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 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 ?s); the other narrow (6.7 ?s). The bandwidth 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.
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