Re: [SI-LIST] : 2.5Gbps across a backplane?

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Thu Oct 12 2000 - 09:56:02 PDT


Bob, as I already said - (I'm no theoretician and use other peoples'
findings, but..) - Broadside coupling in Hi-Speed works fine for long
distances, we have it done and successfully so, and as you pointed out - you
are NOT supposed to have much common mode signal - that is why you use
difpairs with sizable distance between them (3 dielectric thickness
distances are the rule of thumb) and compatible DC coupling or AC coupling
in case of non-direcvtly-compatible ECL logic.
If you have commonm mode - you should remove it in the first place, and not
to substitute it by making your design work even though you have it (of
course reasonable common-mode protection, like common mode termination etc
should be used where you expect it to exist to a degree).

Bob Perlman wrote:

> Hi -
>
> This is a great discussion.
>
> Can someone post either theoretical or experimental evidence to
> show that the coupling type (loose, tight) along the entire path
> must be the same? In particular,
>
> 1) If I have loose coupling in the connectors and tight coupling on
> the boards, what's the consequence?
>
> 2) If I have broadside-coupled traces on the backplane and edge-
> coupled traces on the daughtercards (or vice versa), why would this
> matter?
>
> Granted, there might be a common-mode impedance discontinuity,
> but if we have a significant amount of common-mode voltage on
> these signals, aren't we in trouble already? After all, a lot of the
> 2.5Gbps links I've seen people propose have no meaningful
> common-mode termination in the first place.
>
> Thanks,
> Bob Perlman
>
> > I support Vinu's advice. If the connector is loosely coupled or not
> > coupled at all, then the traces on the backplane should be layed out
> > the same way.
> >
> > Chris
> >
> > -----Original Message-----
> > From: Vinu Arumugham [mailto:vinu@cisco.com]
> > Sent: Wednesday, October 11, 2000 12:41 PM
> > To: Michael Nudelman
> > Cc: Todd Derego; si-list@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : 2.5Gbps across a backplane?
> >
> >
> > Broadside coupling usually means significant coupling between the
> > signal conductors of the differential pair. However, many backplane
> > connectors may have little coupling between the signals of a
> > differential pair.
> >
> > For optimal performance, the type of coupling used on the board may
> > have to be compatible with the type of coupling in the connector.
> >
> > Thanks,
> > Vinu
> >
> > Michael Nudelman wrote:
> >
> > > Todd:
> > >
> > > First, do not use FR-4.
> > > Try something else. We do, and it works without equalization. And
> > > the
> > backplane
> > > is huge. The traces' runs are up to few feet. Also try to use
> > > broadside coupling; if your skin losses are high, this helps a bit.
> > >
> > > Mike.
> > >
> > > Todd Derego wrote:
> > >
> > > > I'm working to take a present 1.25Gbps backplane to
> > > > 2.5Gbps or
> > > > faster. Any thought on transceivers with preemphasis and/or
> > > > adaptive equalization to help get 2.5Gbps or better across a long
> > > > FR4 backplane?
> > The
> > > > eye looks pretty collapsed at 2.5Gbps but I have seen some
> > > > products the drive across a twisted pair and seem to recover a
> > > > signal from something
> > that
> > > > does look much like a signal.
> > > > Thanks!
> > > > Todd
> > > >
> > > > Todd DeRego
> > > > Senior Signal Integrity Engineer
> > > > Lucent Technologies INS
> > > > 200 Nickerson Road
> > > > Marlborough MA 01752
> > > > (508) 786-2168
> > > > tderego@lucent.com
> > > > >
> > > >
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