Improving Time Domain Network Measurements
Removing Measurement Errors
10-6
Edge speed is also critical when using TDR to locate the source of a discontinuity
along a transmission line. Just as the limited risetime of the oscilloscope can
limit the accuracy of this kind of measurement, the risetime of the step source
can also limit accuracy.
The edge speed also affects the spatial resolution of a TDR measurement or its
ability to resolve discontinuities along a transmission line. This can be important
when trying to extract models for an interconnect.
The risetime of the measurement system is limited by the combined risetimes
of the oscilloscope and the step generator. It can be approximated by equation
1.
(1)
In a system with zero minimum risetime, the response of a discontinuity would
not be attenuated at all. A real system has a limited risetime, which acts as a
lowpass filter. If the step stimulus used is too slow, the true nature of the
discontinuity may be disguised or may not even be visible. The cause may be
more difficult to physically locate. Notice in Figure 10-2 that as the risetime of
the step stimulus is decreased, the true nature of the reflection from the DUT
becomes more apparent.
Removing Measurement Errors
Waveform Subtraction has Limitations
In the past, waveform subtraction was used to reduce the effects of some of the
errors discussed above. It was convenient because many digitizing oscilloscopes
provided this feature without the aid of an external controller. A known good
reference device was measured and the reference waveform stored in memory.
The reference waveform could then be subtracted from the waveform measured
from the DUT. The result showed how the DUT response differed from the
reference response. This technique removed error terms common to both the
reference and DUT waveforms, such as trigger coupling, channel crosstalk, and
reflections from cables and connectors.
Waveform subtraction has, however, several shortcomings. First, it requires
that a known good reference DUT exists and is available to measure. In some
cases a good DUT may not be readily available or may not exist at all. Second,
the waveform which results from the subtraction process is a description of how
System risetime Step risetime()
2
Scope risetime()
2
Test setup risetime()
2
++=
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