Improving Time Domain Network Measurements
Sources of Measurement Error
10-3
Sources of Measurement Error
There are three primary sources of error in TDNA measurements: the cables
and connectors, the oscilloscope, and the step generator.
Cables and Connectors Cause Losses and Reflections
Cables and connectors between the step source, the DUT, and the oscilloscope
can significantly affect measurement results. Impedance mismatches and
imperfect connectors add reflections to the actual signal being measured.
These can distort the signal and make it difficult to determine which reflections
are from the DUT and which are from other sources.
In addition, cables are imperfect conductors that become more imperfect as
frequency increases. Cable losses, which increase at higher frequencies,
increase the risetime of edges and cause the edges to droop as they approach
their final value.
Figure 10-1 illustrates how cables and connectors affect TDNA measurements.
The upper waveform is the reflection of a step from a short circuit. Connections
cause the reflections at the peak of the step and along the baseline. Cable loss
yields the rounded transition of the step to its baseline level. Normalization can
correct the measured data, resulting in the lower waveform.
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