[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
>  
>  
> On Mon, 19 May 2008 09:23:10 +0100 David Instone 
> <dave.instone@xxxxxxxxxx <mailto:dave.instone@xxxxxxxxxx>> writes:
> > For the best signal fidelity one has to match and terminate the
> > common
> > mode impedance as well as the differential mode.  Therefore every
> > one
> > has to be using the same impedances for both.  Thus a specification
> > such
> > as PCIe has to specify the common mode impedance as well as the
> > differential. If you specify an impedance you must also specify a
> > tolerance on that impedance; this can be done either with an
> > impedance
> > tolerance in ohms or as a return loss specification.
> >
> > 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
> >
> >
> >
> > Randy May wrote:
> > > Greetings,
> > > I've been trying to understand the relevance of common mode return
> > loss in
> > > high speed specifications.  I thought that most differential
> > receivers would
> > > reject common mode noise on a link, and amplify the differential,
> > making the
> > > differential spec far more important.  An example spec is PCI
> > Express Gen2
> > > which calls for <= -6db from 50MHz to 2.5GHz.  What is the impact
> > if I
> > > violate the common mode return loss spec?  What is the benefit to
> > me beating
> > > it?
> > >
> > > I've also noticed that some spec's have a common mode return loss
> > > requirement on the receiver and not on the transmitter.  Any
> > thoughts on why
> > > this might have been done?
> > >
> > > Thanks
> > >
> > >
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