Stephen - I've seen this before also. besides trace length differences, I believe it can also happen when the source impedance for the pull-up FET and the pull-down FET are different. If the source-Z (aka drive-strength) is different, then this can have an effect on the rise & fall time (less energy initially injected onto the t-line). If the rise and fall times are not equal, then the crossing point won't be in the middle. This should be true for an eye diagram of a single-ended signal or an eye diagram of a differential signal. the short answer for both the trace length answer you gave and the one above : no symmetry. Jim Peterson Honeywell -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Stephen Greenhalgh Sent: Tuesday, January 17, 2006 4:03 AM To: Si-List (E-mail) Subject: [SI-LIST] Eye diagram Recently I saw an eye diagram in which all the crossovers occurred at 70-80% amplitude rather than the ideal 50%. Apart from this obvious defect, the eye looked pretty good. Ever since, I have been wondering what would cause this effect. My first thought was some difference between the +ve and -ve traces of the differential signal path, but I cannot formulate a theory that explains this satisfactorily. For example, if one trace were longer than the other, I would expect both positive-going and negative-going edges on the longer trace to be delayed relative to the shorter one, which would mean that some crossovers were raised, and others lowered, from the 50% level. Is this a known phenomenon that I have somehow completely missed? I would be glad for any explanation or suggestion that anyone can offer. Stephen Greenhalgh ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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