RE: [SI-LIST] : Signal Dispersion

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From: Daniel, Erik S. (Daniel.Erik@mayo.edu)
Date: Tue Sep 05 2000 - 04:56:31 PDT


Bob-

Generally, dispersion occurs any time any of the factors governing wave
propagation are frequency dependent. Many textbooks cover this at an
abstract level -- sometimes optics books do a better job of this as
dispersion is much more common/problematic in that field. Unfortunately, I
don't know of a single book that really covers the wide range of physical
mechanisms that can bring this about. "Skin effects" cause resistance per
unit length and inductance per unit length ("R", "L") to change with
frequency. Dielectric loss causes dielectric conductance ("G") to change
with frequency. A frequency dependent dielectric constant will cause
capacitance per unit length ("C") to change with frequency. If the
frequencies get large enough that the waveguide carrying the signal can
support multiple modes, then each transmission mode is characterized with
its own set of transport parameters (different propagation velocities,
characteristic impedance, etc.), and propagation along multiple modes, not
to mention crosstalk between the modes, can cause dispersion. Other effects
can come into play with "nonideal" structures (e.g., surface roughness
dependent inductance, resistance, etc., which introduce additional frequency
dependence). I'm sure other list members will post other mechanisms. In
any event, if you're more inclined to physics, a good E+M book might be a
good place to start (Jackson's Classical Electrodynamics is my bible), or an
optics book if you are more interested in frequency dependent dielectrics
and mode dispersion, but if you're more inclined to electrical engineering,
perhaps a microwave engineering book, although I don't know of one that
covers all of these effects adequately.

                                        - Erik

> Hi -
>
> A happy Labor Day to all.
>
> I'm looking into effects that can distort very high frequency
> waveforms. There's already been a lengthy discussion of skin
> effect here, and in the past folks have discussed dielectric
> absorption.
>
> One thing I haven't heard much about is dispersion. My
> understanding is that, for long signal traces or cables, slight
> frequency-dependent differences in propagation delay can cause
> the various frequency components in a signal transition to de-align,
> causing the edge to disperse. Some of the causes to which this is
> attributed seem to be:
>
> - different frequencies propagating in different transmission modes
> - changes in dielectric constant as a function of frequency.
>
> Of course, I could be totally wrong; this is just what I've been able
> to piece together from things I've read. Can anyone explain
> dispersion, or point to a good text that does? Under what
> conditions does dispersion become significant?
>
> Thanks,
> Bob Perlman
>
>
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