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

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From: Jinhua Chen (chen@nesa.com)
Date: Thu Oct 12 2000 - 06:32:59 PDT


Hi, Mike

Could you send a copy of pre-emphasis at 3.125G across 20 inches
of FR4 (microstrip) to me? I am very interested in the pre-emphasis
data. In general, the loss is linear to the length of PCB. The pre-emphasis
is trying to compensate for the high frequency PCB loss (dielectric loss +
skin effect loss). Assume the pre-emphasis is designed to compensate for L
inches
of FR4 material. When the real PCB length is shorter than L, the pre-emphasis
may over compensate. When the real PCB length is longer than L, the
pre-emphasis
may under compensate. Different dielectric material might have different
L. It is hard to optimize the signals with pre-emphasis.

This is just my 2 cents!

Regards!

At 01:27 PM 10/11/00 -0700, you wrote:
>Todd,
>
>Looks like you hit a hot button on the SI list, so I'll add
>my two cents, as well. On the issue of 'broadside' coupling,
>while the advantages are as billed, there are two drawbacks.
>One is the common mode impedance, which is probably high,
>resulting in mismatched common mode. Depending on how much
>common mode is in your signal, this will have EMI implications.
>The second drawback is the complexity at the ends of the line.
>one line (at least) of the pair must change planes. Resulting
>asymmetrical parasitics on the dif'l pair layout can cause
>common mode components in the signal if you're not very careful.
>For short runs, it might not be worth it.
>
>My second comment is about pre-emphasis, which was part of your
>initial question. Hoping you don't mind, I will send you a PDF
>showing the effect of pre-emphasis at 3.125G across 20 inches
>of FR4 (microstrip). It's the same material presented at last
>November's 10G Ethernet meeting, but those presentations are no
>longer public. (I didn't want to spam everyone's mailbox with
>435K, but if anyone else wants a copy, please let me know.)
>
>There was another presentation on post-emphasis at the recent
>Fibre Channel plenary meetings, but it's not up on the web site
>yet. The performance was quite impressive.
>
>Lastly, the required frequency content of 2.5Gb/s doesn't go very
>much above 1.25G, so interpret any FR4 loss curves appropriately.
>
>Hope that helps.
>
>Regards,
>Mike Jenkins
>LSI Logic
>
>
>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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>--
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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> LSI Logic Corp, ms/G715 Fax: 408.433.7461 LSI|LOGIC| (R)
> 1525 McCarthy Blvd. mailto:Jenkins@LSIL.com | |
> Milpitas, CA 95035 http://www.lsilogic.com |_____|
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