Chapter 8 Tutorial

Measurement Fundamentals

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misconception is that “since the internal MWW is true bWc, 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 waveshapes is ” ” . Crest factor is the ratio of the peak value to the bWc 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 as shown in the table on page 360. In general, the greater the crest factor the greater the energy contained in higher frequency harmonics. All multimeters exhibit measurement errors that are crest factor dependent. Crest factor errors are shown in the specifications in chapter 9 on page 407. Note that the

crest factor errors

apply for input signals below 100 Hz when

using the

ac filter.

 

You can estimate the measurement error due to signal crest factor as shown below:

Total Error = Errorsine + Errorcrest factor + Errorbandwidth

Where:

Errorsine = DMM’s sinewave accuracy (see page 406)

Errorcrest factor = DMM’s crest factor (see page 407)

Errorbandwidth = Estimated bandwidth error as shown below:

C.F. 2 x F Errorbandwidth = 4 π x BW

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C.F. = Signal crest factor (see the table on page 360)

F = Fundamental input signal frequency

BW = DMM’s -3 dB bandwidth (1 MHz for the HP 34970A)

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HP 34970A manual Km w