Maybe another way to look at it is to ask where the common mode signal power is dissipated. With a common mode signal magnitude spec. or SCD21/SCD11 spec. the common mode signal power could be either radiated or converted to differential. A TX or RX common mode termination is not required with such a spec. With a common mode return loss spec., the bulk of the common mode signal power is converted to heat in the RX or TX common mode termination. Thanks, Vinu Eric Bogatin wrote: > Guys- > > > If there is so much concern about the presence of the common signal on the > interconnect, and its rattling around, shouldn't there be a spec on the > amount of common signal allowed at the receiver? > > > > Anyone have a feel for what a reasonable value is, before it starts to > affect the jitter, as Steve Weir pointed out? > > > > Lynn Greene suggested that any common signal present could be converted back > to diff and screw up the diff signal. If the source of the comm. signal is > from asymmetries in the interconnect, then isn't this the first order > problem to fix? > > > > Shouldn't there be a spec on the SCD21 performance of the interconnect to > evaluate the conversion of the differential signal into common signal? This > is the first order problem, the second order one being the conversion of > "rattling around" common signal back into differential signal, further > screwing up the diff signal. > > > > Steve Weir suggests that a spec for the SCC11 and SCC21 of an interconnect > is related to the fact it is easy to do, not that it is the most reasonable > approach. It is sort of like the joke where the punch line is, "because the > light is better over here" (only if I am publicly encouraged will I provide > the rest of the joke) > > > > I am still trying to understand the importance of the SCC11 and SCC21 spec, > as opposed to a spec on the magnitude of the common signal, or on SCD21 or > SCD11. > > > > Does anyone have any insight on the discussions that went on at the > committee meetings for the specs? > > > > Thanks > > > > --eric > > > > > > > > ******************************************************* > > Eric Bogatin > > Signal Integrity Evangelist > > Bogatin Enterprises > > Setting the Standard for Signal Integrity Training > > 26235 W 110th Terr > > Olathe, KS 66061 > > e: eric@xxxxxxxxxxxxxxx > > v: 913-393-1305 > > cell: 913-424-4333 > > f: 913-393-0929 > > www.BeTheSignal.com > > > > 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 David Instone > Sent: Tuesday, May 20, 2008 4:16 AM > To: olaney@xxxxxxxx > Cc: si-list@xxxxxxxxxxxxx > Subject: [SI-LIST] Re: Relevance of Common Mode Return Loss > > > > Orin, > > As Lynne pointed out in her posting on this subject, part of the common > > mode energy can get converted to differential. Surely then, reflecting > > the common mode back to the Tx, even if the Tx is a good CM match, would > > give it twice the opportunity to interfere with the differential. How > > much this matters of course depends on how imbalanced the differential > > lines are and how imbalanced the signal is. > > Centre tapping the differential terminating resistor, at the Rx, to > > ground only fully terminates the common mode if there is little coupling > > between the lines, if they are coupled then three resistors are > > required, 1 from each line to gnd to terminate the even mode and 1 > > across the lines, which in parallel with the other two terminates the > > odd mode, how necessary this is depends on how tightly coupled the > > lines are. Currently SATA and PCIe implementations that I have seen > > have the Rx termination inside the chip which makes the 3 resistor > > termination difficult to achieve. This makes a common and differential > > return loss specification at the Rx relevant as it enables the traces > > and cable to be designed to match the termination in both modes. > > > > > > 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 > > > > > > > > olaney@xxxxxxxx wrote: > > >> Since we don't want them, common mode signals can be treated >> > > >> differently than for differential mode. Given that common mode is >> > > >> undesirable, at the transmit end we often use a deliberate mismatch >> > > >> (CMC) to reflect this signal back to the transmitter. This energy can >> > > >> be absorbed by the transmitter if there is an adequate common mode >> > > >> backmatch, or it can be left to ring between the driver and choke if >> > > >> that is considered harmless. At the receiver, the intent of providing >> > > >> a common mode termination is simply to prevent unwanted CM energy from >> > > >> returning up the line, giving it an additional chance to radiate. >> > > >> If the common mode signal is terminated before the differential signal >> > > >> passes through a CMC to reach the DM termination at the receiver, then >> > > >> the best of both worlds is achieved: the CM signal is both absorbed >> > > >> and suppressed. The receiver common mode range becomes much harder to >> > > >> violate. For coding with a zero at DC (accepts AC coupling), a center >> > > >> tapped inductor is an easy way to provide the CM termination: >> > > >> >> > > >> Orin Laney >> > > >> >> > > >> >> > > > > > > ------------------------------------------------------------------ > 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