[SI-LIST] Re: Students - matching 1 mil IEEE1394/ethernet guidelines and DM to CM conversion
- From: Bill Owsley <wdowsley@xxxxxxxxx>
- To: steve weir <weirsi@xxxxxxxxxx>
- Date: Sat, 2 Jun 2007 19:06:10 -0700 (PDT)
Steve,
I would if I could, but alas I have been mandated (pay check dependent) to
produce product that goes out the door with the minimum fuss possible and with
no returns for redesigns. There has been no interest (funding) to pay for
demonstrating in a public forum what is considered by some employers as a
competitive advantage. Let rational engineering rule.
ps. And you mentioned in the note below the very area I was thinking of for
my assertion, and the students apparent area of work. And smaller route, the
more the effect, such as within the integrated magnetic RJ's and the wiring to
them, but I found the effect first in the discrete magnetics, some good and
some not so good. Remove the package, or x-ray the internals, to see the
routing.
Curious how the original problem limitations dropped away during the
discussions that followed. I found it to be one of the more stimulating
discussions on the list, and as one contributer suggested, one ripe for
research in the many areas of diff-pair use.
As for those atrocious integrated magnetic RJ's, I'm afraid I may have
provided the reason for developing them when the choice of discrete magnetics
manifested a remarkable ability to convert DM to CM. A different vendor had a
much better product, but a different pin out and so could not be substituted.
We shipped a most embarrassing product wrt EMC, but not for long for as soon as
the integrated RJ ramped up volumns, in it went.
steve weir <weirsi@xxxxxxxxxx> wrote:
Bill, Scott delivered a very good paper on this subject at DesignCon
2005. While there are methods to reduce the effects, what was found
from a good test platform is that the random behavior only serves to
determine the distribution of the skew. It does not normalize it
towards zero. Unless specific measures are taken: choice of weave,
routing pattern wrt the weave etc ( some of which are or were recently
under patent dispute ), the 3-10ps / inch skew that Scott mentioned is
very real. When best methods are employed, that can be reduced to
perhaps 1.5-2ps/inch. On top of this there is the bandwidth limitation
of any particular transmission line design with whatever materials we
choose. What this says to me is that only very short lines can benefit
from a pretty tight match. But most cases I am familiar with, those
would be lines going to magnetics. When dealing with manufacturing
variation atrocities like many of the RJ's out there with integrated
magnetics, 1 mil, 10 mils, even 100 mils mismatch is pretty hard to for
a guy like me to see. If it's really free I see no harm. I am really
pressed, and would be delighted to see a real statistically valid study
that showed that a particular physical path match length provides a
measureable and meaningful benefit.
Best Regards,
Steve.
Bill Owsley wrote:
> Most systems are quite functional (SI) when presented to EMC for testing, and
> quite often fail. And as so many have very eloquently (where's my spell
> check) explained, there is not likely any 1 mil mis-match in a matched pair
> that caused it. So given my realm of influence, (which certainly is not the
> weave of FR4. Is there a felt or random pattern FR4?) I ask for continuous
> incremental improvements (that don't get me necktie party). I suspect that
> the weave variation of FR4 over any run of interest would have a plus and
> minus shift that on the average would come out near the nominal - remember
> odd/even number of twists for a pair, that odd number twist would unlock
> pandoras box. And certainly the other sources mentioned that are not the
> diff-pair trace length can be the dominate source of problem, but they were
> not part of the trace length constraint.
>
> Thanks to those that brought up the BER, RJ, any other jitter, eye diagrams
> and that stuff of SI world. All those numbers scare me in some fashion, they
> are so big.
> And it seems that a better match in trace lengths given the phase percentage
> mentioned in another note and other descriptions of the effects, or lack of,
> for excessive constraints for differential signalling, that the little X mark
> in the middle of eye diagram seems to shrink, the supposed flat segments of
> the eye diagram are a little bit flatter. In a tightly couple pair, the
> forward crosstalk to the other signal of the pair is a little bit closer in
> phase with better matching and so does not cause as much of a slight shift in
> the crossover or switching point, leading to less jitter. Some of the
> multilevel signalling has such small differences in the discrete levels that
> any small improvement in the little effects that degrade these levels would
> seem to be better. Does a 1 mil request/constraint do that and at what cost?
> Well now, that all depends on where you would like to be in the market.
>
>
>
> Chris Cheng wrote:
> Show me a case where 1 mil difference will break SI.
> Then.
> Show me a case where 1 mil difference will break EMI but not SI.
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Bill Owsley
> Sent: Friday, June 01, 2007 2:29 PM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Students - matching 1 mil IEEE1394/ethernet
> guidelines and DM to CM conversion
>
>
> If any students are still with us, the ongoing interchange might indicate
> that these subjects are indeed interesting and can be somewhat complex in
> that there are a number of variable to keep in mind - all at once.
> And maybe enough information to get your project done well.
> You're welcome... < > really stupid grin within the brackets
>
> And this all started with a simple help me with my project question.
>
> Since this is an SI list, the EMC aspects seem a little less important. I'm
> reminded of a class on how to use one of those CAD tools for schematic
> capture, layout, SI, EMC. The SI guys got to go home a day early since their
> concern in class was millivolts. The EMC guys had to stay over a day to work
> on the microvolts part.
>
> And there is at least one layout group that has for me, a short rope and tall
> tree. But I love them anyway.
>
> ---------------------------------
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