RE: [SI-LIST] : Transmission Lines Formulae

Muranyi, Arpad (arpad.muranyi@intel.com)
Wed, 13 Jan 1999 12:35:00 -0800

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.

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 <lweemei@dso.org.sg> on 01/12/99 06:01:00 PM

To: "'si-list@silab.eng.sun.com'" <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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