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DC
Voltage
Ranging
Low
Voltage
Ranges
(100|iV-
lOVFR). The
basic
range of
the
4708
is
±10V
Full
Range
(±19.999999V
Full Scale),
derived
directly
from
the
working
reference.
The IV
and
100m
V
ranges
are
achieved
by
attenuation:
The
lOOmV
range
attenuator
is
also
used
for
10m
V,
ImV
and
100|iV
ranges,
and
the
digital
input
to
the
f)recision
divider
is
scaled
to
provide
the correct
working
reference values.
Range
Working
reference values
lOmV
—
2V - +2V
ImV
—
200mV
+200mV
lOOpV
_20mV-4 +20mV
High Voltage
Ranges
(100V
and
1000V)
The lOOV
range
is
a
direct
amplification
of
the
working
refer-
ence.
The lOOOV
range
employs
step-up
AC
transformation.
Output
Switching.
In addition
to
switching
between
functions, the
output switching
circuits isolate
terminals
on
OUTPUT
OFF.
Remote/Local
Sense and
Guard
switching
is
incorporated.
AC
Voltage Output
The
working
reference
for
AC
Voltage Output
is
a
stable
DC
voltage, accurately variable
at
high
resolution
between
+0.1V
and
+2V
DC.
AC
Reference Generator
The
higher accuracy of
AC/
AC
comparison
(over
AC/DC)
is
exploited
by
converting
the
DC
Working
Reference
into
a
stepped
waveform whose
characteristics
match
those
of a
sine-
wave.
The
amplitude
of
this
'Quasi-sinewave'
is
precisely
controlled
by
the
DC
Working
Reference
value.
Sinewave Source
Frequency
Synthesizer
From
the
frequency value
set into
the
MODE/FREQUENCY
display, the
processor
controls the synthesizer
using
an encoded
9-bit
command. The
synthesizer
translates
the
command
into
a pulse
train
at
a
crystal-derived
frequency
between
240kHz
and
4MHz,
to
be
divided
down
for
use
as
phase-reference
for the
Quadrature
Oscillator.
N.B.
If
required, the
Frequency
Synthesizer,
can
be locked
to
an
externally
supplied
IMHz
or
lOMHz
frequency, input
via
J53
on
the
rear panel.
Quadrature
Oscillator
The
oscillator's
output frequency
is
set
close
to
any demand,
between
lOHz and
IMHz,
by
selecting the
RC
time constants of
its
dual
integrators;
and
then
by
correcting
to
the actual
demand
by
phase-comparison
with
the
output
from
the synthesizer.
The
output
sinewave
purity
and
constant
amplitude
are precisely
defined
by
a
sophisticated control loop,
and
the
RMS
value of
the
sinewave
is
adjusted
to
be roughly
proportional
to the de-
manded
output voltage or
current.
Timing
data
is
output from
the
source
to
synchronize
the actions
of
the
AC
Reference Gen-
erator
and
AC/AC
Comparator.
Voltage-Controlled Amplifier
(VCA)
This has
variable gain,
amplifying
the
output
from
the
Sinewave
Source and providing a
buffered drive
to the
output
circuits.
Its
gain
is
determined
by
the
measured
difference
between
the
RMS
values
of
the
sensed
calibrator
output
and
the
AC
Reference;
so
the
VCA
provides
the correcting fine
adjustment
for the
output
amplitude
loop.
AC
Voltage
Ranging
1V
Range
This
is
the basic
AC
voltage
range of
the
4708.
As
the
AC
working
reference
is
variable
between
0.1
V
and
2V RMS,
it
is
compared
in 1:1 ratio
with
the
sensed
output.
The IV
Buffer
output
is
thus
passed
directly
to
the output
1+
and
I-
terminals.
lOOmV,
l0mV
and
lmV
Ranges
The
1
V
Buffer output
is
reduced
by
precision attenuators before
being connected
to
the terminals, the
level
being sensed
before
attenuation.
10V,
100V and 1000V Ranges
The IV
Buffer output
is
amplified
on
each of
these ranges.
A
separate amplifier
is
provided
for the
lOV
range, the output
sense
signal
being obtained
at
the terminals
and
attenuated
before
comparison
with
the reference.
A
common
power
amplifier
is
used
for
both
1
00
V
and
1
000
V
ranges.
On
the
1
00
V
Range
the
output
is
fed
directly to the terminals,
on
the
lOOOV
Range
the
output
is
stepped
up by
a
transformer.
On
both
ranges, the
sensed
terminal voltage
is
reduced
to the
reference
level
by
precision
attenuators.
Output Sensing
On
the
IV
range
and
above,
the
output
is
sensed
at
the front panel
Hi and
Lo
terminals.
With
Remote
Sense
selected,
these
are
isolated
from
1+
and
I-,
but
in
Local Sense
Hi
is
internally
connected
to I+,
and
Lo
to
I-.
As
described above,
the
lOV,
1(X)V
and
1(XX)V ranges' sense
signal
is
attenuated before
comparison
with
the reference.
AC/
AC
Comparator
The
comparator
generates
an
error
voltage proportional
to the
difference
between
the
RMS
values
of
the
AC
reference
and
the
sensed
output.
It
alternately
samples
a
number
of
cycles
from
its
'Ref and
'Sense'
inputs,
computes and
integrates
the
squares of
their
instantaneous values,
and
uses a
'Sample and
Hold' technique
to
subtract
one from
the other,
this
being
the
'error'
voltage
to
control the
VCA.
The
loop
thus controls
the
4708
output so
that the
RMS
value of
the
comparator's
sense
input equates
to that
of
its
reference
input.
1-5
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