
300kHz
pulses
through IC9d.
This
is
controlled
by
the pulse multiplier, the
channel spacing
switch
and
the
Range
switch
in
conjunction with
the
channel spacing
control circuitry
shown
in
the
upper
right
hand
corner of
Fig. 5.
4.25
For
example,
if
operating
on Range
5
and channelized mode,
a
5kHz
channel
spacing
is
selected, then
each
pulse from the
spin
wheel on slow
tune causes
one
pulse
to
be
released through IC9d,
this
clocks the 'N' counter
by
one,
which
instructs
the
'N' divider
to
change
the instrument
output frequency
by 5kHz.
If
the
channel spacing
is
changed
to
10kHz,
then
two
pulses
will
be
released through
IC9d giving
a
10kHz change
of output frequency.
Similarly
for
20kHz
channel spacing,
4
pulses will
be
released.
for
each
pulse from the
spin
wheel.
9081
&
9082
Vo.l
.
2
4-4
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