Combined Ac And Dc Signal Measurements; Affects Of Offset In Ac Measurements; Bandwidth Limitations In Ac Voltage Measurements; Crest Factor - Fluke 8010A Instruction Manual

OPERATION
APPLICATIONS
2>75.
Combined
AC
and
DC
Signal Measurements
2-76. It
is
sometimes
necessary to
measure an
ac signal
riding
on a dc
level.
An
example
of
this
type of
signal
is
shown
in
Figure
2-16.
To
obtain an
accurate
measurement of the
total
rms
value for these
signals,
perform the following
steps:
1.
Measure
the ac
component
of the
signal
using
the
AC V
function.
2.
Measure
the
dc
component
of the
signal using
the
DC V
function,
3.
Use
the
following formula, with
the values
obtained
in
Steps
1
and
2,
to determine
the total
rms
value of the
signal:
Total
RMS =
\/(ac
component rms)^
+
(dc
component)^
Value
2-77. Affects of
Offset
in
AC
Measurements
2-78.
Your Multimeter
is
a
true-rms-responding
multimeter,
and
the display
may
indicate a reading of
1
to
2
digits
(because of amplifier noise)
when
the input
is
shorted
in
the
AC V
or
AC mA
functions.
The accuracy
of
your Multimeter
is
not affected by
this
internal
offset
even
when
measured
inputs are at the specified floor
of the
multimeter (5% of the
range
selected).
When
the
rms
value of the
two
signals (internal offset
and
5%
of range
input)
is
calculated, as in the following equation, the
insignificant effect
of the
offset
is
shown:
Total
rms
digits
=
\J2'^
10()^
=
100.02
The
display of
your Multimeter
will
read 100
digits.
2-79.
Bandwidth
Limitations
in
AC
Voitage
Measurements
2-80. Signals
with rectangular
waveforms
contain
component
frequencies that are
much
higher
than
the
fundamental frequency used to describe them.
The
component
frequencies, for
rise
times
less
than
1
microsecond, exceed the 200
kHz
bandwidth
of
your
Multimeter (and
will
produce
a
low
reading
display).
For
example, a
10-kHz square
wave
with a
rise
time of
less
than
1
microsecond
will
produce
a display reading that
is
approximately 1% low. This
error will
be proportionally
higher
for
higher frequency square waves.
WAVEFORM
CREST FACTOR
SQUARE WAVE
^
1.0
SINE
WAVE
%
1.414
TRIANGLE
SAWTOOTH
1.732
MIXED
FREQUENCIES
1.414
to 2.0
SCR
OUTPUT
OF
100%
-
10%
~'\j~
1.414 to 3.0
WHITE
NOISE
mm
3,0 to 4.0
AC COUPLED
PULSE TRAIN
n
3.0Vb/a -1
j.
b"'
*1
SPIKE
>9.0
Figure
2-15. Crest Factor
Figure 2-16.
Total
RMS
Value
2-15/2-16