[SI-LIST] Re: Dispersion
- From: Fred Balistreri <fred@xxxxxxxxxxxxx>
- To: deibele@xxxxxxx
- Date: Thu, 12 Dec 2002 09:34:51 -0800
Except that in a PCB it is not possible to have dispersion without loss.
Dispersion without loss is defined only for wave guides. In a PCB the
resulting change in er carries with it losses in the dielectric
characterized
by the loss tangent. The losses and resulting change in er are a
function
of frequency, which results in dispersion and attentuation. There is
also
additional attenuation with frequency caused by the so called skin
effect.
I believe the loss in the conductor is often associated with the
attenuation
constant. While the losses in the dielectric are often attributed to the
phase constant. The propagation constant is a function of the two and is
frequency dependent. The degree of change depends on the materials used.
Best regards,
C Deibele wrote:
>
> Jeff,
>
> You bring up some very good points -- The real definition of dispersion is the
> fact that the phase velocity (omega divided by k) is different than the group
> velocity (d omega / dk). When the phase velocity is different from the group
> velocity, an impulse "disperses" or gets "fatter" when looked in the time
> domain.
>
> In fact, even though I don't like the textbook in general, Jackson's book
> "Classical Electrodynamics" has a great theoretical treatment of the subject
> that can be fairly well understood even with a cursory overview.
>
> Dispersion, in general, has absolutely nothing to do with losses. For
> example, a waveguide (here I mean a pipe, circular or rectangular), is
> absolutely dispersive. the phase velocity is different from the group
> velocity.
>
> Regarding loss -- when the loss varies w.r.t. frequency, this causes
> dispersion. While the loss variance may be accounted for in any myriad of
> techniques, the end result is dispersion.
>
> In fact, I've designed lots of equalizers to rid systems of dispersive
> properties.
>
> I agree, if a material is lossy, it is dispersive. This is easy to measure in
> the laboratory. Take a short piece of coax and put in a *great* square wave,
> and measure the rise time on a scope. Now, take a 100 meter section of coax
> and insert it in place of the short piece. The rise time measurement will be
> *much* worse. This is an effect of dispersion.
>
> In essence, lossy implies dispersive. but dispersive does _not_ imply lossy.
> A perfectly conducting waveguide is a great example of this property.
>
> And yes, knobbing is perfectly legal. While this isn't the perfect definition
> for measuring the dispersion, one can see the phase dispersive qualities. The
> magnitude is also very important. So, one has to consider the bandwidth of
> the source -- and relate that back to the measurement.
>
> So, if one corrects the magnitude to be flat, and corrects the "knobbed" phase
> to be flat, the system is absolutely non-dispersive.
>
> Craig
>
> >===== Original Message From "Loyer, Jeff" <jeff.loyer@xxxxxxxxx> =====
> >This part of the thread (discussion of dispersion) began when I asked the
> question below. It seems that we are back to the original question.
> >
> >ORIGINAL QUESTION:
> >When you use the term "dispersive", are you talking about losses (resistive,
> skin effect, dielectric), or about differences in phase velocities (page 170
> of Pozar's book)? I've heard others refer to loss effects as dispersive and
> have had
> >confusion as a result. Are both uses of the term "dispersive" correct?
> >
> >The explanation of how to measure dispersion (S21 magnitude) implies you
> believe "dispersion" and what I would have termed "effects of conductor and
> dielectric losses" are the same. I have trouble with that, since stripline
> insertion loss
> >magnitude definitely varies with F, and that effect is explained without
> dispersion. I believe "dispersion" is a separate effect than conductor and
> dielectric losses. The only tie between them that I've heard of is that Steve
> Corey (who I am
> >loath to contradict) stated "if a material is dispersive, it is also lossy".
> It may be that the converse holds (if a material is lossy, it is also
> dispersive), but I believe the 2 effects are separate (even if one can't occur
> without the other).
> >Maybe Steve would clarify this?
> >
> >I couldn't follow the explanation of "knobbing" electrical delays until S21
> phase is flat. Is that legal? ;-)
> >
> >Jeff Loyer
>
> Craig Deibele
> Spallation Neutron Source
> 701 Scarboro Road
> Room 301 MS 6473
> Oak Ridge, TN 37830
> mailto:deibele@xxxxxxx
> office: +1 865.574.1969 cell: +1 865.719.4381 fax: +1 865.241.6739
>
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--
Fred Balistreri
fred@xxxxxxxxxxxxx
http://www.apsimtech.com
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