Hello Eric, A few thoughts...... Given that one has meet the transmit specs for a given Serdes interface some specs call out for the differential and common mode return loss as seen at the receiver (SDD11 and SCD11) in order to design the receiver interface. Does this not infer the fidelity of the received signal for receiver design? This can be measured although not trivial. I would think any S21 measurements would be difficult to make at the receiver (how do we get a probe on the chip rx). Regards, -Jory ----- Original Message ---- From: Eric Bogatin <eric@xxxxxxxxxxxxxxx> To: steve weir <weirsi@xxxxxxxxxx>; si-list@xxxxxxxxxxxxx Sent: Friday, May 16, 2008 2:29:45 PM Subject: [SI-LIST] Re: Relevance of Common Mode Return Loss Steve- I can understand how excessive common signal can contribute to increased jitter of the received differential signal in the receiver due to, as you say the finite CMRR. So, shouldn't the spec be on the amount of common signal that is present, rather than on the common insertion or return loss of the interconnect? If the worry is on the amount of common signal present, then shouldn't the spec be about the SCD21 value, which would indicate how much common signal would be converted from the differential signal? Other 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: steve weir [mailto:weirsi@xxxxxxxxxx] Sent: Friday, May 16, 2008 1:39 PM To: Eric Bogatin Subject: Re: [SI-LIST] Re: Relevance of Common Mode Return Loss Eric, CMRR is a finite number in any receive amplifier. Larger common signal components that are still within the CMR adversely impact jitter compared to smaller common signal components. Best Regards, Steve. Eric Bogatin wrote: > 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 > > > > -- Steve Weir Teraspeed Consulting Group LLC 121 North River Drive Narragansett, RI 02882 California office (866) 675-4630 Business (707) 780-1951 Fax Main office (401) 284-1827 Business (401) 284-1840 Fax Oregon office (503) 430-1065 Business (503) 430-1285 Fax http://www.teraspeed.com This e-mail contains proprietary and confidential intellectual property of Teraspeed Consulting Group LLC ---------------------------------------------------------------------------- -------------------------- Teraspeed(R) is the registered service mark of Teraspeed Consulting Group LLC ------------------------------------------------------------------ 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