Improving Time Domain Network Measurements
Removing Measurement Errors
10-7
the DUT response differs from the reference response. Hence, there is no way
to view the actual DUT response without the errors introduced by the test
system.
Finally, the most significant shortcoming is that measurements are limited to
the risetime of the test system. Determining the DUT response at multiple
risetimes is cumbersome. Either multiple step generators or multiple risetime
convertors are necessary and a separate reference waveform is required for
each risetime.
Normalization Improves on Error Correction
A digital error-correction method known as normalization can significantly
reduce or remove all of the above types of errors from TDNA measurements.
Taking full advantage of its powerful internal microprocessor, the HP 54750A
Series digitizing oscilloscopes with TDR capability include normalization as a
standard feature.
Normalization can predict how the DUT will respond to an ideal step of the user-
specified risetime. Only one step generator and one normalization process are
required. No risetime convertors are necessary, and the normalization
standards are not related to the DUT.
Unlike a risetime converter, normalization can also increase the bandwidth (i.e.,
decrease the risetime) of the system by some amount depending on the noise
floor. This means that when more bandwidth is critical, such as when trying to
locate a discontinuity along a transmission line, the waveform data acquired by
the oscilloscope can be "squeezed" for every bit of useful information it contains.
Examples of What Normalization Can Do
The following two examples illustrate what normalization can accomplish:
Example 1 Correcting for the TDR measurement errors introduced by
connecting hardware.
Consider trying to model a device at the end of some imperfect test fixture as in
Figure 10-3.
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