**From:** Offer Kaye (*sofferk@techst02.technion.ac.il*)

**Date:** Sun Feb 18 2001 - 00:24:26 PST

**Next message:**Chris Padilla: "RE: [SI-LIST] : Simple question about propagation speed vs. diele ctric"**Previous message:**Mike LaBonte: "Re: [SI-LIST] : Question regarding Host Bus simulation simulation"**In reply to:**Patrick_Carrier@Dell.com: "RE: [SI-LIST] : Simple question about propagation speed vs. diele ctric"**Next in thread:**Doug McKean: "RE: [SI-LIST] : Simple question about propagation speed vs. dielectric"**Reply:**Doug McKean: "RE: [SI-LIST] : Simple question about propagation speed vs. dielectric"

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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**Next message:**Chris Padilla: "RE: [SI-LIST] : Simple question about propagation speed vs. diele ctric"**Previous message:**Mike LaBonte: "Re: [SI-LIST] : Question regarding Host Bus simulation simulation"**In reply to:**Patrick_Carrier@Dell.com: "RE: [SI-LIST] : Simple question about propagation speed vs. diele ctric"**Next in thread:**Doug McKean: "RE: [SI-LIST] : Simple question about propagation speed vs. dielectric"**Reply:**Doug McKean: "RE: [SI-LIST] : Simple question about propagation speed vs. dielectric"

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