**From:** C Deibele (*deibele@fnal.gov*)

**Date:** Fri Sep 01 2000 - 16:31:51 PDT

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George_Tang@Dell.com wrote:

*>
*

*> This is great from a theoretical stand point. But in reality,
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*> the trace impedance, Zo, is fixed at a certain range by either
*

*> the driver or receiver. You will need to make the widest
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*> possible trace while keeping the same impedance. This brings
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*> us back to the original discussion.
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*>
*

*> George Tang
*

*>
*

I disagree completely, George. What in the world are you

talking

about?? I do not know how one can make a constant impedance

by

varying line widths. You certainly know that impedance is a

distributed quantity. Varying line widths varies the

impedance.

How then, can one make the widest possible trace while

keeping

the same impedance constant?

It is clear from the most basic funamentals of

transmissionline

theory, see Pozar for example, that one only one geometry

will

enjoy the least transmission attenuation. The driver and

receiver

are irrelevant to that standpoint.

The driver and receiver may dictate a range of impedances

that can

be used. but other than that, one can easily transverse

from the saddle

point of the least attenuation.

For example, George, in coax, the best impedance is 77

ohms. That is

to say, 77 ohms has the least copper attenuation. so, are

you

saying that the driver and the receiver change this

intrinsic property

of physics?

Your point George, is equivalent to saying that the source

and

driver dynamically change Maxwell's Equations, which I am

certain you will

agree must satisfy Maxwell's Equations.

In fact, probably the only time one may disregard

the simulations I presented are for wavelengths much longer

than the geometry in question. And in these circumstances,

then skin depth is of importance.

I think Pozar goes through the coax example in Chapter 2....

I hope everyone was on their toes...I made a mistake in my

text. The text is only valid for the coax case of a >>

delta.

For the point of Bill Owsley, you are correct, but I think

we are concerned for the case of "ALL THINGS BEING EQUAL".

So, if for the original problem, given a certain trace

width,

does it help to reduce copper loss if one widens the trace

or thickens the trace? I can imagine that the effect of

surface

roughness might be reduced if the trace would be thickened.

Craig Deibele

-- Craig Deibele, PhD, PE Fermilab--Beams Division PO Box 500 MS 341 Batavia IL 60510my opinions only....not of the gov't...nor of the lab...

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