# RE: [SI-LIST] : A basic question

From: Ingraham, Andrew ([email protected])
Date: Fri Mar 17 2000 - 06:56:47 PST

>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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