[SI-LIST] Re: Stack up for EMI reduction, plane resonance and u-s trip radiation etc etc

  • From: "Michael E. Vrbanac" <vrbanacm@xxxxxxxxxx>
  • To: weirsp@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Wed, 11 Feb 2004 23:18:19 -0600

Continuing the dialog...  BTW, I'm getting two copies of your messages....

>It is certainly going to depend on what the board geometry is and what
>excitation is available to get it going.

But, of course....


> >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
>
>Please no hard and fast rules.  As I said, I prefer fences for ESD, but
>give them their due for capping the tuned cavity.

Agreed.  I've used them successfully for other things but they aren't the right
answer for everything.

> >  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.
>
>I think that we differ here.  I find the logic circular, so maybe you care
>to expand.  The standing wave is a result of the reflection.  So,  there is
>no way that I see to add or remove a short.  It was intrinsic.  Surely if I
>put a pin through coax, I am going to heat the heck out of the drive
>amplifier.  I don't see why you believe that a rectangular cavity is
>fundamentally different.

Then think about this.  I once worked on a television station project where 
the
nitrogen filled 6 1/8" dia hardline coax carried over 200KW of video power 
through a
short section of t-line that had a huge, deliberately placed metal slug 
between the
center conductor and the outer tube (shield) essentially an RF short 
designed to
protect the transmitter from lightning strikes. This section fed a large 
rectangular cavity
which is otherwise known as a diplexer.  By using your explanation, it 
would seem to
stand to reason that the transmitter would have been destroyed upon 
applying power
to this t-line section or at least the metal slug would have been 
disintegrated.  It was not.
I was able to visually inspect this after power up tests and re-examining 
the feedlines
for another reason. The surface temperature of the section was as cool as 
the room it was
in and cool to the touch.  By comparing the results of the output power 
measured
at the transmitter and a remote measurement unit on the other side of this 
t-line section
showed no meaningful power loss between those measurement devices. Given the
explanation you mentioned, I'm having some trouble with the logic.

>I hope you do well with the patent.  In the meantime I take it that you
>agree that the planes don't provide a lot of CM reduction for HF currents
>in the ICs.

Patent? Phooey.  Take a look at all the RF devices out there already doing
a form of it.  That's what I meant by the technology already being there.

re: CM reduction
Why would they?  CM means that the planes are in phase with each other.
In that case, they are only closely associated and parallel conductors on the
same "signal net".  There is no mechanism to cause a reduction by the planes
alone.  Actually, to be specific, the planes can't do a whit of good in 
reducing CM
emissions by themselves. The mechanism that generates the CM current must be
fixed.  Its usually one of the "unruly devices" we were talking about a 
couple of
weeks ago.

>Michael that's a long paragraph, but in the end the RD version seems to be
>"reduce the plane height", to which we already agreed.
>
>My point, and throw all the stones at it you like is that Q comes down
>directly with L, L is only coming down with height.  C increases as a side
>effect of that.

Yes, but we are limited to how much it can do by that method alone by
physical constraints.  So what do we do after that? Its obviously not yet 
enough
because we are reaching the point where electromigration could cause shorting
in boards with planes spaced too closely together.  But there are other ways.

>What I was challenging was the specific benefit to Q of raising Er, as that
>affects capacitance directly but only Z and Q at less than inverse square
>root.  It is less than square root, because lowering the SRF also reduces
>the skin resistance.

re: lowering Q
Ok. We are agreed on that. We cannot do that by increasing R, though.

>If I could get a really high Er, then I could damp out the parallel
>resonance with the discrete caps, but it takes a whole lot more C than a
>plane can practically give.  So inductance reduction, which means height
>reduction is the order of the day for Q reduction.  Istvan's solution is
>his DMB concept using either ARIES, or controlled ESR decoupling
>caps.  Each of those approaches has its own drawbacks.

re: DMB or ARIES
I am not familiar with either of these, at least by those names.  I know about
the controlled ESR decoupling.

re: increasing C
Sure.  You keep lowering the Z by doing that and make it formidable to drive
with typical I/O drivers.  That's what I've been saying.  Sounds like we 
agree. Still, we
can only reduce the height to a limited amount to avoid other problems.  We 
get some
improvement but this will eventually "run out of gas" so to speak. Even at 
that, it does
nothing for the common mode stuff that comes out of the poorly selected 
package/
active device combination not to mention any poor design of each.

> >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.
>
>Follow the field lines West young man!!!  Due to the higher permittivity,
>the field concentration  close to the top of the trace is much greater than
>for a surface trace.  Consequently, it doesn't take a lot of the stuff to
>knock 15db or more off of the signal versus a surface microstrip.
>

Take a look at the gray hair and you'll know that "young" isn't quite the right
description. Hey, but I think young! <grin>  Also, I've been "West" ... and
chose not to be there, at least for now.

Seriously though, after helping folks on EMI stuff for a long time, large 
quotes on dB
reduction make me smile and say "yeah, right."  I'm sorry but that's just a 
bit much
to swallow (an 83% reduction?).  There are only a few ways to get 
reductions like that
and that isn't one of them (been there, done that).  The physical 
construction of the
problem just doesn't allow for that much, maybe 6 dB max if you're 
lucky.  I know that
the dielectric will help concentrate the E field lines but emissions really 
aren't generally
E-field sourced, they are H-field sourced in the usual sort of electronic 
equipment most
of us deal with.  If I have my terminology correct, you are talking about 
being so close that
we are talking about the "quasi-static region" (I think Harrington coined 
that term) instead
of being in the near field. Anyway, you can know they are H-field sourced 
by the impedances
that are involved in the circuitry and the predominately circuit loop 
topology.  But I understand
the point you are trying to make. I guess I shouldn't be so pessimistic. If 
someone did really
get 15dB out of that, maybe those "prayer beads" finally worked and their 
prayers were
answered for a change.  Really, I'm glad for the person who experienced 
that miracle but
honestly, I think they did something else in the process to achieve it.

Best Regards,

Michael E. Vrbanac


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