Attached is the top layer data showing the 30 mil mismatch and this is compensated at the end of routes by adding loops. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of qazi Sent: Tuesday, August 26, 2008 10:45 AM To: 'istvan Novak' Cc: 'Jimmy'; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Ringing in Return Loss data Thank you guys. Yes I do not have any mismatch in routing other than that the phase matching of the P and N of the pairs are slightly out of sync. It is on the order of .03" and that is the only mismatch I see in the design. Istvan, do you have any reference document for the effect that you mentioned? I would like to read this in little bit details. -Qazi -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of istvan Novak Sent: Monday, August 25, 2008 7:35 PM To: 'qazi' Cc: Jimmy; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Ringing in Return Loss data Qazi, Just to clarify: if this is Return Loss and not Insertion Loss, the periodic ripple (ringing) is inevitable. Actually it is inevitable in both, just in Insertion Loss it is hardly visible if the trace is close enough to the reference impedance. Return Loss, on the other hand, periodically drops to zero (minus infinite on the dB scale), wherever the frequency corresponds to multiples of half wavelength over the trace length. At even multiples of quarter-wave resonance the Return Loss reaches its maximum. Unless the trace is completely ideal, we can not get a frequency independent characteristic impedance, so even if the trace's characteristic impedance is close to the reference impedance, the nulls at multiples of the half-wavelength resonance make the remaining small mismatch very obvious. The Insertion Loss, on the other hand, will 'flatten out' to the naked eye if the characteristic impedance comes to within a couple of percent of the reference impedance. Regards, Istvan Novak SUN Microsystems Jimmy wrote: > 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 > > > > ------------------------------------------------------------------ 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 -- PRIVACY WARNING: For auditing purposes, a copy of this message has been saved in a permanent database. -- 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 -- 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