HP 54753A User Manual page 198

TDR Fundamentals
Instrument Configuration
Figure 9-22
System Risetime Affects the TDR Results
In analyzing TDR results so far, we have assumed that the time constant and
therefore risetime (t ) created by a discontinuity were known and therefore
r lpf
the value of the inductor L or capacitor C was also known. In most TDNA
measurements, only the combined step generator and sampler risetime (t
r
) and the reflected waveform are known. From this information, you may
system
want to derive the value of the discontinuity L or C. Again three cases exist in
this analysis. If t << t , then, as stated earlier, the TDR response
r system r lpf
approaches the ideal result and a value for the L or C can be calculated (as in
Figure 9-7) from the measured time constant of the exponential decay or rise
to the final value. If t is of the same order of magnitude as t , then
r system r lpf
calculating the L or C discontinuity becomes much more difficult due to the
interaction of the time constants.
One way to find the value of the L or C in this case is to use a SPICE simulation
program to model the response and vary the L or C value until the maximum
reflection on the SPICE simulation program and the TDR waveform match.
Accuracy depends on using realistic waveforms in the SPICE simulation. When
t >> t , such as small reflections, it is possible to relate the reflected
r system r lpf
signal to the value of the L or C by assuming the L or C is driven by a current
or voltage source. This is equivalent to saying that for the frequencies contained
in the step, the impedance of a discontinuity does not significantly alter the
impedance of the circuit loading it. Using this approximation, we can relate the
maximum slope of the step to the maximum reflection from the discontinuity.
If the TDNA step is Gaussian (or can be normalized to an approximately
Gaussian step), then it can be shown that the maximum slope of the step is 27%
9-24