Nice and practically helpful summary, TaeKwang. On Tue, May 20, 2008 at 2:23 PM, Jeon, Tae-Kwang <Tae-Kwang.Jeon@xxxxxxx> wrote: > Hi, > > Some industrial committees already have had the SCDxx spec documented in > the standard. You may want to take a look at the FibreChannel, SATA, SAS > and OIF. Although 4G FC has no SCDxx, 8G FC adapted the SCDxx for both > Tx and Rx. As far as I know, the impedance balance spec specified in > SATA in the past has been replaced by SCD spec and the impedance > imbalance spec shown in SAS before has been replace by SCD too from the > most recent revision. OIF left the SCD spec table blank at the moment, > however, it seems they are considering the spec. I'm not quite sure why > PCIE does not have the spec, but it is interesting that all standards > that I mentioned earlier have the data rate of at least 6Gb/s for the > spec. Who knows if the next generation of PCIE spec would have the SCD > spec. > > BTW, when it comes to the return loss, we must consider not only the > termination resistor but both the load capacitance and the parasitic > capacitance because the zero frequency and the pole frequency are > determined by RC after all. Therefore, if you have bigger capacitance, > the pole frequency will get lowered which may result in failure to meet > the spec. > > Regards, > TK > > -----Original Message----- > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] > On Behalf Of Eric Bogatin > Sent: Tuesday, May 20, 2008 8:01 AM > To: 'David Instone'; olaney@xxxxxxxx > Cc: si-list@xxxxxxxxxxxxx; eric@xxxxxxxxxxxxxxx > Subject: [SI-LIST] Re: Relevance of Common Mode Return Loss > > Guys- > =20 > > 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? > > =20 > > Anyone have a feel for what a reasonable value is, before it starts to > affect the jitter, as Steve Weir pointed out? > > =20 > > 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? > > =20 > > 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. > > =20 > > 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) > > =20 > > 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. > > =20 > > Does anyone have any insight on the discussions that went on at the > committee meetings for the specs? > > =20 > > Thanks > > =20 > > --eric > > =20 > > =20 > > =20 > > ******************************************************* > > 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 <http://www.bethesignal.com/> > > =20 > > 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 > > =20 > > 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=20 > > 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=20 > > much this matters of course depends on how imbalanced the differential=20 > > lines are and how imbalanced the signal is.=20 > > Centre tapping the differential terminating resistor, at the Rx, to=20 > > ground only fully terminates the common mode if there is little coupling > > > between the lines, if they are coupled then three resistors are=20 > > required, 1 from each line to gnd to terminate the even mode and 1=20 > > across the lines, which in parallel with the other two terminates the=20 > > odd mode, how necessary this is depends on how tightly coupled the=20 > > lines are. Currently SATA and PCIe implementations that I have seen=20 > > have the Rx termination inside the chip which makes the 3 resistor=20 > > termination difficult to achieve. This makes a common and differential=20 > > return loss specification at the Rx relevant as it enables the traces=20 > > and cable to be designed to match the termination in both modes. > > =20 > > =20 > > Regards > > Dave Instone > > +44 (0)1235 824963 > > =20 > > OXFORD SEMICONDUCTOR LIMITED > > 25 MILTON PARK > > ABINGDON > > OXFORDSHIRE > > OX14 4SH > > Registered in England no 2733820 > > Registered Address: As above=20 > > =20 > > =20 > > =20 > > olaney@xxxxxxxx wrote: > > > Since we don't want them, common mode signals can be treated=20 > > > differently than for differential mode. Given that common mode is=20 > > > undesirable, at the transmit end we often use a deliberate mismatch=20 > > > (CMC) to reflect this signal back to the transmitter. This energy can > > > > be absorbed by the transmitter if there is an adequate common mode=20 > > > backmatch, or it can be left to ring between the driver and choke if=20 > > > 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. =20 > > > 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=20 > > > 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: > > > =20 > > > Orin Laney > > > =20 > > > =20 > > =20 > > > > ------------------------------------------------------------------ > 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: =20 > //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 > =20 > > ------------------------------------------------------------------ > 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