Chundi, The short answer is that the three models attempt to represent the variation you can expect in performance across all the chips, and you should be using all of the models. A longer answer begins with a description of what these models represent. In brief, IC fabrication is not absolutely exact. All the etching, imaging, and doping processes have some small amount of variation. The variation exists between fabrication lots, within a single wafer, and even within each IC. The physical variations result in electrical variations in performance. Each transistor on an IC behaves slightly different than the transistor next to it. Generally, the variation on a single IC is small, but it does exist. For large production ICs that are fabricated over several wafer lots over differetn days, weeks, months, or years, the variation can get large. Electrically, the variation manifests itself in all kinds of ways. The parasitic capacitances are way, as you suggest. But other things happen as well, such as the gain, resistance, leakage, etc. Considering an HSPICE transistor model with all its parameters, nearly every one of those parameters has some amount of variation in performance due to the manufacturing tolerance. Beyond fabrication tolerance, the models often also try to account for voltage variations and temperature extremes. So, if you have a 1.0 V IC with a 10% voltage tolerance, the slow model often represents the performance of the IC when 900 mV is appplied, etc. It is impractical to provide models to people that account for every possible variation. Instead, manufacturers simplify the variations by offering three representative cases: slow, typical, fast (or weak, typical, strong). These models do not exactly represent what you will see in all your ICs, but they come reasonably close. If you take a detailed look in the lab at any IC, it will not perform exactly as indicated by a "typical" model, but it should come close. If you then take another IC, it too will come close, but it will behave differently. If you take many such Ics in the lab, measure them, and compare them to the models, all the ICs should perform within the range indicated by the slow/typical/fast models. So, if you are trying to develop a robust system that will operate with any of the ICs you are likely to get, then you should be running your simulations with all the models variations. As Steve indicates, though, typical models are a great place to start for SI analysis. We commonly use the typical models to make decisions about terminations, trace design issues, etc. Once we get reasonable performance using the typical models, we then run a matrix of simualations that use an assortment of slow, typical, and fast models and make sure everything still works. In many cases, the inclusion of these model extremes breaks the link whereby we no longer meet margin, and we are forced to adjust our design. Good luck. Pat Zabinski Mayo Clinic -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of chundi srikanth Sent: Friday, March 05, 2010 5:16 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Query about Hyperlynx tool Hi All, Iam using Hyperlynx Boardsim tool. I have a query on this tool. There is provision in the tool to select the IC modelling like slow-week,typical and fast-strong modelling. So as far as my knowledge is concerned these modellings vary on the basis of RC parasitics. I would like know whether my assumption is correct? And generally which modelling we need to select and what basis we need to select this for single-ended and differential high-speed signal simulations. Thanks Chundi ------------------------------------------------------------------ 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