For the best signal fidelity one has to match and terminate the common mode impedance as well as the differential mode. Therefore every one has to be using the same impedances for both. Thus a specification such as PCIe has to specify the common mode impedance as well as the differential. If you specify an impedance you must also specify a tolerance on that impedance; this can be done either with an impedance tolerance in ohms or as a return loss specification. Regards Dave Instone +44 (0)1235 824963 OXFORD SEMICONDUCTOR LIMITED 25 MILTON PARK ABINGDON OXFORDSHIRE OX14 4SH Registered in England no 2733820 Registered Address: As above Randy May wrote: > Greetings, > I've been trying to understand the relevance of common mode return loss in > high speed specifications. I thought that most differential receivers would > reject common mode noise on a link, and amplify the differential, making the > differential spec far more important. An example spec is PCI Express Gen2 > which calls for <= -6db from 50MHz to 2.5GHz. What is the impact if I > violate the common mode return loss spec? What is the benefit to me beating > it? > > I've also noticed that some spec's have a common mode return loss > requirement on the receiver and not on the transmitter. Any thoughts on why > this might have been done? > > Thanks > > > ------------------------------------------------------------------ > 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