RE: [SI-LIST] : "skin effect/depth calculation results"

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From: George_Tang@Dell.com
Date: Fri Sep 01 2000 - 16:44:25 PDT


You find out how thick you can make your board to. You determine how many
layers your board will have. You then determine what the thickest dimension
you can have for the critical layers. You find out what Zo should be if the
receiver has termination built-in that you cannot change. You use that
impedance to determine your trace width. You may find that you have too
much copper loss, so then you try to play with the stack-up to see if you
can make other layers thinner and your critical layers thicker to use wider
traces. This is if you do not want to add external termination on top of
the internal receiver termination, so you use the impedance specified by the
IC vendors . . .

George Tang

-----Original Message-----
From: C Deibele [mailto:deibele@fnal.gov]
Sent: Friday, September 01, 2000 4:32 PM
To: George_Tang@exchange.dell.com; si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : "skin effect/depth calculation results"

George_Tang@Dell.com wrote:
>
> This is great from a theoretical stand point. But in reality,
> the trace impedance, Zo, is fixed at a certain range by either
> the driver or receiver. You will need to make the widest
> possible trace while keeping the same impedance. This brings
> us back to the original discussion.
>
> 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  60510

my opinions only....not of the gov't...nor of the lab...

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