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

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Mon Oct 16 2000 - 13:12:58 PDT


Edward,

I am not really sure how equalization helps to get rid of crosstalk. Unless I
misunderstood the question (long sentences have that effect on me; when I reach
the end, I forget the beginning :-)

The point of equalization, as I understand it, is to flatten the response of a
transmission system in a used bandwidth. For passive equalization, this will
come at the expence of total gain.

And the bandwidth is really wide - say, SONET has the upper limit of
...whatever, for OC48 it is 2.5GHz, but the lower limit stays 8kHz.

But to protect from crosstalk you cannot really use it. You need something
else; the best protection is a good clean layout.

Did I miss something in your question?

Mike.

"Dr. Edward P. Sayre" wrote:

> Folks:
>
> A subtle issue came up in the recent T11 (Fibre Channel) meetings. How do
> you spec an interconnect when adaptive equalization parts will be used to
> dig out the signal? If you have really good eye-diagrams response then do
> you need equalization? Conversely, if you need equalization because of
> excessive deterministic jitter (read that insufficient rise time bandwidth)
> or eye closure (read that too much attenuation) or both, then what
> constitutes an acceptable way to specify the interconnect, be it backplane,
> cable or connector?
>
> Now, the question of cross talk corruption of the signals is another
> matter, especially when equalization is involved. Cross talk can be of no
> consequence or it can be of terrible consequence, especially if there is a
> requirement for byte (a 4 bit nibble) aligned serial data streams. Our
> work indicates that connector cross talk in differential connectors appears
> to be essentially dV/dt noise due to any asymmetries in the geometry of the
> connector or interconnect (vias count here!). Although small, and
> generally in the (1 - 4%) range, it is obviously a problem when many lines
> are switching and especially if the time alignment is not within a small %
> of the rise time between individual differential components. If the signal
> is broken into its even (common mode) and odd (differential) mode
> components, it can be shown that very soon the common mode cross talk
> dominates the cross talk.
>
> So, when is cross talk an equalization issue from a differential eye
> diagram point of view; and can equalization help from an EMI point of view?
> I look forward to folks' ideas on this matter.
>
> ed sayre
>
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
> | NORTH EAST SYSTEMS ASSOCIATES, INC. |
> | ------------------------------------- |
> | "High Performance Engineering & Design" |
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
> | Dr. Ed Sayre e-mail: esayre@nesa.com |
> | NESA, Inc. http://www.nesa.com/ |
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