[SI-LIST] Re: Relevance of Common Mode Return Loss

> By the way how would one measure the interconnect/package/on-chip circuitry 
> to evaluate the conversion of the differential signal into common signal 
> (SCD21)?  
>
> -Jory
>
>   
Jory,

One would perform measurement correlated modeling and simulation.  It's 
simple enough to correlate a 3D full-wave modeling package to 
interconnect, package and substrate measurements. Nothing new here.  If 
you understand your manufacturing tolerances, it's then simple enough to 
perform worst-case modeling of the Scd21 and Sdc21. 

I'm with Eric.  I see no reason why common mode return loss is specified 
for receivers, unless it is used in a specification to force a 
requirement for common mode termination at the receiver. Otherwise, it 
can limit some good alternative package and receiver designs, especially 
at ultra-high frequencies, where it makes sense to block common modes 
from the receiver inputs, in return for much better differential 
insertion loss characteristics.


Scott

>
> 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:
> http://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:    
>         http://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:
> http://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:     
>               http://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:
http://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:     
                http://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

Other related posts: