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Principles of
Operation
Inputs/Outputs
Simplified
Functionai Diagram. This
shows
the
division
and
flow
of
functions
within
the
4708
Inputs
The 6802
microprocessor
controls the
output
in
response
to
three
main
inputs:
i)
Front panel keys
ii)
IEEE
488
bus messages
in
'Remote'
operation
iii)
Corrections placed
in
non-volatile
memory
during
'
Autocalibration
'
.
These modify
the
values
which
control the output.
After processing,
the
computing system changes
the
output
of
the
instrument
to
respond
to the
input
instructions.
Reference Voltages
A
20V
DC
'Master'
Voltage Reference
establishes the
funda-
mental accuracy of
the instrument.
From
this
20V,
a precision
electronic divider derives
an
adjustable
'Working'
reference
voltage
between
OV
and 20V,
whose
value
depends on
digital
inputs
from
front
panel keys
and
calibration
memory.
Precision Electronic Divider
In the
out-guard
section the selected
output
value,
including
calibration corrections,
is
set into
a
digital
comparator
as
a 25-
bit
number. This
is
counted
out
by
a
crystal
controlled binary
counter, resulting
in
a
125Hz
square
wave whose mark
:
period
ratio
accurately represents the
output value
selection.
When
transferred into
guard,
it
chops
the
Master Reference
voltage.
A
7-pole
active
low-pass
filter
integrates the
chopped
reference,
to
generate
the ripple-free
DC
Working
Reference
Voltage.
DC
Voltage
Output
The
working
reference
for
DC
Voltage Output
is
a
stable
DC
voltage, accurately variable
at
high
resolution
between
0
and
+20V.
1-4
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