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

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Sun Feb 18 2001 - 12:33:17 PST


> EM waves propogate by the movement of photons, electrical signals
> propogate by the movement of charge carriers- only electrons in our case,
> but in semiconductors also sometimes holes. EM waves usually move at a
> speed very close to their speed in vacuum (3*10^8 m/s), electrical signals
> usually around "only" 2/3 of that value.
> So no, they are definitly not the same.
 
Huh???

In stripline, the signal is carried by the EM wave in the dielectric, which
indeed moves at a velocity determined by the dielectric constant or
refraction index. NOT at all at the speed of light in a vacuum. Yes there
is some frequency dependency if the medium is dispersive, and FR4 is a
little, but not overwhelmingly so.

The reason a signal may appear to take more time to reach the end of a wire
than given by its EM wave velocity (1/sqrt(mu*Er) stuff), is primarily
because of parasitic capacitive and inductive effects of ICs, vias, etc.,
coupled with the fact that the total signal is the summation of all
reflections, forward- and backward-moving waves, etc. Each of those
component waves propagates at exactly the velocity given by the dielectric
constant of the EM wave (non-magnetic dielectric).

Andy

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