It could very well be the simulations are correct and your measurements are tainted by poor measurement technique. Probing boards that were never designed to be probed can be very problematic. Probing is a SI problem and the key to good SI is keeping an eye on the return currents. The ground lead in your probing system looks like an inductor which can cause a resonant circuit with the input capacitance of the scope probe. Whenever a resonant circuit is stimulated with an edge ringing will occur. If your probe point and local ground return point are not very close to each other, say less than 0.100" and your "ground lead" is almost nonexistent in length then you will have ringing in your scope's display. When you probe a board you need to think in terms of how closely your scope probing system approximates a coaxial connection to the board. How well are the fields contained in the probing system? Hope this helps. Tom Dagostino Teraspeed Labs 13610 SW Harness Lane Beaverton, OR 97008 503-430-1065 tom@xxxxxxxxxxxxx www.teraspeed.com Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 401-284-1827 www.teraspeed.com -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Santangelo, Steven Sent: Tuesday, June 08, 2010 7:23 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Correlation to lab measurement Hi All, I'm trying to correlate some lab measurements to my simulations in Cadence PCB SI (SpectraQuest). I have a 400MHz unidirectional LVDS interface between two Xilinx devices that is terminated in 100ohms inside one of the devices. My lab measurements show a pretty good reflection occurring on the rising and falling edges of the waveform. I'm trying to determine if this is being caused by the ball to I/O pad trace length in the package (plus my BGA breakout). My simulation only shows a slight bend in the waveform rather than the non monotonic ring that I see in the lab. The bend does correlate well time wise (location on the edge) with the lab measurement. I don't really know much about the limitations of IBIS models so I'm wondering if some of the discrepancy could be due to that. FYI: I'm using the models that Xilinx "ibiswriter" puts out. The rise/fall times of the waveforms are in the neighborhood of 300pS. Thanks Steve ------------------------------------------------------------------ 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