220 Keysight InfiniiVision 3000T X-Series Oscilloscopes User's Guide
13 Acquisition Control
An oscilloscope's rise time is not the fastest edge speed that the oscilloscope can
accurately measure. It is the fastest edge speed the oscilloscope can possibly
produce.
Oscilloscope Bandwidth Required
The oscilloscope bandwidth required to accurately measure a signal is primarily
determined by the signal's rise time, not the signal's frequency. You can use these
steps to calculate the oscilloscope bandwidth required:
1 Determine the fastest edge speeds.
You can usually obtain rise time information from published specifications for
devices used in your designs.
2 Compute the maximum "practical" frequency component.
From Dr. Howard W. Johnson's book, High-Speed Digital Design – A
Handbook of Black Magic, all fast edges have an infinite spectrum of frequency
components. However, there is an inflection (or "knee") in the frequency
spectrum of fast edges where frequency components higher than f
knee
are
insignificant in determining the shape of the signal.
f
knee
= 0.5 / signal rise time (based on 10% - 90% thresholds)
f
knee
= 0.4 / signal rise time (based on 20% - 80% thresholds)
3 Use a multiplication factor for the required accuracy to determine the
oscilloscope bandwidth required.
See Also Choosing an Oscilloscope with the Right Bandwidth for your Application, Keysight
Application Note 1588
(http://literature.cdn.keysight.com/litweb/pdf/5989-5733EN.pdf)
Required accuracy Oscilloscope band width required
20% f
BW
= 1.0 x f
knee
10% f
BW
= 1.3 x f
knee
3% f
BW
= 1.9 x f
knee
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