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

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From: Bob Lewandowski (Bob.Lewandowski@Vixel.com)
Date: Wed Oct 11 2000 - 12:28:14 PDT


I've done this several times recently, and have been able to get very good
results mixing loosely coupled and tightly coupled lines. You have to have good
control of your board materials, i.e. dielectric constant and thickness, as well
as line width and spacing. The trick is in the transition region between
tightly and loosely coupled. Connectors produce 'huge' discontinuities by
comparison, even coax to pcb connectors. I also find large variations in the
answers that different analysis software gives for line width and spacing at the
extremes of coupling.

---Bob Lewandowski
    Vixel Corporation

"Heard, Chris" wrote:

> 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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