[SI-LIST] Re: Relevance of Common Mode Return Loss
- From: "Kihong Joshua Kim" <joshuakh@xxxxxxxxx>
- To: "Jeon, Tae-Kwang" <Tae-Kwang.Jeon@xxxxxxx>
- Date: Tue, 20 May 2008 17:57:17 -0400
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 SATA in the past has been replaced by SCD spec and the impedance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
>
>
>
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