[SI-LIST] TDR S-parameter and correlation

  • From: "Dmitriev-Zdorov, Vladimir" <vladimir_dmitriev-zdorov@xxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 30 Aug 2007 15:03:07 -0700



As I understand, the issue is about "general type" model, not just a
segment of T-line.


The answer to your first question is YES provided that (a) DUT's
S-parameters were measured correctly, (b) the equivalent circuit is
correctly built from S-parameters and (c) the way you assign ports in
your equivalent circuits is consistent with how the measurements was


Topology cannot be uniquely derived from S (or Y/Z/G/H... etc.)
parameters unless you have it predefined, like in case of T-line. There
could be many circuits with quite different topology producing the same
S/Y/Z... parameters.


Yes, S-parameters only characterize the model from outside ports, but
this is exactly what other pieces of your design see from this model:
this is sufficient to use the model in many simulation procedures,
unless you are interested in voltages/currents inside the model itself.


The problems above named (b) and (c) are sometimes not well understood.


First of all, any measured S-parameters (think of touchstone file)
contain the data in a limited frequency range, while any equivalent
circuit is 'defined' from DC to infinite frequency. Hence, they cannot
be equivalent if S-parameters do not cover sufficiently wide range of
frequencies, presumably from the lowest frequency where they start
changing up to the highest frequency where they stop changing and
approach to constant level. In all other cases, be prepared that the
circuit does not accurately capture the model behavior at very low and
high frequencies. Building equivalent circuit requires rational
polynomial fitting. This may be done by different tools with different
accuracy. In many cases, passivity enforcement is required on the post
fit stage to prevent unstable model behavior, especially if the upper
frequency in touchstone data is not sufficient. "Passivation" adds some
inaccuracy to the fitted model. Typically, representing the
poles/residues with circuit elements does not bring much error. These
are main sources of discrepancy we may have between given S-parameters
and equivalent circuit. Plus, time domain simulation of the equivalent
circuit, performed with finite resolution, adds LTE (local truncation


The problem (c) may sometimes be insidious. Imagine the S-parameters of
DUT were measured for two ports, without a 'common' ground. Essentially,
these measurements only characterize how the wave may propagate between
these two ports. Then, an equivalent circuit was built with four
external nodes making two ports. Now, someone use this model not only by
apply input to the first port and measure output at the second, but also
making arbitrary connections between any of external nodes of this
model. The result: behavior inconsistent with the original device. (Same
also possible with common ground). That is, we need to make sure the
model is used the same way the measurements were done for S-parameters.






Msg: #15 in digest

Date: Thu, 30 Aug 2007 12:30:19 -0700

From: "ZHENGGANG CHENG" <zhenggang.cheng@xxxxxxxxx>

Subject: [SI-LIST] TDR S-parameter and correlation



My question is:


If we TDR  (assuming the TDR method is 100% correct) the equivalent
circuit converted using S-parameter, will the result is exactly the same
as real TDR of the same DUT? (assume the converting error and bandwidth
are not issues) Assume this DUT has many large discontinuities inside.


To me, the real TDR can distinguish all the discontinuities inside a
DUT; however, the S-parameter is only the characterization at the ports
rather than inside. Will two equivalent circuits give the same
S-parameter but have two different topologies?


Look froward to your replies.








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