RE: [SI-LIST] : Merits of low dielectric constant

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From: Zabinski, Patrick J. (zabinski.patrick@mayo.edu)
Date: Fri Jun 23 2000 - 07:32:05 PDT


Hassan,

Specifically to the capacitive loading issue, I am
as perplexed as you. It simply doesn't make sense for
long transmission lines (long being defined as 2*Td > Tr).
For very short lines where the line acts as a simple
lumped parasitic, then the comments make some sense.

Outside of the capacitive loading comments, low Er materials
do offer benefits at higher frequencies, including:

* to maintain 50 ohms, the lower Er materials require wider
(for stripline/microstrip) or bigger (for coax) conductors,
which reduces losses

* based on the same geometries, the lower Er materials have a
higher bandwidth before exciting the higher order modes

* given the same signal pitch (in strip/micro), the lower
Er generally results in lower crosstalk

* lower Er results in higher velocities, which translate
into lower prop delay

* others?

Anyway, I am not sure where the capacitive loading
comment came from, but there are benifits in lower Er
materials.

Pat

> I attended a presentation by a high-frequency (1GHz < f <
> 65GHz) coaxial
> cable vendor, and the presenter claimed that their cables use
> a material
> with a very low dielectric constant and therefore are ideal
> for high-speed
> application as they give rise to low capacitive loading. He
> gave a formula
> showing the capacitance (I think per unit length) decreasing
> as you decrease
> dielectric constant. This claim, however, perplexed me as I
> don't know how a
> cable's capacitance per unit length would give rise to a
> capacitive loading.
> All I know from my transmission line classes, a lossless
> transmission line
> with Z0 = sqrt(L/C) would transmit signals exactly the same
> way regardless
> of the value of the p.u.l. capacitance C as long as the ratio L/C is
> maintained. Am I missing something here?
>

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