Re: [BP] Dielectric Weave effects
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
>
>
>
>