[SI-LIST] Re: Students - matching 1 mil IEEE1394/ethernet guidelines and DM to CM conversion
- From: steve weir <weirsi@xxxxxxxxxx>
- To: wdowsley@xxxxxxxxx
- Date: Sat, 02 Jun 2007 10:17:03 -0700
Bill, Scott delivered a very good paper on this subject at DesignCon=20
2005. While there are methods to reduce the effects, what was found=20
from a good test platform is that the random behavior only serves to=20
determine the distribution of the skew. It does not normalize it=20
towards zero. Unless specific measures are taken: choice of weave,=20
routing pattern wrt the weave etc ( some of which are or were recently=20
under patent dispute ), the 3-10ps / inch skew that Scott mentioned is=20
very real. When best methods are employed, that can be reduced to=20
perhaps 1.5-2ps/inch. On top of this there is the bandwidth limitation=20
of any particular transmission line design with whatever materials we=20
choose. What this says to me is that only very short lines can benefit=20
from a pretty tight match. But most cases I am familiar with, those=20
would be lines going to magnetics. When dealing with manufacturing=20
variation atrocities like many of the RJ's out there with integrated=20
magnetics, 1 mil, 10 mils, even 100 mils mismatch is pretty hard to for=20
a guy like me to see. If it's really free I see no harm. I am really=20
pressed, and would be delighted to see a real statistically valid study=20
that showed that a particular physical path match length provides a=20
measureable and meaningful benefit.
Best Regards,
Steve.
Bill Owsley wrote:
> Most systems are quite functional (SI) when presented to EMC for testin=
g, 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 mat=
ched pair that caused it. So given my realm of influence, (which certain=
ly is not the weave of FR4. Is there a felt or random pattern FR4?) I a=
sk for continuous incremental improvements (that don't get me necktie par=
ty). 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 nu=
mber twist would unlock pandoras box. And certainly the other sources me=
ntioned that are not the diff-pair trace length can be the dominate sourc=
e of problem, but they were not part of the trace length constraint. =20
> =20
> Thanks to those that brought up the BER, RJ, any other jitter, eye di=
agrams and that stuff of SI world. All those numbers scare me in some fa=
shion, they are so big.
> And it seems that a better match in trace lengths given the phase per=
centage mentioned in another note and other descriptions of the effects, =
or lack of, for excessive constraints for differential signalling, that t=
he little X mark in the middle of eye diagram seems to shrink, the suppos=
ed flat segments of the eye diagram are a little bit flatter. In a tight=
ly 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 a=
s much of a slight shift in the crossover or switching point, leading to =
less jitter. Some of the multilevel signalling has such small difference=
s 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 wher=
e you would like to be in the market.
> =20
> =20
> =20
> Chris Cheng <Chris.Cheng@xxxxxxxx> 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 indica=
te 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.=20
> 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 schemat=
ic 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 d=
ay to work on the microvolts part.
>
> And there is at least one layout group that has for me, a short rope an=
d tall tree. But I love them anyway.
>
> ---------------------------------
> Fussy? Opinionated? Impossible to please? Perfect. Join Yahoo!'s user p=
anel and lay it on us.
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at:=20
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
> =20
> ---------------------------------
> It's here! Your new message!
> Get new email alerts with the free Yahoo! Toolbar.
>
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>
> List technical documents are available at:
> http://www.si-list.net
>
> List archives are viewable at: =20
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
> =20
>
>
>
> =20
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
