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 Truevolt Series DMMs 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 f
1
= 1/t
p
.
This gives an immediate impression of the high-frequency content by identifying where this crossing occurs as a
function of crest factor: f
1
=(CF
2
)(prf).
92 Agilent Truevolt Series DMM Operating and Service Guide
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