182 Keysight InfiniiVision 2000 X-Series Oscilloscopes User's Guide
12 Acquisition Control
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)
Memory Depth and Sample Rate
The number of points of oscilloscope memory is fixed, and there is a maximum
sample rate associated with oscilloscope's analog-to-digital converter; however,
the actual sample rate is determined by the time of the acquisition (which is set
according to the oscilloscope's horizontal time/div scale).
Required accuracy Oscilloscope bandwidth 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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