# Re: [SI-LIST] : Transmission Lines Formulae

Mike Degerstrom ([email protected])
Fri, 15 Jan 1999 14:12:42 -0600

Arpad makes a very good point. I'll take this one step
further and say that getting an ohm or two of better accuracy
is a second or third order effect when looking at the whole
SI picture. Other things that are generally much more important
than absolute accuracy of Zo design formulas (in addition to Arpad's
point on moding) include:

1) manufacture's pcb\cable\etc. Zo control
2) net topology
3) SSN (ground bounce)
4) output buffer manufacturing variability
6) power/ground delivery

Have I missed anything?

Despite having said all of this, I still like to have
a method which gives me better than one-half of an ohm
Zo accuracy. For this I use EM simulators. I like
to have the accuracy, not for general SI design, but
for things such as validating new measurement techniques.

Mike

On Jan 13, 12:35pm, Muranyi, Arpad wrote:
> Subject: RE: [SI-LIST] : Transmission Lines Formulae
> I find it a little disturbing that so many of us SI engineers are hung up on
> the
> hair splitting accuracy issues of these various formulas, (and numerical
> methods
> when it comes to which field solver is better). I just want to remind
> everyone
> that in a bussed signaling environment we also have to take the switching
> modes
> into consideration. Some of the basic equations I have seen don't even
> mention
> these effects.
>
> A nominally 60 Ohm trace can easily look like 55 or 65 Ohms when the signals
>
> switch in odd or even mode. (These numbers can get even worse with tighter
> spacing and larger ground plane distances). So what is the point of
> splitting
> hair over whether an equation gives us 59.5 or 61.5 Ohms for a "60 Ohm"
> trace?
>
> In our work we need to consider an impdance range which includes not only
> geometric dimensions but modal effects also. I agree that one still needs
> accurate equations to get good results to find these ranges, but to account
> for
> all modal effects one has to solve eigenvector problems which can't be done
> on
> the back of an envelope anyway...
>
> So I favor good field solver tools over simple equations when it comes to
> accuracy.
>
> Intel Corporation
> ============================================================================
> ===
>
>
> The formulae provided in different reference books are not very accurate,
> but are subject to several approximations. It is good to use these
> formulae only for obtaining a ball park figure. For greater accuracy, one
> should use a 2-D, or 3-D field solver. Again, the PCB building processes
> have common tolerances of no better than +/- 10%, unless one is willing to
> pay big bucks for tighter tolerances.
>
> The best approach I have found out is to have the PCBs built based on your
> calculations/field solver analysis, and then perform an actual impedance
> measurement with TDR. The difference between calculated and actual
> impedance value is the adjustment you need to make for the PCB
> manufacturing process. Hence, it pays to stick with one fab for all your
> PCB needs.
>
> HyperLynx provides a reasonably accurate 2-D field solver as part of their
> Crosstalk analysis tool, and it is very easy to use.
>
> Regards, Ravinder
> PCB Development and Design Department
> IBM Corporation - Storage Systems Division
> ***************************************************************************
> Always do right. This will gratify some people and astonish the rest.
> .... Mark Twain
>
>
>
>
> Lum Wee Mei <[email protected]> on 01/12/99 06:01:00 PM
>
> To: "'[email protected]'" <[email protected]>
> cc: (bcc: Ravinder Ajmani/San Jose/IBM)
> Subject: [SI-LIST] : Transmission Lines Formulae
>
>
>
>
>
> While working on my Z calculation for transmission line, I noticed that
> different reference books provide different variations of the
> transmission line formulae be it microstrip or stipline.
>
> As a designer, I am expected to be proffesional in my work and able to
> explain the rationale why I use the formulae from this reference book
> and not the other. Can someone enlighten me on which formula to use and
> the reason, if any?
>
> BTW, an engineer in another dept of mine mentioned that I need not have
> to bother with manual Z calculation because the PNC SI tool is able to
> extract the information. I have attended the PNC workshop and do not
> find it friendly to use. Moreover, the accuracy of the output depend
> heavily on the accuracy of the input. That is just my feeling ;)p, I do
> not know about the rest of you who have use this PNC SI tool?
>
> Hope to hear from anyone of you.
> Thanks and regards.
>
>
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>-- End of excerpt from Muranyi, Arpad

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