Keysight 4000 X-Series, Руководство по продвинутой подготовке

Страница: 160 / 170

160 4000 X-Series Oscilloscopes Advanced Training Guide
B
Oscilloscope Bandwidth Tutorial
Required Bandwidth for Digital Applications
Rule of Thumb
As a rule of thumb, your scope’s bandwidth should be at least five times higher
than the fastest digital clock rate in your system under test. If your scope meets
this criterion, then it will be able to capture up to the fifth harmonic with minimum
signal attenuation. This component of the signal is very important in determining
the overall shape of your digital signals.
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
knee
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 .”
1
He refers to this frequency
component as the “knee” frequency (f
knee
). 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.
For signals with rise time characteristics based on 10% to 90% thresholds, f
knee
is
equal to 0.5 divided by the rise time of the signal. For signals with rise time
characteristics based on 20% to 80% thresholds, which is very common in many of
today’s device specifications, f
knee
is equal to 0.4 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.
f
BW
>= 5 x f
clk
f
knee
= 0.5/RT (10% 90%)
f
knee
= 0.4/RT (20% 80%)