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

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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