[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: Fri, 13 Feb 2004 02:12:49 -0600

At 11:24 PM 2/11/2004 -0800, you wrote:
> >>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.
>
>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.

Sorry, you don't have the winning number.... please play again. <grin>

... not 6 MHz... that's the bandwidth.  Try up around 600 MHz....
these pitiful little boards have nothing of the energy that stuff had...

and you made my point.

> >>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.

Ok. Noted.

> >>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.
>
>
>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.

Been there, done that and wondered what was so special... definitely
different but I wouldn't call it special.

> >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.

I hope the experiment was consistent enough such that the microstrip
was the same distance from the plane in both cases... still that much
change doesn't make sense.  I agree it'll help some but not that much.
I would believe it if the distances were different like in a stackup like:

surface microstrip
dielectric (distance A)
embedded microstrip
dielectric (distance A)
plane reference

>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?
>
>Regards,
>
>
>Steve.

Yep.  And agree on the measurement method...

Regards,

Michael


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