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206
According to Dr. Howard W. Johnson’s book “High-Speed Digital Design–A Handbook of Black Magic”, all
fast edges have wirelessly continuous frequency components. However, there is a turning point (or “inflection
point”) in the fast edge spectrum at which frequency components above f
knee
are negligible in determining the
signal shape.
f
knee
=0.5/signal rise time (based on 10% - 90% threshold)
f
knee
=0.4/signal rise time (based on 20% - 80% threshold)
3) The multiplication factor for the desired accuracy is used to determine the required oscilloscope bandwidth.
Desired
Accuracy
Desired Oscilloscope
Bandwidth
20%
f
BW
=1.0xf
knee
10%
f
BW
=1.3xf
knee
3%
f
BW
=1.9xf
knee
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