3Capturing Data
70 Agilent 1000B Series Oscilloscopes User’s Guide
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, Agilent Application Note 1588
(http://cp.literature.agilent.com/litweb/pdf/5989-5733EN.pdf)
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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