The question was asked, "which of these differential vias is better?" Ignoring all the philosophical issues of the meaning "better", I put together a simple 3D-model of each of the vias in Microwave Studio and took a look at S11 and S21 (among other things). I have several questions regarding the setup of the simulation and the interpretation of the results: 1) Is a differential via (pair) a two-port or four-port model? The model I put together has two vias, with pads/connections (a "P" and an "N" pair) on the top layer and on an inner layer. The model includes a number of reference planes tied together with additional "ground vias". In modeling this, do I attach a single port between the "P" and "N" leads at the micro-strip, and a second port between the "P" and "N" striplines (hence a two-port model)? Or, should I attach a port from a reference plane to each "P", and from a reference plane to each "N", hence having a four-port model? Exciting the two-port model in MWS is straight forward - I haven't figured out how to stimulate the four port simulation to give me "differential" operation - any hints? 2) Assuming I get #1 figured out, do I want to normalize the resulting S-parameter to 50 Ohms? If a 4-port model is the right one, I'm assuming normalizing to 50 Ohms is the "right thing to do". However, if the answer to #1 is a 2-port model, shouldn't the normalization be done to 100 Ohms? 3) Finally, regarding S11 - I'm assuming I can use this to determine how "transparent" my vias are - that is, how close each via is to matching the characteristic impedance of the rest of my transmission line (normalized to 50 or 100 Ohms). True or not true? Can I use S11 to calculate the "Z0" of the via? I'm intuitively drawn to wanting to equate S11 to the reflection coefficient, and therefore use that to calculate the Z0 of the via (given that S11 is normalized to 50 or 100 Ohms). However, since S11 is always positive, this implies I could never have a via whose impedance is less than that of the transmission line - and I don't believe that. Or, do I need to look at the phase information as well as S11 to determine positive vs. negative reflection? Thanks in advance for the help! Ralph Wilson Alcatel-Lucent ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu