[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 17:30:47 -0600

Steve,

Nima's email was the original that started the thread several days 
ago...  I have the
copy here in my mailbox and referred to it when I wrote the last email....

Continuing the dialog...  launching from your comments... and I'll truncate 
the rest of the
last message...

re:  "Considerable energy from what would be only a standing wave can and will
find its way out under many conditions. Agreed that CM is a separate issue of
plane potential versus the chassis."

Without doubt...  The funny thing is after solving countless (meaning I 
lost count!)
emissions problems, only once could I ever verify that plane resonance was 
the real
issue.  I say that as a result of "board scanning examinations" and 
"distributed plane
potential measurements.  With these tests, in the one case only, was a 
defined standing
wave pattern observed.  The rest of the time those issues were always 
resolved by
very well defined design corrections that directly addressed the real issue 
that was
present.  I say that only to make the point that I don't do "kludge fixes" 
except at
gunpoint. <grin>  I hate EMI problems resurrecting themselves to haunt me 
and my
clients.

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 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.  Given that, it is granted that the 
more consistent
a particular electrical feature appears on a plane the more likely a 
stronger reaction to
producing something quite significant (resonance-wise) will occur.

re: "Ah yes, those devilish little details again.  As in frequencies 
present in
the package now go way beyond what we can convey through the package
interconnect to the PWB.  At frequencies substantially above 100MHz I know
of no way to tightly couple any IC to the plane beneath.  The best I know
how to do is shield the overall structure.  If you have a secret for
overcoming package impedance at 100's of MHz, and beyond, I am all ears.

Hmmm... well, I agree with the 100MHz when all we are talking about is 
"pins and balls"
for package electrical connections.  I do not agree that is the end of the 
matter.  I've
designed with RF power devices and if we understand how we can get 
significant power out
of those devices without melting the leads off from the RF current density 
issues caused
by the skin effect, we can certainly understand how to get a lower powered 
package
connected to the reference plane better.  My point is to say is that we 
have the technology
already... we just aren't using it to our advantage. When we finally do, I 
predict that many
of these issues will simply "go away".  Our problem is that we are fixated 
on "pins and
balls" for package connection methodology.  Its like we want to use 
something comparable
to "wirewrap" technology for sub-100nS signal transmission. When aspect 
ratio of a suitable
conductor is used, the inductance will drop and we'll all look back on it 
and say "gee, why
didn't we think of this sooner?"

re: "I have always seen this as a matter of reducing inductance for the first
choice.  If we hold L constant and increase C by altering Er, then the SRF
falls by 1/sqrt(Er_new/Er_old), and for a fixed resistance, Q also falls by
a similar amount.  It is actually a little worse than this, as the reduced
SRF drops R due to skin effect.  However, for the same constant R Q falls
directly with reduction in L."

But all you are doing is dropping the resonant frequency by increasing C... 
not surprising.
Dropping L under those conditions does drop Q and that's good but I think 
there is more to it.

Perhaps we can look at this another way by looking at all the 
variables.  Actually, you
need to handle the R first since that sets the baseline for overall 
impedance, the reactive
portion is frequency dependent, R is not... (R + jwL or R - jwC).  This 
means that skin effect and
surface area of the conductor must be accounted for in the problem.  To set 
the "baseline" lower,
the R must drop.  But a problem exists... dropping R always raises Q 
since:  Q = X / R. Hi-Q creates
a more efficient (less losses) resonance.  How do we solve this?  We make 
the "drive point impedance"
too low for the driver to drive at the unwanted frequency.  Why?  The 
impedance mismatch not
only causes inefficient power transfer to the resonant structure, it also 
loads the "driver" down
too far to be able to drive it effectively.  So what do we do?  Since Z = 
sqrt (L/C), decreasing L
drops the impedance as well as increasing C.  Since we have already known 
that we can easily
increase C (to a point) through closely spaced power planes, we can use 
this method to decrease
the power plane "transmission line" impedance.  We also know that we can 
add to this method
by locally loading the planes with selected capacitance but at higher 
frequencies this becomes
more difficult (but not impossible) for a variety of reasons.By providing 
these additions, RF current shunt
paths are provided to "ground", this is equivalent to the old term 
"bypassing". (That term is sometimes
used in place of  "decoupling" intending to mean the same thing but the 
circuit functions are not the
same.)  The only thing that's left is "L".  In reality, we (industry) 
actually are leaving this term largely
untouched as I was implying earlier.  When we explore what determines "L" 
and how to reduce it, we
have our generic answers on how to solve the problem.  So from my 
viewpoint, we can say we treat
"L" first but in reality we've left it for last.  (I do agree we probably 
should have solved "L" first).

re: "Agreed, we can design well tuned or intentionally poorly tuned 
antennae in microstrips.
The point that had been circulating concerned the containment properties of 
prepreg."

Prepreg?  I was under the impression that pre-preg was (for FR-4 class 
dielectrics) uncured
epoxy glass dielectric substrate that was cured during the "press-up" 
during fab.  Why would
more recently cured FR-4 (cured pre-preg) act any different than pre-cured 
FR-4 (core) if the
basic materials are the same and the glass/epoxy ratio is the same?

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.

Best Regards,

Michael E. Vrbanac





------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List technical documents are available at:
                http://www.si-list.org

List archives are viewable at:     
                //www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: