FW: [SI-LIST] : A basic question

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From: Peterson, James F (FL51) (james.f.peterson@honeywell.com)
Date: Fri Mar 17 2000 - 09:39:12 PST


"If you change the geometry but not the dielectric, you affect BOTH L and C
equally but in opposite directions, such that Pd really doesn't change."

Exactly - and that's the part I was missing (L changes also). Thanks all,
for the clarification.
It's amazing how all of this fits together.

-----Original Message-----
From: Ingraham, Andrew [mailto:Andrew.Ingraham@compaq.com]
Sent: Friday, March 17, 2000 9:57 AM
To: 'si-list@silab.eng.sun.com'; 'Peterson, James F (FL51)'
Subject: RE: [SI-LIST] : A basic question

>So, if what Ken says below is true, then Pd does not equal (LC)^(1/2)!!!

Um, why not?

Pd (delay/unit length) really DOES equal (LC)^(1/2).

If you change the geometry but not the dielectric, you affect BOTH L and C
equally but in opposite directions, such that Pd really doesn't change.

Fundamentally, Pd must not change, because the electromagnetic field
propagates through the dielectric between the conductors; very little of it
penetrates into the conductors. Thus the dielectric medium determines Pd.
Then, C and L are related to one another according to Pd=(LC)^(1/2).

>Propagation delay is strictly a function of the permittivity and
>permeability of
>the medium (er and ur).
>Characteristic impedance is a funtion of both of these, plus the geometry
of
>the
>conductors carrying
>the current. While changes in the conductors will affect Zo, it will not
>affect
>prop delay.

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