As Steve stated, you have a mismatch somewhere in your routing. Suggest you look at layer changes and number of VIAs in each of the diff pair to see if both are the same. If they do change layers, check how that changes regarding your planes and there distance from them. Check the driver strength so see if these are being overdriven and you are having ring back from undershoot/overshoot. Lastly, if you can find nothing wrong question if all the input into the simulator is correct. Omega -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of qazi Sent: Monday, August 25, 2008 8:20 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Ringing in Return Loss data Gurus, I can explain ringings on a TDR data but can anyone tell me why I am seeing this on a Return Loss data for a layout that is geometrically correctly done? This is a simulation result based on a .brd file in allegro and I was wondering if I was doing something wrong here as far as modeling the device or the SMAs. This board uses Fr-4 and the traces are around 4.5 inches long diff pairs. I know the impedances are kept constant (same geometry) throughout the whole path and return path is ideal. Rgds, Qazi -- PRIVACY WARNING: For auditing purposes, a copy of this message has been saved in a permanent database. ------------------------------------------------------------------ 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