110 34410A/11A/L4411A User’s Guide
4 Measurement Tutorial
Estimating High–Frequency (Out–of–Band) Error
A common way to describe signal waveshapes is to refer to their “Crest
Factor”. Crest factor is the ratio of the peak value to rms 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.
Notice that crest factor is a composite parameter, dependent upon the
pulse–width and repetition frequency; crest factor alone is not enough to
characterize the frequency content of a signal.
Traditionally, DMMs include a crest factor derating table that applies at all
frequencies. The measurement algorithm used in the 34410A/11A/L4411A
multimeters is not inherently sensitive to crest factor, so no such derating is
necessary. With this multimeter, as discussed in the previous section, the focal
issue is high–frequency signal content which exceeds the multimeter’s
bandwidth.
For periodic signals, the combination of crest factor and repetition rate can
suggest the amount of high–frequency content and associated measurement
error. The first zero crossing of a simple pulse occurs at:
This gives an immediate impression of the high-frequency,content by
identifying where this crossing occurs as a function of crest factor:
UG_ED5.book Page 110 Thursday, March 1, 2012 11:28 AM
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