I somewhat
disagree. Typically, you won’t find a diff pair that suffers from the weave
effect. The probability is low. But once in a while, “in a galaxy far far
away…” Oh no no no J . Anyhow, when you
do, the effect is right at the top of the Pareto list. It can be hundreds of
ps on a long route. I have personally seen 50 ps on 8 inch routes on
computer -like boards. For many years, I tossed this off as
“anecdotal” but have come to understand the cause was inhomogeneous
dielectric effects. Not withstanding the weave effect, I can compensate most
conductor geometries with “good” design practice and yes, even twists,
turns, and connectors. Most of the time these design practices
serendipitously even be choke out the weave effect.
The good news is
that back plane have wider trace widths which are less sensitive. The line
cards are a different story.
…
Rich
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