Hi folks- I thought Randy asked an interesting question about the common return loss spec. I don't recall seeing any responses. I'd like to hear comments. I have seen a number of specs that call out a differential impedance spec and a common impedance spec, as well as a differential and common return and insertion loss spec. When I asked engineers about it, all I could got was, this was the behavior of the reference systems we built and so we expect all future systems to meet this spec. Other than the potential of saturation of receivers if the common signal gets too large, and the EMI problem if the common signal gets out on external cables, is there another compelling reason to spec the common impedance or return or insertion loss for a cable or interconnect link? If the differential performance is met, is there a performance reason for a common signal spec? Thanks for your comments. --eric ************************************** Dr. Eric Bogatin, Signal Integrity Evangelist Bogatin Enterprises, LLC Setting the Standard for Signal Integrity Training 26235 w 110th terr Olathe, KS 66061 v: 913-393-1305 f: 913-393-0929 c:913-424-4333 e:eric@xxxxxxxxxxxxxxx www.BeTheSignal.com Upcoming Signal Integrity Classes Winnersh, UK, TSI, May 20, 2008 San Diego: EPSI, BBDP, July 28-31, 2008 San Jose, SICT, Aug 12-13 San Jose, EPSI, BBDP, Sept 29-Oct 2 **************************************** -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Randy May Sent: Thursday, May 08, 2008 1:27 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Relevance of Common Mode Return Loss 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