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

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From: Vinu Arumugham (vinu@cisco.com)
Date: Fri Oct 13 2000 - 12:16:27 PDT


Bob,

Whenever there is a change in the conductor configuration along a
transmission line, there is an impedance discontinuity.
Such a discontinuity may be masked by the discontinuity caused by vias for
the connector. If the vias are improved (or eliminated), I would expect to
see the impedance mismatch caused by changes in the conductor
configuration.

Thanks,
Vinu

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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