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.
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
> hair splitting accuracy issues of these various formulas, (and numerical
> when it comes to which field solver is better). I just want to remind
> that in a bussed signaling environment we also have to take the switching
> into consideration. Some of the basic equations I have seen don't even
> 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
> hair over whether an equation gives us 59.5 or 61.5 Ohms for a "60 Ohm"
> 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
> all modal effects one has to solve eigenvector problems which can't be done
> the back of an envelope anyway...
> So I favor good field solver tools over simple equations when it comes to
> Arpad Muranyi
> 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@example.com> on 01/12/99 06:01:00 PM
> To: "'firstname.lastname@example.org'" <si-list@silab.Eng.Sun.COM>
> 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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