[SI-LIST] Fwd: Re: Re: Stack up for EMI reduction, plane resonance and u-s trip radiation etc etc
- From: steve weir <weirsp@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sat, 14 Feb 2004 17:38:24 -0800
>Date: Sat, 14 Feb 2004 14:59:16 -0800
>To: "Michael E. Vrbanac" <vrbanacm@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
>From: steve weir <weirsp@xxxxxxxxxx>
>Subject: Re: [SI-LIST] Re: Stack up for EMI reduction, plane resonance and
>u-s trip radiation etc etc
>
>Michael,
>
>At 03:54 PM 2/14/2004 -0600, Michael E. Vrbanac wrote:
>>I suppose this could go on forever.... I'll snip severely to shorten
>>things... and then make them long again
>>... but then again isn't that what a "forum" is for... discussion?
>
>It has been a good discussion that I have enjoyed.
>
>>>If your point is that we could arrange an inefficient coupling by
>>>selectively choosing lambda, I agree. But, I disagree that arbitrarily
>>>putting a brick wall at the board edge does that, as lambda is then set
>>>by the board geometry, and not by design against some excitation that we
>>>want to present an inefficient coupling to. And that's where I think
>>>the position and reactance of that plate in your analogy is very important.
>>
>>re: points of agreement
>>Yes, you can do it that way, too. Generally, you don't get to choose
>>lambda. It gets chosen for
>>you based on the physical reality.
>
>We agree.
>
>>You choose the position based on that reality. I think the value
>>of what we are talking about rests in the reasons why this analogy works.
>>
>>re: brick wall at the board edge
>>Who's talking about that? I thought we were talking about fences, etc.
>
>I don't know how tight a fence we are talking about that you wish to
>represent the impedance with the fence as significant. If my fence is
>close to the board edge, ( that has been my assumption in this
>conversation ), then the board geometry defines lambda as we appear to
>agree, but has nothing to do with the excitation source.
>
>If you are advocating specific fence patterns significantly in-board of
>the edge to tune against excitation sources, I think that is a different
>thread.
>
>>and the effects that they
>>had on things. That is another matter. Such things, like "brick walls"
>>used judiciously are quite
>>effective and at least one very good paper was written about their use by
>>some folks working for
>>a cell phone vendor. I'll let you discuss it with them.
>>
>>re: about arbitrariness.
>>For the record, I do not arbitrarily put things anywhere. So, if you
>>please, let's not talk about arbitrariness
>>unless we are getting rid of it.
>
>I hope not. Ambiguity sucks.
>
>
>>>I understand where you are trying to go with this. I am sorry you felt
>>>the need to go all the way back to Ampere's law for it.
>>>
>>>Let's see what we can agree upon, and what's left in our differences.
>>>
>>>We agree that Ampere and Biot-Savart still apply.
>>>We agree that mu is effectively 1.0
>>>So this means that we must agree that the distribution of B relative to
>>>the trace position has not changed.
>>>I think we also agree that if we perform a 1.0000 meter measurement and
>>>a 1.0002 meter measurement the EMI numbers will be indistinguishable.
>>>
>>>Stopping right there, your posit is that B is the problem for radiation
>>>and if we agreed on this, you would be absolutely correct.
>>>
>>>However, B is dominant to low impedance coupling, like nearby traces or
>>>if there is some object that is going to reradiate.
>>
>>re: the easy stuff
>>Yes to the first three. The fourth...yes, that's what I think will
>>happen, but your measurements
>>will not be valid. Let's not short-circuit a good answer by skimping on
>>the procedure. For the
>>measurement to be valid, the microstripline's distance to the plane must
>>be that same in both cases.
>>Otherwise, that answer will be as bogus as any other myth I've seen.
>
>I had assumed that.
>
>Only the trace width will change to maintain constant Z due to the change
>in effective Er. Agree? Object?
>
>
>>re: wave impedance
>>I am sorry that this is not clear to you. In many laboratory studies,
>>I've proven this point ad nauseum
>>and will not argue it here with you. I'll point you however to do some
>>study about shielding, type of shielding,
>>why they work and when they won't and what are the operating
>>characteristics of the materials, bonds, and
>>wave impedances involved. We aren't going to go anywhere on this until
>>this is understood.
>
>I am happy to look at any information you have.
>
>
>>re: object going to "re-radiate"
>>Sigh... by definition (of "re-radiate") since there is no current loop,
>>the re-radiation case is an E-field
>>induced structure. It has to be. And its a "high-impedance structure",
>>no circuit loop structure
>>except by parasitics, no/very low current flow, maximum voltage without
>>load. It will re-radiate in
>>proportion to its match to free-space following Gauss' Law for electric
>>fields. Any reduction in charge
>>on the object will be due to small amounts of leakage current through the
>>environmental parasitics.
