**From:** Sweetman, Eric (Eric) (*esweetman@lucent.com*)

**Date:** Mon Feb 19 2001 - 07:31:09 PST

**Next message:**Stuart Adams: "[SI-LIST] : damping resistors ??"**Previous message:**David Instone: "Re: [SI-LIST] : Multi-drop backplane trace, with Z0=50 ohms - but my TDR says 40 ohms (fwd)"**Next in thread:**Bo: "[SI-LIST] : Evaluating packages: A simple task or is it?"**Reply:**Bo: "[SI-LIST] : Evaluating packages: A simple task or is it?"

I once had an E&M Physics professor who, not facetiously, stated that the

energy of an EM signal travels through the dielectric medium (or free

space); the conductors only provide the boundary conditions.

Eric Sweetman

Lucent Technologies Bell Labs

Princeton, NJ

esweetman@lucent.com

*> ----------
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*>
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*> Date: Sun, 18 Feb 2001 10:24:26 +0200 (IST)
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*> From: Offer Kaye <sofferk@techst02.technion.ac.il>
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*> Subject: RE: [SI-LIST] : Simple question about propagation speed vs. diele
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*> ctric
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*>
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*> Pat, that's the same equation as mentioned in the previous replies, it's
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*> simply that the equation: V=C/sqrt(Er) assumes that mu=1 (that's realtive
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*> mu), which is a pretty good assumption since most materials we (EE's)
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*> use are dielectric, not magnetic, in nature. In Optics, for example, that
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*> assumption is usually false.
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*>
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*> One question to everyone who answered Keith's question- why are you
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*> assuming Keith meant the propogation speed of Electromagnetic signals?
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*> Because while the above equation is true for EM signals, and as noted in
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*> another answer, the sqrt(Er) can simply be called the "refractive index
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*> (nr)" of the material, how do you know Keith didn't mean the propogation
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*> speed of electrical signals in a PCB conductor (or other conductor, for
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*> that matter)?
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*> In that case, the speed is inversly proportional to sqrt(Er), so if Er
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*> increases V(sig) decreases. However, the exact formula changes according
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*> to:
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*> 1. Geometry (stripline, microstrip, etc.)
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*> 2. Er used should be *effective* Er, which can be hard to calculate.
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*> 3. Loading and line terminations, which can add constants and other
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*> factors to the equation.
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*>
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*> Last but not least, remember the basic difference between EM wave
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*> propagation and electrical signals propagation speed: EM waves depend on
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*> the Er of the medium through which they are travelling, electrical signals
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*> depend on the Er of the surrounding dielectric.
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*>
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*> Hope this helps... :)
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*>
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*> Offer Kaye
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*> sofferk@techst02.technion.ac.il
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*>
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*>
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*> On Thu, 15 Feb 2001 Patrick_Carrier@Dell.com wrote:
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*>
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*> > There's also that old equation:
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*> > v = c/(sqrt(mu * epsilon))
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*> >
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*> > So as epsilon, or Er, gets bigger, v gets smaller.
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*> >
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*> > --Pat
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*> >
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*> > -----Original Message-----
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*> > From: Kowal, Keith [mailto:kowal@dnpg.com]
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*> > Sent: Thursday, February 15, 2001 2:19 PM
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*> > To: 'si-list@silab.eng.sun.com'
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*> > Subject: [SI-LIST] : Simple question about propagation speed vs.
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*> dielectric
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*> >
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*> >
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*> >
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*> > Folks,
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*> > IN general , is it true or false, signals travel faster the lower the
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*> Er
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*> > ,, or not //// why or whynot?
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*> >
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*> > keith
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*> > Digital Networks, LLC
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*> >
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*> >
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*>
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*>
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*>
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*>
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*>
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**Next message:**Stuart Adams: "[SI-LIST] : damping resistors ??"**Previous message:**David Instone: "Re: [SI-LIST] : Multi-drop backplane trace, with Z0=50 ohms - but my TDR says 40 ohms (fwd)"**Next in thread:**Bo: "[SI-LIST] : Evaluating packages: A simple task or is it?"**Reply:**Bo: "[SI-LIST] : Evaluating packages: A simple task or is it?"

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