[SI-LIST] Re: Dispersion
- From: "D G" <dgun@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 17 Dec 2002 04:09:29 +0800
Jeff,
I thought I answered your question about ADS. I can add a little more detail.
The documentation is at:
http://eesof.tm.agilent.com/docs/
Microstrip lines include dispersion. You can select from three dispersion
models. However, some of the short components (like bends) do not include
dispersion.
Striplines do not include dispersion (as far as I can tell).
PCB multicoupled lines do not include dispersion.
Momentum simulation includes dispersion.
One trick you can use to simulate permittivity change with frequency is to use
a frequency-dependent equation for Er in the substrate models. I've done this
in the past. Something like "Er=er_nom+freq*disp_factor" would work if the
change in Er is linearly dependent on frequency. You can do the same with loss
tangent (TanD).
- Daniel
From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
> OOOOOOOOOH! Now I'm getting somewhere, I think. Attached (for Bryce, anway.
> Anyone else who would like the data will have to e-mail me individually
> since it won't get through the reflector) you will find the data which
> started this whole fiasco (or at least some of it). In it, I am VNA'ing a
> long (~14") microstrip trace (actually 1/2 of a differential pair), and
> comparing those measurements with simulations (Hspice, using W-element for
> the T-line).
>
> What I noticed was that measured S21 phase derivative varied with frequency.
> See "1tz_S21P" sheet.
>
> In it (top graph) you will note that the simulation and measurement phases
> line up at the beginning and ends, but not in the middle. Trying to explain
> this was my original intent. Below S21 phase, I've also plotted a calculated
> Tp. For the measurement, this clearly varies with frequency (Tp gets less at
> higher F).
>
> Some things I would tend to conclude at this point are:
> 1) Yes, I'm seeing dispersion, as defined by Bryce (and his "official
> definition" seems to be an accurate, concise definition, based on what I've
> read and heard).
> 2) It is possible to have losses without dispersion, at least in simulations.
> On page "1tz", it's clear that S21 magnitude is varying with frequency as a
> result of losses, both in measurements and simulations. But, there is no
> dispersion apparent in the simulations.
> 3) Dispersion is causing my Tp for this 14" line to vary from 2.35nS (at low
> F) to 2.15nS (at 6GHz).
>
> So, of my original 2 questions, the 2nd one is answered (how dispersion would
> show itself in measurements). The first question - can Hspice or ADS
> simulate dispersion effects - remains open.
>
> P.S.
> I'm not sure there exists enough disk space in the world for a "book" this
> forum might produce :-)
>
> Thanks again for everyone's help...
>
> Jeff Loyer
>
>
>
> -----Original Message-----
> From: Bolton, Bryce D [mailto:BoltoBD@xxxxxxxxxxxxxxxxxxxxxx]
> Sent: Friday, December 13, 2002 9:59 AM
> To: si-list
> Subject: [SI-LIST] Re: Dispersion
>
>
>
>
> Hi Folks,
>
> I often enjoy hearing discussions and interpretations about concepts and
> practices. It adds a huge amount of understanding to the theory. My
> favorite books and mentors show the theory and verbalize it.
>
> Given any system H(s), we can derive:
>
> - insertion loss, also known as magnitude response.
> - phase response
> - group delay (derivative of phase)
> - dispersion (derivative of group delay) <- official definition of
> dispersion.
>
> If I were to observe a phase or group delay plot, I would have an idea of
> how much spectral component spreading or dispersion would occur at each
> frequency. But, my eyes aren't that good at doing derivatives, so it might
> be easier to simply look at the dispersion plot. So, when we say
> "dispersion" we are conceptually talking about this general spreading, that
> we know exists in the 2nd derivative plot, even though most of us don't make
> dispersion plots of an H(s) every day. One could talk about group delay or
> phase delay just as well, and say that they cause dispersion. It's just
> another way of talking about the same property. But, if we are talking
> about dispersion specifically, then one might want to know exactly how much
> spectral spreading was introduced by a given trace, etc. And, one would
> want to know how to minimize that number to meet a spec.
>
> I think it would be quite valuable if we could share some examples of actual
> measurements--especially with plots and descriptions.
>
> Signal integrity is broad, and I often think that this list would be
> benefited by an online book that just had everything clarified. We could
> just say--look at section 1.2 to find your answer. We could improve the
> "book" as a group.
>
> Thanks,
> Bryce
>
>
>
>
>
>
> -----Original Message-----
> From: C Deibele [mailto:deibele@xxxxxxx]
> Sent: December 13, 2002 10:18 AM
> To: Fred Balistreri
> Cc: Loyer, Jeff; si-list
> Subject: [SI-LIST] Re: Dispersion
>
>
>
> 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
> >>
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> >
> >--
> >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
>
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> -- Binary/unsupported file stripped by Ecartis --
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> -- File: 1tz_phase.ZIP
> -- Desc: 1tz_phase.ZIP
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--
Daniel
ZZZ-dgun-ZZZ-@xxxxxxxxxxxxxxx
(remove the Z-'s to reply--they're what I do when I read spam)
--
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