True Rms Accuracy And High-Frequency Signal Content - Agilent Technologies L34410A User Manual

Measurement Tutorial 4
An ac–coupled true rms measurement is desirable when you are measuring
small ac signals in the presence of large dc offsets. For example, this situation
is common when measuring ac ripple present on dc power supplies. There are
situations, however, where you might want to know the ac+dc true rms value.
You can determine this value by combining results from dc and ac
measurements, as shown below:
For the best ac noise rejection, you should perform the dc measurement using
an integration time of at least 10 power–line cycles (PLCs).
True RMS Accuracy and High Frequency Signal Content
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A common misconception is that "since an ac multimeter is true rms, its sine
wave accuracy specifications apply to all waveforms." Actually, the shape of
the input signal can dramatically affect measurement accuracy for any
multimeter, especially when that input signal contains high–frequency
components which exceed the instrument's bandwidth.
As an example, consider a pulse train, one of the most challenging waveforms
for a multimeter. The pulse–width of that waveform largely determines its
high–frequency content. The frequency spectrum of an individual pulse is
determined by its Fourier Integral. The frequency spectrum of the pulse train
is the Fourier Series that samples along the Fourier Integral at multiples of the
input pulse repetition frequency (prf).
34410A/11A/L4411A User's Guide 107