Ralph- You may gain a little insight in single ended and differential via design by looking at the talk I am giving at the Agilent ADS roadshow, on "Designing transparent vias". I cover both single ended and differential via design. The schedule for the tour is posted at: http://www.home.agilent.com/agilent/eventDetail.jspx?cc=US <http://www.home.agilent.com/agilent/eventDetail.jspx?cc=US&lc=eng&ckey=8737 87&nid=-536900532.536905430.08&id=873787> &lc=eng&ckey=873787&nid=-536900532.536905430.08&id=873787 If this link doesn't go through, there is another link posted on my web site, www.bethesignal.com <http://www.bethesignal.com/> , under the "where's eric" tab at the top of the splash page. I also posted a copy of the handouts for this talk on my web site under the features articles tab. The bottom line is that to make a transparent differential via, you want to make the return loss, SDD11, as large a negative dB value as you can, and make the insertion loss, SDD21 as close to 0 dB as you can. As a quick estimate, and to provide guidance on which knobs to tweak, and in which direction, you can fit the via to an ideal differential pair model, with a diff stub attached, as needed, to extract the differential impedance and the degree of coupling. In looking at a number of stack up configurations, I typically find that if nothing special is done in the design, the coupling, Z21, is small and the diagonal element, Z11, is very low. To match the via to a 100 ohm diff impedance usually requires increasing the single ended impedance by such things as removing the NFPs and increasing the antipads clearance holes. As I show in my talk, since there still is considerable coupling, you can't just take two 50 Ohms single ended vias and have them be optimized for 100 Ohms differential vias. You have to include the impact for the coupling. Feel free to drop me a note off line if you have any questions about the method I used. I hope to have this talk posted as an online lecture in the next month. --eric ************************************** Dr. Eric Bogatin, President Bogatin Enterprises, LLC Setting the Standard for Signal Integrity Training 26235 w 110th terr Olathe, KS 66061 v: 913-393-1305 f: 913-393-0929 c:913-424-4333 e:eric@xxxxxxxxxxxxxxx www.BeTheSignal.com **************************************** -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Ralph A Wilson III Sent: Wednesday, May 02, 2007 8:02 AM To: si-list Subject: [SI-LIST] Understanding S-Parameters 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 ------------------------------------------------------------------ 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