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

  • From: steve weir <weirsp@xxxxxxxxxx>
  • To: "Michael E. Vrbanac" <vrbanacm@xxxxxxxxxx>, si-list@xxxxxxxxxxxxx
  • Date: Wed, 11 Feb 2004 23:24:37 -0800

At 11:18 PM 2/11/2004 -0600, Michael E. Vrbanac wrote:
>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 
>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 

Not at all, because the reactance of that plate did not interfere with the 
tuned circuit driving the cable.  A 6MHz television channel is a whole lot 
different than the broadband of a PWB, or a broadband CATV 
amplifier.  Aren't bandpass filters wonderful?  This actually goes right 
back to the guy with his 400MHz radio and "Bob's" inductor.

>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 
>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.

But, if it can be done and it isn't, you really should go do something and 
make money at it.

>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 
>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.

Exactly, unless someone once to start doping the foil, we are stuck on R.

>>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.

DMB is a term that Istvan has coined to represent different filter sections 
all with matched, or nearly matched damping.
ARIES is an embedded annular resistor, sort of like the Ohmega stuff.

>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 
>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 
>description. Hey, but I think young! <grin>  Also, I've been "West" ... and
>chose not to be there, at least for now.

You mean you don't look forward to lala land, the "Capital of the Third 
World".  Although all the industry is north and south of the basin.

>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?).

Microstrip to buried microstrip that is the idea.  It is not an 83% 
reduction of the overall assembly.

>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.

Well, as much as we disagree on this point, the best way to get everyone on 
the same page is to do controlled measurements and publish them.  Could we 
agree to set this up using two boards built with just an SMA launch and a 
single microstrip trace?



>Best Regards,
>Michael E. Vrbanac

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