If one has no choice but to trust just simulation, I have had good luck with SiSoft and SIwave in answering questions such as this capacitor location one. In my opinion, it does matter where the capacitor is placed. Extracting every detail (planes, vias, pads, traces, etc.) of the physical PCB design with a highly accurate field-solver increases the chances that you get the correct answer to questions such as this. We can't always validate the simulations we do, but in cases where I could, these simulation tools provided results that correlated extremely well with lab measurements in both the time and frequency domains. Best regards, Steve Stephen P. Zinck High-Speed Signal Integrity Consulting Interconnect Engineering Inc. P.O. Box 577 South Berwick, ME 03908 Phone - (207) 384-8280 Email - szinck@xxxxxxxxxxxxxxxxxxxxxxxxxxx Web - www.interconnectengineering.com -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Istvan Novak Sent: Thursday, May 13, 2010 9:02 PM To: leeritchey@xxxxxxxxxxxxx Cc: fei xue; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: AC series capacitor position in high speeddifferential signals Lee, I agree with you that people should not blindly trust only simulation results. Those who follow my work know that for me a question answered means that measurements, simulations and understanding all have to agree to a sufficient degree. That having said I cant argue with your statement when you say that your measurements show negligible reflections in your boards: I trust it is true for the boards you built. What prompted my posting was that your original comment could be interpreted as a suggestion that the capacitor placement makes no difference under any circumstances as long as the channel is linear. My point was that there are situations (and we dont need to think about something exotic, just cases when pad discontinuity and/or via stub discontinuities are not removed for any reason) when the location of capacitor makes a difference in the received eye parameters by a noticeable way. With very typical parameters we can end up having 15-20% fluctuation of eye closure horizontally and vertically as we slide the capacitor along the channel. This can be simulated, but can also be measured on real system channels. Regards, Istvan Novak Oracle Lee Ritchey wrote: > I would maintain from my measurements that the reflections are negligibly > small. > > I would also maintain that simulation results, no matter what the tool or > operator, should not be trusted until they are validated by measurements to > insure the modelling is accurate. I've seen too many simulations done > incorrectly which were used to develop design rules that were either > ineffective or caused problems that I, for one, will not use the results of > unvalidated simulations. > > Each engineer can make up his or her own mind on this, but my experience > says validate simulation results before betting any money on them. > > I'm not trying to offend anyone who does simulations, just advising proceed > with caution. > > Lee > > > >> [Original Message] >> From: Istvan Novak <istvan.novak@xxxxxxx> >> To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx> >> Cc: fei xue <harrison_cls@xxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx> >> Date: 5/12/2010 8:55:32 PM >> Subject: Re: [SI-LIST] Re: AC series capacitor position in high >> > speeddifferential signals > >> Lee, >> >> As it was pointed out in earlier threads, location does make a >> difference unless reflections are >> negligibly small. Assuming linearity, when we move components around, >> reciprocity prevails, >> but voltage transfer function from source to load will change, which in >> turn impacts eye >> parameters. >> >> Regards, >> >> Istvan Novak >> Oracle-America >> >> >> Lee Ritchey wrote: >> >>> This has been answered before on this forum. Since the circuits are >>> linear, it does not matter from a signal integrity point of view. >>> >>> Lee >>> >>> >>> >>> >>>> [Original Message] >>>> From: fei xue <harrison_cls@xxxxxxxxxxxx> >>>> To: <si-list@xxxxxxxxxxxxx> >>>> Date: 5/12/2010 11:35:49 AM >>>> Subject: [SI-LIST] AC series capacitor position in high speed >>>> >>>> >>> differential signals >>> >>> >>>> Hello all, >>>> We often can get different guideline of placing capacitor position when >>>> >>>> >>> placing AC series capacitor on high speed differential signals, like >>> > PCIe, > >>> SAS or LVDS signals. sometimes we followed the guideline to put >>> > capacitors > >>> near driver, sometimes near multi-connection connectors or sometimes >>> > put it > >>> near receivers. >>> >>> >>>> Could anybody tell me what is the consideration of capacitor placing >>>> >>>> >>> position? Thanks! >>> >>> >>>> Harrison >>>> >>>> >>>> >>>> >>>> > > > > ------------------------------------------------------------------ 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 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 Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu