HP 54753A User Manual page 211

Improving Time Domain Network Measurements
Sources of Measurement Error
The risetime of the step is also extremely important. In most cases the step
generator used for TDNA will have a fixed risetime. A hardware filter known
as a risetime convertor can be used in some systems to change the risetime.
To determine how the DUT will actually respond in it's intended application,
you should test it at edge speeds similar to those it will actually encounter.
Consider the example of a BNC connector (Figure 10-2). Only about 3% of a
350 ps risetime edge (top waveform) is reflected by a BNC connector whereas
6% of a 100 ps risetime edge (middle waveform) is reflected and about 8% of a
50 ps risetime edge (bottom waveform) is reflected.
Figure 10-2
Variable edge speed helps determine the amount of reflection in actual applications. The top
waveform (tested to 350 ps) shows less reflection than the middle waveform (tested to 100 ps) or
the bottom waveform (tested to 50 ps)
In the case of this measurement, the results obtained using a 50 ps risetime step
stimulus do not apply for a connector that sees edges that are always slower
than 350 ps. The connector might be acceptable for 350 ps edges but not for
50 ps edges. Measurements made at inappropriate risetimes can yield invalid
conclusions.
10-5