Tektronix PRIMER P6101B Manual page 8

Primer
Figure 1.5. Probes and oscilloscopes are designed to make measurements to
specification over an operating bandwidth. At frequencies beyond the 3 dB point,
signal amplitudes become overly attenuated and measurement results may be
unpredictable.
Bandwidth and Rise Time Limitations
Bandwidth is the range of frequencies that an oscilloscope
or probe is designed for. For example, a 100 MHz probe
or oscilloscope is designed to make measurements within
specification on all frequencies up to 100 MHz. Unwanted
or unpredictable measurement results can occur at signal
frequencies above the specified bandwidth (Figure 1.5).
As a general rule, for accurate amplitude measurements, the
bandwidth of the oscilloscope should be five times greater
than the frequency of the waveform being measured. This
"five-times rule" ensures adequate bandwidth for the higher-
frequency components of non-sinusoidal waveforms, such as
square waves.
Similarly, the oscilloscope must have an adequate rise time
for the waveforms being measured. The rise time of an
oscilloscope or probe is defined as the rise time that would be
measured if an ideal, instantaneous-rise pulse were applied.
For reasonable accuracy in measuring pulse rise or fall times,
the rise time of the probe and oscilloscope together should be
three to five times faster than that of the pulse being measured
(Figure 1.6).
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Figure 1.6. Rise time measurement error can be estimated from the above chart.
An oscilloscope/probe combination with a rise time three times faster than the pulse
being measured (3:1 ratio) can be expected to measure the pulse rise time to within 5%.
A 5:1 ratio would result in only 2% error.
In cases where rise time isn't specified, you can derive rise
time (Tr) from the bandwidth (BW) specification with the
following relationship:
Tr = 0.35/BW
Every oscilloscope has defined bandwidth and rise time
limits. Similarly, every probe also has its own set of bandwidth
and rise time limits. And, when a probe is attached to an
oscilloscope, you get a new set of system bandwidth and rise
time limits.
Unfortunately, the relationship between system bandwidth
and the individual oscilloscope and probe bandwidths is not
a simple one. The same is true for rise times. To cope with
this, manufacturers of quality oscilloscopes specify bandwidth
or rise time to the probe tip when the oscilloscope is used
with specific probe models. This is important because the
oscilloscope and probe together form a measurement system,
and it's the bandwidth and rise time of the system that
determine its measurement capabilities. If you use a probe
that is not on the oscilloscope's recommended list of probes,
you run the risk of unpredictable measurement results.