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 >> >> ------------------------------------------------------------------ >> 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 ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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