[SI-LIST] Re: Relevance of Common Mode Return Loss
- From: olaney@xxxxxxxx
- To: dave.instone@xxxxxxxxxx
- Date: Mon, 19 May 2008 13:45:28 -0700
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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> 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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