Published by Eric Bogatin on 05 Jul 2012
Like a doctor who only sees patients with a medical problem and rarely talks to healthy people, I see hundreds of examples of S-parameter files either from measurements or simulations, all with some sort of problem.
While the quality of the information stored in the S-parameters is unaffected by the port labeling, how we interpret each S-parameter matrix element is strongly affected by the port labeling scheme.
For example, in one assignment, the insertion loss is the S21 matrix element, while in the other port assignment, the insertion loss is S31.
If you generate your S-parameter file using one assignment, but it is interpreted by someone else with the other port assignment, the analysis will be incorrect. Unfortunately, because so few users of S-parameters know what to look for in the S-parameters or are so afraid of looking at the actual data stored in the touchstone file, rarely is this root cause ever identified until too much time has been wasted.
Instead, I get a frantic call that the measurement and simulation is off, or that the eye predictions for a channel are completely collapsed, or that the model from their connector or package vendor shows that the signal is too small getting through the channel and how should they re-design the channel.
Following a few simple guidelines, this simple and common problem can be identified before it becomes a real problem and completely eliminated.
For more information on quick fixes to this problem, please see the paper that Alan Blankman and I wrote for Test and Measurement World, or check out the SPSI class I teach, which goes through this analysis in much more detail.