Agilent Technologies Truevolt Series Operating And Service Manual page 91

True RMS Accuracy and High–Frequency Signal Content
A common misconception is that because an AC multimeter is true rms, its sine wave accuracy specifications apply to
all waveforms. Actually, the shape of the input signal dramatically affects measurement accuracy for any multimeter,
especially when that input signal contains high–frequency component sbeyond the instrument's bandwidth.
For 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).
The figure below shows the Fourier Integral of two very different pulses: one of broad width (200 µs); the other narrow
(6.7 µs). The bandwith of the ACV path in the DMM is 300 kHZ; therefore, frequency content above 300 kHz is not
measured.
Notice that the sin(πfT)/πfT spectrum of the narrow pulse significantly exceeds the effective bandwidth of the
instrument. The net result is a less accurate measurement of the narrow, high–frequency pulse.
In contrast, the frequency spectrum of the broad pulse has fallen off significantly below the multimeter's 300 kHz
(approximately) bandwidth, so measurements of this pulse are more accurate.
Reducing the prf increases the density of lines in the Fourier spectrum, and increases the portion of the input signal's
spectral energy within the multimeter's bandwidth, which improves accuracy.
In summary, error in rms measurements arise when there is significant input signal energy at frequencies above the
multimeter's bandwidth.
Agilent Truevolt Series DMM Operating and Service Guide 91