>>It is not a B-field structure.
>>
>>>>C = (area * permittivity) / distance
>>>
>>>We disagree in that the above equation does not work for fringing. It
>>>works near the center of the "infinite plane of charge".
>>
>>Hmph. Are we changing physics now?
>
>Absolutely not.
>
>>I am sorry that I provided the "at a point" version of the equation....
>>should I have expanded it to deal with vector quantities in three
>>dimensions? I am stating the simplified version
>>just to point out the principle. But the equation still applies, its
>>proven physics... to make it work in the fringing
>>case we have to work on the spatial aspects of the problem.
>
>Sure, and as soon as we do that, we are not going to see what looks like a
>uniform vector pointing along one axis. Fringing matters.
>
>If we agreed that the dominant phenomena is of H surrounding the trace,
>and that by setting Z constant in both cases, then we would have long ago
>agreed on your position that the higher Er above the trace having made no
>change in H would have zero effect for an isolated trace, and might even
>do unwanted things near structures above the surface.
>
>
>>We don't have to necessarily do that to get a general understanding of
>>the problem. Here's the facts:
>>1. We assume that the microstripline of 4-5 mils width is closely
>>associated with the reference plane
>>(4-5 mils) and we're using a dielectric constant of around 4.
>
>Agreed.
>
>>2. The order of greatest to least E-field flux density is as follows:
>> a. between the microstripline and the reference plane
>> b. off the middle to lower edge of the microstripline to the
>> reference plane
>> c. off the middle to upper edge of the microstripline to the
>> reference plane
>> d. off the backside of the microstripline
>
>Agreed
>
>>3. (a) and (b) are already captured in the surface case and would be in
>>the embedded case.
>
>Agreed
>
>>4. (b) constitutes more than 50% available in all fringing fields due to
>>dielectric presence and would
>>in the embedded case as well.
>
>Agreed
>
>>5. (c) has some field lines captured but they are weakly coupled, the
>>rest are not captured.
>
>Agreed
>
>>6. (c) can be exploited by additional coverage by dielectric but is much
>>less than 50% of total
>>fringing field lines.
>
>Agreed
>
>>7. (d) cannot be exploited to any meaningful degree by additional
>>coverage by dielectric
>
>Do you disagree that the flux density in the dielectric above d. and
>wrapping back to the plane is higher than in air or a vacuum?
>
>>8. if an interposed reference plane were added to make the structure a
>>stripline then field capture
>>would be near 100% (assuming no apertures) depending on "top" dielectric
>>characteristics and plane
>>distance and distance to the edge of the reference plane. The flux
>>density would more evenly spread
>>over the two surfaces of the "now stripline" center conductor.
>
>Agreed
>
>>Therefore, the few remaining flux lines in the fringing fields would have
>>to constitute the entire change
>>of 15dB reduction or there was another mechanism, or there was a
>>combination, heretofore unidentified.
>
>I think the basis of disagreement here is what appears to be a point of
>view on your part that the energy radiated somehow remains proportional to
>the total that we started with had we had a trace suspended far from any
>reference plane, what you keep calling loosely coupled, sic wire wrap.
>
>The point of view that I start with is that what is captured by a. b. and
>c. is already out of the equation. Those lines aren't going into the
>far-field and I can forget about them. So, what is left is d. The
>relative attenuation will be the result of how many lines from d. go into
>the far-field in the first case with the microstrip on the surface, and
>the second when it is buried.
>
>>OR
>>
>>we really are not closely associated with the reference plane and loosely
>>coupled. Such a case might
>>deliver a larger than normal reduction due to the excessive fringing
>>fields that would result. In such a case,
>>the "rectangular" shape would begin to look more rounded from a greater
>>distance and the fringing fields
>>would be must greater. In such a case, it might be advisable to add the
>>extra dielectric layer to contain the
>>fringing fields... downside... more cost. This just might be the "trick"
>>or the "other mechanism" I'm
>>talking about. The problem is that even a 15dB reduction in that case
>>may still result in very poor test
>>readings. What is not said here is that why were they looking for a 15dB
>>reduction in the first place?
>>Probably because something was really not working well for the loosely
>>coupled case.
>>
>>However, back to the tightly coupled model, is that is doesn't work that
>>way. Yes, I've tested this
>>before as a "cost reduction idea" for a company. Zilch. Zip. Nada.
