RE: [SI-LIST] : differential impedance calculations

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From: Larry Miller (ldmiller@nortelnetworks.com)
Date: Thu Nov 02 2000 - 07:42:06 PST


Hello, all,

Well, Mr. Grasso, you could be right inasmuch as I did not have the
opportunity to attend the presentation in person.

However,

1) The preceding part of the paper presents equations that deal with the
measurements (principally TDR) that show how differential lines work, and
these look very OK.

2) It appears to require a field solver to get the impedances required for
the formulae if you do not want to do TDR measurements. This greatly limits
usefulness for a "paper design" where you do not have a test board available
to measure (for example, I do have the TDR instrument(!) )

3) The only closed-form calculations I see in the paper are the National
ones, and National (in AN-905) points out that these formulae, used on
Slides 45 and 46 are from empirical measurements (i.e., curve-fitting to
measured data) and your mileage may vary.

4) Now maybe Dr Bogatin was slagging these calculations in the verbal
presentation, but there is nothing in the slides to indicate that or that
these National formulae are NOT pretty good, such as curves showing the
error between the formulae and measured data. If this is the case, I would
surely like to know it!

Like many others on this reflector, I have been trying to cope with
differential pair logic (in particular, souped-up LVDS) to model
interconnects for back- and mid-planes for supra-gigabit serial backplane
design. Between having to deal with very high trace density (1000's of diff
pairs) with long runs, huge skin effect losses, and limits on backplane
thickness, this has been a pretty tough exercise, and the literature seems
pretty darned slim.

Working with chip, connector and backplane vendors has not been particularly
satisfactory for planning a design. This is again because nobody seems to
have rolled all the math into a big ball. Maybe it cannot be done. In any
case, we find ourselves doing very pricey iterations that are little better
than cut-and-try. (An informed cut-and-try, to be sure.)

In particular, I have had a hard time finding closed-form equations that
take all of these factors into account, or even applications that do. Most
of the (expensive) tools touted on this reflector have gotten very mixed
reviews. The one Gary mentioned seems to --partly-- solve the problem, and
that seems to be the state of things. I think that there is a sort of
consensus here that the board-level modelers/analyzers are either not very
accurate or require so much input that they are hardly worth the trouble.
(Pardon me for a moment while I duck!)

Having met Dr Johnson when I was working on the IEEE Gigabit Ethernet
committee a couple of years ago (IEEE 802.3z), I requested him to do a 10 Gb
update on "Digital Black Magic". I don't know if he is working on such a
thing.

Now I will stand back for the onslaught.....

Larry Miller

> -----Original Message-----
> From: chasgra [SMTP:chasgra@email.msn.com]
> Sent: Wednesday, November 01, 2000 9:04 PM
> To: Miller, Larry [SC7:322:EXCH]; Charles Grasso; Gary Peterson; si-list
> Cc: eric
> Subject: Re: [SI-LIST] : differential impedance calculations
>
> Hi Larry - Thanks for your comments.
>
> If you are referring to slides 45 & 46 I think you misunderstand the
> intent.
>
> Dr Bogatin was, I believe, trying to show comparisons between Ist order
> approximations and a field solver. I do not belive any assumptions were
> being made. Indeed as I recall he did nake the statement that one should
> not always trust approximation BECAUSE of the assumptions.
>
> Charles Grasso
>
>
> ----- Original Message -----
> From: Larry Miller <mailto:ldmiller@nortelnetworks.com>
> To: Charles Grasso <mailto:chasgra@msn.com> ; Gary Peterson
> <mailto:Gary_Peterson@Emcore.com> ; si-list
> <mailto:si-list@silab.eng.sun.com>
> Cc: eric <mailto:eric@bogent.com>
> Sent: Wednesday, November 01, 2000 2:04 PM
> Subject: RE: [SI-LIST] : differential impedance calculations
>
>
> Unfortunately, the piece by Dr Bogatin makes and refers to the very
> assumptions that Dr Johnson refers to.
> All roads lead to the National App Note AN-905.
> (I have been battling this same fight myself.)
>
> Larry Miller
>
> -----Original Message-----
> From: Charles Grasso [SMTP:chasgra@msn.com]
> Sent: Wednesday, November 01, 2000 2:23 PM
> To: Gary Peterson; si-list
> Cc: eric
> Subject: Re: [SI-LIST] : differential impedance calculations
>
> Hello - There is an excellent presentation available for
> download called
> "Differential Impedance finally made Simple" authored by Dr Bogatin
> that
> should help answer this question.
>
> Please goto <http://www.ewh.ieee.org/rmcemc> Click on Last
> Meeting and
> follow the instructions for the presentation download.
>
> Enjoy!!
>
> Charles Grasso
> Ansoft Corporartion
> p: 303-666-1400
> f: 303-666-1499
> ----- Original Message -----
> From: "Gary Peterson" <Gary_Peterson@Emcore.com>
> To: <si-list@silab.eng.sun.com>
> Sent: Wednesday, November 01, 2000 12:50 PM
> Subject: [SI-LIST] : differential impedance calculations
>
>
> >
> >
> > I seem to recall Dr. Howard Johnson mentioning that impedance
> calculation
> > formulas made assumptions (since closed-form solutions aren't
> possible).
> >
> > Can someone comment on assumptions made for the equation for
> differential
> > impedance shown below? Seems to me that as traces get closer
> together,
> the
> > single ended impedance would be affected. When I calculate
> single-point
> > values for differential impedance using Hyperlynx LineSim, I do
> see the
> > single-ended impedance change when the spacing between the pair
> gets
> small.
> >
> > Thanks,
> >
> > Gary Peterson
> >
> >
> >
> > (Embedded image moved to file: pic27894.pcx)
> >
> >
>
>
>
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