Re: [BP] Dielectric Weave effects

[Joel Goergen <joel@xxxxxxxxxxxxxxxxxxx>]

Title:

All,
The differential pair skew per inch is from many factors, the least of which is glass construction.  The greatest, outside of the connector, is trace length mismatch from route direction changes (45s or rounds).  The second greatest is from length matching where a toroidal effect is not considered.

I would offer that there comes a time when we have to stop writing the standard for poor design practices.  A designer should always be targeting less then 5ps point to point.  My feeling is that 20ps is fine and does not violate any US patents in any system I have been involved with.  I wouldn't want to even try and debug a system with 50ps skew ... that would be horrid and unpredictable.

As I said ... sooner or later we have to consider reasonable engineering as a solution.

-joel

Scott McMorrow wrote:

I am not currently aware of any systematic study of differential pair skew on production backplanes, due to the problem of accurate instrumentation of hundreds of channels.  However, there are occasional anecdotal measurements that have been made by some connector vendors on customer backplanes which have shown 40 to 50 ps skew from time to time on high BER channels.  Teradyne and Teraspeed Consulting will begin conducting a systematic study of backplane length differential pair skew this summer, on a variety of standard backplane stackup and trace configurations.  That data should be available by fall, and allow us to set some fundamental skew limits on weave-parallel routed tracks and off-angle tracks.  I believe that Intel Labs is also involved in a similar study.   However, I believe thate Rich is correct. A theoretical upper skew bound of 6 ps/in does currently exist for many stackup, material and trace geometry combinations.  In addition, a lower skew asymptotic bound for optimal routing strategies w.r.t. the weave has been shown to exist, but has not been adequately quantified by anyone that I am aware of.  However, this may eventually prove to be elusive due to low level defects in the fiberglass structure and non-uniform epoxy Er.

It is my belief that when laminate weave skew is placed into the mix and common mode conversion is properly accounted for, non-NRZ encoding schemes may have enough UI headroom to ultimately outperform NRZ signaling, in the worst cases.  However, this is only a hunch and not yet substantiated.  However, I think it behooves the committee to consider laminate weave skew in the specification.  It is my considered position that a total differential skew specification of 20 ps in an end-to-end system is not achievable on epoxy/glass composites without extra ordinary efforts and license of (or violation of) at least one major patent in this area.

There is yet another possibility that may hold promise.  Electronic skew pre-compensation may very well allow channels to work in the presence of moderate amounts of differential skew.

Regards,

Scott

Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



b_panos wrote:

Yea, S McMorrow et al.
The thing is, it really depends on things like material, bundle size, glass
dia. to trace width and separation, as well as the orientation and the
length of trace. To be quite honest, if skew is bad enough, it wouldn't meet
ISI levels and the eye wouldn't be recoverable. The RT of a 10G signal is
going to be on the order of 50ps. so any skew approaching this level, your
RL is going to look terrible, not to mention your S21 results. Just my
2cents

Regards
Bill


-----Original Message-----
From: owner-stds-802-3-blade@ieee.org
[mailto:owner-stds-802-3-blade@ieee.org]On Behalf Of Mellitz, Richard
Sent: Friday, June 10, 2005 1:43 PM
To: STDS-802-3-BLADE@listserv.ieee.org
Subject: [BP] Dielectric Weave effects


Hi All,
I started reviewing the draft doc and came across a differential skew
spec of 20 ps. Has any one actually measured skew on the published
backplanes?

It's possible to get up 6ps/inch skew in a pair due to bundle weave. Do
we need to address this issue?

These effects have been published before.

Rich Mellitz
Intel Corporation