[SI-LIST] Re: Fwd: Re: Re: Stack up for EMI reduction, plane resonance and u-s trip radiation etc etc
- From: <ludvik@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sat, 14 Feb 2004 17:53:29 -0800
Sorry, one more test.
LK
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx =
[mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of steve weir
> Sent: Saturday, February 14, 2004 17:38
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Fwd: Re: Re: Stack up for EMI reduction, plane
> resonance and u-s trip radiation etc etc
>=20
>=20
> >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 =3D (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 =3D 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
> >>
>=20
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