Table of Contents
Advertisement
B
Oscilloscope Bandwidth Tutorial
But if you need to make accurate measurements on high- speed edges, this
simple formula does not take into account the actual highest- frequency
components embedded in fast rising and falling edges.
Step 1: Determine fastest actual edge speeds
A more accurate method to determine required oscilloscope bandwidth is
to ascertain the maximum frequency present in your digital signals, which
is not the maximum clock rate. The maximum frequency will be based on
the fastest edge speeds in your designs. So the first thing you need to do
is determine the rise and fall times of your fastest signals. You can usually
obtain this information from published specifications for devices used in
your designs.
Step 2: Calculate f
You can then use a simple formula to compute the maximum "practical"
frequency component. Dr. Howard W. Johnson has written a book on this
topic, "High- speed Digital Design – A Handbook of Black Magic."
refers to this frequency component as the "knee" frequency (f
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
the shape of the signal.
For signals with rise time characteristics based on 10% to 90% thresholds,
f
knee
rise time characteristics based on 20% to 80% thresholds, which is very
common in many of today's device specifications, f
divided by the rise time of the signal. Now don't confuse these rise times
with a scope's specified rise time. We are talking about actual signal edge
speeds.
Step 3: Calculate scope bandwidth
The third step is to determine the oscilloscope bandwidth required to
measure this signal, based on your desired degree of accuracy when
measuring rise times and fall times.
various degrees of accuracy for scopes with a Gaussian frequency
response.
96
knee
f
knee
f
knee
is equal to 0.5 divided by the rise time of the signal. For signals with
are insignificant in determining
knee
= 0.5/RT (10% 90%)
= 0.4/RT (20% 80%)
Table 1
shows multiplying factors for
2000 X-Series Oscilloscopes Advanced Training Guide
1
He
). All fast
knee
is equal to 0.4
knee
Advertisement
Table of Contents
