RE: [SI-LIST] : Simple question about propagation speed vs. diele ctric

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From: Offer Kaye (sofferk@techst02.technion.ac.il)
Date: Sun Feb 18 2001 - 00:24:26 PST


Pat, that's the same equation as mentioned in the previous replies, it's
simply that the equation: V=C/sqrt(Er) assumes that mu=1 (that's realtive
mu), which is a pretty good assumption since most materials we (EE's)
use are dielectric, not magnetic, in nature. In Optics, for example, that
assumption is usually false.

One question to everyone who answered Keith's question- why are you
assuming Keith meant the propogation speed of Electromagnetic signals?
Because while the above equation is true for EM signals, and as noted in
another answer, the sqrt(Er) can simply be called the "refractive index
(nr)" of the material, how do you know Keith didn't mean the propogation
speed of electrical signals in a PCB conductor (or other conductor, for
that matter)?
In that case, the speed is inversly proportional to sqrt(Er), so if Er
increases V(sig) decreases. However, the exact formula changes according
to:
1. Geometry (stripline, microstrip, etc.)
2. Er used should be *effective* Er, which can be hard to calculate.
3. Loading and line terminations, which can add constants and other
factors to the equation.

Last but not least, remember the basic difference between EM wave
propagation and electrical signals propagation speed: EM waves depend on
the Er of the medium through which they are travelling, electrical signals
depend on the Er of the surrounding dielectric.

Hope this helps... :)

Offer Kaye
sofferk@techst02.technion.ac.il

On Thu, 15 Feb 2001 Patrick_Carrier@Dell.com wrote:

> There's also that old equation:
> v = c/(sqrt(mu * epsilon))
>
> So as epsilon, or Er, gets bigger, v gets smaller.
>
> --Pat
>
> -----Original Message-----
> From: Kowal, Keith [mailto:kowal@dnpg.com]
> Sent: Thursday, February 15, 2001 2:19 PM
> To: 'si-list@silab.eng.sun.com'
> Subject: [SI-LIST] : Simple question about propagation speed vs. dielectric
>
>
>
> Folks,
> IN general , is it true or false, signals travel faster the lower the Er
> ,, or not //// why or whynot?
>
> keith
> Digital Networks, LLC
>
>

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