User Manual

Chapter 4

Measurement Setup

Central to setting the proper range of the instrument is the Crest Factor. Crest factor is the ratio of the peak

value of a waveform to the RMS value of the waveform. For example, a perfect sine wave crest factor RMS

value is 0.707*Peak. The crest factor is the inverse of the normalized RMS value, 0.707

−1

= 1.414. In many

cases, like the current waveform of a AC-DC power supply, the crest factor is a larger value. For accurate

measurement, the signal measured should stay within the measurement range of the instrument. For example, a

100V RMS signal with a crest factor of 5 has peaks of 500V. If the range selected is smaller than 500V than

inaccuracy will occur as the signal will be clipped.

4.0.1 Crest factor

The Crest Factor, the ratio of the peak to the RMS value, has two settings, 6 or 3. This setting applies to both

current and voltage simultaneously, and modies the available ranges listed in Section 4.0.2. Essentially, by

knowing the RMS signal level and the peak value, the setting of this parameter is determined. For an 117V

RMS AC line signal with little distortion and a purely real load (power factor = 1), the peak value is

1.414 ∗ 117V = 165V . 1.414 is the approximate Crest Factor of a Sine wave. So, the appropriate setting will be

3 (CF3).

CrestF actor(CF ) =

P eak

RM S

As this meter also measures the DC level of the signal, this must also be accounted for when choosing the Crest

Factor. If the the signal will exceed the range of the unit (crest factor * range) then the measurement value will

be clipped and the value inaccurate. Conversely, by choosing a Crest Factor and Range combination that

accommodates the peak value of the signal, a larger crest factor than set may be measured. The trade-o is

then the accuracy, and is listed in the specications, Chapter 10.

Example (Range=150 V, CF = 6): The maximum input signal and crest factor are as follows:

rms

= 100 Range = 150V V

maxrange

= 900V

So, a signal with a peak that is 9 times that of the RMS value (Crest Factor 9) is measurable using these

settings.

4.0.2 Set Measurement Range

The appropriate measurement range (voltage and current range) must be set for accurate measurement.