>>
>>Also, as you probably know, its easy to see all sorts of 15dB or more
>>reductions in a EMI debug lab
>>and yet see no change on the test range.
>
>
>Agreed that one must set up experiments very carefully. They are useless
>without an adequate control specimen.
>
>
>>>>3. Permittivity only increased by a factor of four, a 12 dB change max.
>>>>This sets the theoretical
>>>>top end of the performance range. A dielectric constant greater than 9
>>>>would likely be necessary
>>>>to bring us any hope of reaching 15dB. FR-4 is only about 4.2 - 4.5
>>>>(generally) depending on the
>>>>material makeup.
>>>
>>>Let's stick with Er = 4, for FR4 as close enough. So the issue is the
>>>12 dB value.
>>>So, the flux density of the electric field lines can only increase by a
>>>factor of 4. However as previously noted, we are already high
>>>influenced by fringing. The amount of total flux that is above the
>>>center of the conductor is limited.
>>
>>You're almost there. The total flux above the center of the conductor is
>>limited... and less than 50%
>>of the total value. The majority of the rest are captured.
>>
>>re: highly influenced by fringing
>>Only in the loosely coupled case is this possible... in the tightly
>>coupled case (IMHO, proper design practice)... no.
>>
>>>Agreed that this isn't a strip-line. The key is to look at the
>>>distribution of the lines, and the significant concentration near the
>>>trace edges before and after the submersion.
>>
>>Hey, gimme a break. I'm not that stupid. What have I been talking about
>>all this time? The
>>current density is greatest at the edges... hint, hint. The flux lines
>>off the signal conductor will
>>align themselves in proportion to the capacitance they see at their
>>"attachment point".
>
>The conversation can get someplace so long as we both assume the other is
>speaking in good faith. If I restate what appears to be obvious, it is
>because I don't want any doubt to exist.
>
>
>>The only way it will work is if the microstripline isn't hardly that at
>>all and the signal conductor
>>is relatively distant to the reference plane. Then perhaps the 15dB down
>>might work... but then
>>again, I don't design like that. I get rid of the problems and many
>>others by tight coupling to
>>the reference plane in the first place. I even do this with differential
>>pairs at multi-gigabit but I
>>know how to tweak it to get precisely what I want. It works, too.
>>Wonderful BERs (years of
>>16 corner testing) and great signal shape, and performance.
>>
>>>>5. If we were to assume half of the remaining field lines were totally
>>>>removed from the problem by
>>>>completely immersion in the FR-4 dielectric, this would only amount to
>>>>a 6dB reduction over the
>>>>surface case... hence this was the number I quoted you earlier.
>>>
>>>I think the fallacy here is that if we removed the ground plane the
>>>power would only go up by 6db. I disagree that the converse is true
>>>that by taking an arbitrary measure the power will only go down by 6db.
>>
>>Its no fallacy. Its just reasoning based on conservation of charge. The
>>"balance sheets of physics"
>>must always balance. Half the field lines, half the E-field. Pretty
>>simple. The only quibble is that
>>we usually calculate power transferred to the antenna so in that case we
>>are dealing with E^2 so
>>a factor of 4 or 12dB just like I said, not 15.
>
>Again the model that I keep interpreting from your statements is one where
>0 db is a result of all field lines going into the far field. But they
>never did. As you have repeatedly stated, a, b, and c for either the
>surface or the buried microstrip capture more than 50% of the lines. The
>0 db reference can only be what results from the lines that reach the
>far-field in the reference case, the surface microstrip.
>
>>>>6. One last thing we haven't thought about is, what happens now when a
>>>>conductor is placed
>>>>above the embedded microstrip? Say a component leadframe? The added
>>>>dielectric now works
>>>>in a different manner us when we make "conductive changes" in near
>>>>proximity to the embedded
>>>>microstrip. Oops! We didn't account for that did we? The field lines
>>>>will change direction more
>>>>strongly than before! Could it affect the measurements? Of course.
>>>
>>>Absolutely, life does get more complicated. But, still we are still far
>>>better off than if the trace were on the surface. Fringing, fringing,
>>>fringing.
>>
>>Oh, yes, it does... sometimes. But I'd "choose wisely" (taking a line
>>from Indiana Jones). That chalice
>>you're holding might not be the "Holy Grail". What seems like an
>>advantage might just be a curse especially
>>in the loosely coupled case. Fringing, fringing, fringing....
>
>I would never advise someone to use wirewrap for anything with significant
>frequency content. It would indeed be a case of someone who "chose poorly".
>
>Regards,
>
>
>Steve.
>
>
>>Regards,
>>
>>Michael
>>
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