[SI-LIST] Re: Relevance of Common Mode Return Loss
- From: Vinu Arumugham <vinu@xxxxxxxxx>
- To: Eric Bogatin <eric@xxxxxxxxxxxxxxx>
- Date: Tue, 20 May 2008 12:18:52 -0700
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
>
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>
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> -----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
>
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>
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>
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>
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>
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>
> 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
>>
>
>
>>
>>
>
>
>>
>>
>
>
>
>
>
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