[SI-LIST] Re: Dispersion
- From: Fred Balistreri <fred@xxxxxxxxxxxxx>
- To: C Deibele <deibele@xxxxxxx>
- Date: Fri, 13 Dec 2002 10:05:31 -0800
I'm not sure about your analogy below. Dispersion occurs when
broadband signal waves are launced into imperfect medium.
The sine waves making up the broadband signals do not travel
at the same speeds because the medium characteristics change over
frequency.
If the sine wave frequencies occur where the medium changes
then the sine waves will not maintain their phase relationship
and the signal gets distorted, we call this dispersion. In
PCB's dispersion occurs at high frequencies where the dielectric
constant changes. The degree of dispersion in PCB's is totally
due to the imperfections of the material used. Specifically the
dielectric. While it is theoritcally possible (in an equation)
to have a change in er without an accompying loss, in reality
this does not happen. A change in er over frequency always includes
a change in the loss tangent..ie loss in the dielectric. If you
can show me a material that changes er over frequency with constant
loss then I will agree with you. Otherwise in a practical sense
and in my humble opinion I do not agree with your conclusion.
Best Regards,
C Deibele wrote:
>
> I disagree.
>
> Take a trace, with two ports, and put an open circuited "T" in the
> middle of it. This is a perfect source of dispersion. The entire
> basis for schiffmann phase shifters is easily realizable on pcb's.
> Loss has nothing to do with these forms of dispersion.
>
> Dispersion means that the phase velocity is different from the
> group velocity. or the slope of the curve of omega versus k
> (frequency versus wavenumber) is different from the ratio of omega
> divided by k.
>
> Craig
>
> >===== Original Message From Fred Balistreri <fred@xxxxxxxxxxxxx> =====
> >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
> >>
> >> ------------------------------------------------------------------
> >> To unsubscribe from si-list:
> >> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
> >>
> >> or to administer your membership from a web page, go to:
> >> //www.freelists.org/webpage/si-list
> >>
> >> For help:
> >> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
> >>
> >> List archives are viewable at:
> >> //www.freelists.org/archives/si-list
> >> or at our remote archives:
> >> http://groups.yahoo.com/group/si-list/messages
> >> Old (prior to June 6, 2001) list archives are viewable at:
> >> http://www.qsl.net/wb6tpu
> >>
> >
> >--
> >Fred Balistreri
> >fred@xxxxxxxxxxxxx
> >
> >http://www.apsimtech.com
>
> 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
--
Fred Balistreri
fred@xxxxxxxxxxxxx
http://www.apsimtech.com
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
