Steve, Nima's email was the original that started the thread several days ago... I have the copy here in my mailbox and referred to it when I wrote the last email.... Continuing the dialog... launching from your comments... and I'll truncate the rest of the last message... re: "Considerable energy from what would be only a standing wave can and will find its way out under many conditions. Agreed that CM is a separate issue of plane potential versus the chassis." Without doubt... The funny thing is after solving countless (meaning I lost count!) emissions problems, only once could I ever verify that plane resonance was the real issue. I say that as a result of "board scanning examinations" and "distributed plane potential measurements. With these tests, in the one case only, was a defined standing wave pattern observed. The rest of the time those issues were always resolved by very well defined design corrections that directly addressed the real issue that was present. I say that only to make the point that I don't do "kludge fixes" except at gunpoint. <grin> I hate EMI problems resurrecting themselves to haunt me and my clients. Personally, I've actually seen more trace resonances (and resonance artifacts due to geometrical features of those traces) than plane resonances. In relation to this, there was some comment about fences and that contributed to plane resonances but I doubt that would be a hard and fast rule and might actually be something that could happen but is not guaranteed. My point comes from the fact that if an RF current null of a standing wave appears at an "conveniently placed RF short" (at the null) , the standing wave will act as if "nothing were there", making the point about fences causing or facilitating plane resonances less than certain. Given that, it is granted that the more consistent a particular electrical feature appears on a plane the more likely a stronger reaction to producing something quite significant (resonance-wise) will occur. re: "Ah yes, those devilish little details again. As in frequencies present in the package now go way beyond what we can convey through the package interconnect to the PWB. At frequencies substantially above 100MHz I know of no way to tightly couple any IC to the plane beneath. The best I know how to do is shield the overall structure. If you have a secret for overcoming package impedance at 100's of MHz, and beyond, I am all ears. Hmmm... well, I agree with the 100MHz when all we are talking about is "pins and balls" for package electrical connections. I do not agree that is the end of the matter. I've designed with RF power devices and if we understand how we can get significant power out of those devices without melting the leads off from the RF current density issues caused by the skin effect, we can certainly understand how to get a lower powered package connected to the reference plane better. My point is to say is that we have the technology already... we just aren't using it to our advantage. When we finally do, I predict that many of these issues will simply "go away". Our problem is that we are fixated on "pins and balls" for package connection methodology. Its like we want to use something comparable to "wirewrap" technology for sub-100nS signal transmission. When aspect ratio of a suitable conductor is used, the inductance will drop and we'll all look back on it and say "gee, why didn't we think of this sooner?" re: "I have always seen this as a matter of reducing inductance for the first choice. If we hold L constant and increase C by altering Er, then the SRF falls by 1/sqrt(Er_new/Er_old), and for a fixed resistance, Q also falls by a similar amount. It is actually a little worse than this, as the reduced SRF drops R due to skin effect. However, for the same constant R Q falls directly with reduction in L." But all you are doing is dropping the resonant frequency by increasing C... not surprising. Dropping L under those conditions does drop Q and that's good but I think there is more to it. Perhaps we can look at this another way by looking at all the variables. Actually, you need to handle the R first since that sets the baseline for overall impedance, the reactive portion is frequency dependent, R is not... (R + jwL or R - jwC). This means that skin effect and surface area of the conductor must be accounted for in the problem. To set the "baseline" lower, the R must drop. But a problem exists... dropping R always raises Q since: Q = X / R. Hi-Q creates a more efficient (less losses) resonance. How do we solve this? We make the "drive point impedance" too low for the driver to drive at the unwanted frequency. Why? The impedance mismatch not only causes inefficient power transfer to the resonant structure, it also loads the "driver" down too far to be able to drive it effectively. So what do we do? Since Z = sqrt (L/C), decreasing L drops the impedance as well as increasing C. Since we have already known that we can easily increase C (to a point) through closely spaced power planes, we can use this method to decrease the power plane "transmission line" impedance. We also know that we can add to this method by locally loading the planes with selected capacitance but at higher frequencies this becomes more difficult (but not impossible) for a variety of reasons.By providing these additions, RF current shunt paths are provided to "ground", this is equivalent to the old term "bypassing". (That term is sometimes used in place of "decoupling" intending to mean the same thing but the circuit functions are not the same.) The only thing that's left is "L". In reality, we (industry) actually are leaving this term largely untouched as I was implying earlier. When we explore what determines "L" and how to reduce it, we have our generic answers on how to solve the problem. So from my viewpoint, we can say we treat "L" first but in reality we've left it for last. (I do agree we probably should have solved "L" first). re: "Agreed, we can design well tuned or intentionally poorly tuned antennae in microstrips. The point that had been circulating concerned the containment properties of prepreg." Prepreg? I was under the impression that pre-preg was (for FR-4 class dielectrics) uncured epoxy glass dielectric substrate that was cured during the "press-up" during fab. Why would more recently cured FR-4 (cured pre-preg) act any different than pre-cured FR-4 (core) if the basic materials are the same and the glass/epoxy ratio is the same? About that subject, it would seem to me that it would only largely affect the launch angle off the microstrip trace due to it being more deeply embedded in a "non-air" dielectric in addition to a reflection off the dielectric boundary back toward the inside of the board due to the wave impedance mismatch. E and H fields penetrate a dielectric like that with the E field lines being distorted but not necessarily altered a great deal. Containment? I don't get it. Best Regards, Michael E. 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