**From:** Larry Miller (*ldmiller@nortelnetworks.com*)

**Date:** Thu Nov 02 2000 - 07:42:06 PST

**Next message:**marko.pulli@nokia.com: "[SI-LIST] : PCI Bus propagation delay measurement"**Previous message:**Omar Al-Taher: "FW: [SI-LIST] : High Speed AC Coupling"**Maybe in reply to:**Gary Peterson: "[SI-LIST] : differential impedance calculations"**Next in thread:**Eric Bogatin: "RE: [SI-LIST] : differential impedance calculations"**Reply:**Eric Bogatin: "RE: [SI-LIST] : differential impedance calculations"

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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**Next message:**marko.pulli@nokia.com: "[SI-LIST] : PCI Bus propagation delay measurement"**Previous message:**Omar Al-Taher: "FW: [SI-LIST] : High Speed AC Coupling"**Maybe in reply to:**Gary Peterson: "[SI-LIST] : differential impedance calculations"**Next in thread:**Eric Bogatin: "RE: [SI-LIST] : differential impedance calculations"**Reply:**Eric Bogatin: "RE: [SI-LIST] : differential impedance calculations"

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