True Rms Accuracy And High-Frequency Signal Content; Estimating High-Frequency (Out-Of-Band) Error - DATATEC EDU34450A Guia Del Usuario

The instrument's AC voltage and AC current functions measure the AC-coupled true RMS value. In this Keysight
instrument, the "heating value" of only the AC components of the input waveform are measured (DC is rejected). As
seen in the figure above; for sinewaves, triangle waves, and square waves, the AC-coupled and AC+DC values are
equal since these waveforms do not contain a DC offset. However, for non-symmetrical waveforms, such as pulse
trains, there is a DC voltage content, which is rejected by Keysight's AC-coupled true rms measurements. This can
provide a significant benefit.
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 at s-mode.

True RMS Accuracy and High-Frequency Signal Content

A common misconception is that "since an AC multimeter is true RMS, its sinewave 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. Error in RMS measurements arise when there is significant input signal energy at frequencies above the
instrument's bandwidth.

Estimating High-Frequency (Out-of-Band) Error

A common way to describe signal waveshapes is to refer to their "Crest Factor". Crest factor is the ratio of the peak
value to RMS value of a waveform. For a pulse train, for example, the crest factor is approximately equal to the
square root of the inverse of the duty cycle.
Notice that crest factor is a composite parameter, dependent upon the pulse-width and repetition frequency; crest
factor alone is not enough to characterize the frequency content of a signal.
Traditionally, digital multimeters include a crest factor derating table that applies at all frequencies. The
measurement algorithm used in the EDU34450A is not inherently sensitive to crest factor, so no such derating is
necessary. With this instrument, as discussed in the previous section, the focal issue is high-frequency signal
content which exceeds the instrument's bandwidth.
For periodic signals, the combination of crest factor and repetition rate can suggest the amount of high-frequency
content and associated measurement error. The first zero crossing of a simple pulse occurs at
106 Keysight EDU34450A User's Guide