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. A common way to describe signal wave shapes is crest factor. Crest factor is the ratio of the peak value to RMS value of a waveform.

For a pulse train, the crest factor is equal to the square root of the, inverse of the duty cycle; SQRT(period divided by the pulse width). In general, the greater the crest factor, the greater the energy contained in higher frequency harmonics. All multimeters have limited bandwidth and are not able to detect the harmonics above their bandwidth. Measuring the lower harmonics only - disregarding the high frequency components will generate a low RMS reading. Crest factor errors are predictable and always reduce the reading. Often, you will see crest factor errors stated with a "negative sign". As long as you know the crest factor, you can add this value to the reading to get the correct value.

Additional Error (adder) Due to Crest Factor
Crest Factor  
1 - 2  0.05%
2 - 3 0.15%
3 - 4 0.30%
4 - 5 0.40%

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