Quick Reference
2
Keysight InfiniiVision 1000 X-Series Oscilloscopes User's Guide
57
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 band wid th 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
