Keysight 1000B Series, User Manual

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68 Keysight 1000B Series Oscilloscopes User’s Guide
3
Capturing Data
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 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