[SI-LIST] Re: risetime effects of plane breaks
- From: "Jeff Walden" <jwalden@xxxxxxxx>
- To: "Si-List" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 20 Jan 2005 22:17:59 -0500
To chime in with my friend Scott, consultant (yes, I'm another that you have
to be careful about making eye contact with), & fellow user of MWS et al, I
will generalize & over simplify .. although obscured in lots of words.
Create an edge(s) in metal near intense RF current and you create a RF
radiator. That is an external result. Also, the edge(s) can be characterized
as LC networks apart of the transmission lines aka signal traces, i.e. it is
also a filter. Typically a high pass (& L or C coupled) depending on
location and orientation. I think of this as an internal effect ... internal
to the signal's integrity and risetime. Regardless of the orientation of the
edge wrt the signal trace, the radiation off the edge is free.
Being a minimalist evidenced by the scarcity of my comments on this list, I
think the above paragraph is all that needs to be said. Bu-u-ut since I have
started, Scott thoroughly explains split plane permutations. To me, I hear
split plane, I hear slot antenna. I'll extend that thought to any
discontinuity (aka edge) introduced in a reference plane or some metallic
structure that gets referenced by RF current ... be that current apart of
narrow band RF signals or that comprising digital signals.
By example, a cutout at the edge of PCB for a mechanical hold down & created
perhaps to isolate digital ground from chassis ground. If traces are near to
that area such that the "return current" passes along that edge, the edge
will radiate. The edge can be of any shape straight, zig zag, etc. ... RF
current flows all around it for a combined electrical length.
If a PCB or other kind of widgets as permanent or separable interconnects in
isolation are well behaved but placed in a metal enclosure and some part of
the enclosure is near traces carrying high speed digital signals, RF current
will couple to and flow on the inside surface. This now insidious RF current
flows until it finds an edge of the enclosure coupling to other signal lines
along the way. Shielding can make a good design bad as it can easily
increase cross talk and EMI. The current moves along & encounters edges for
vents, painted overlapping surfaces, shields that do not quite extend to a
ground plane ... well, they are edges & they radiate. To me they are all the
same & are scalable.
If the edge is electrically short wrt the RF content of the digital signal,
then the filtering and radiation effects will be minimal as it is coupling
to the higher and less intense RF spectra. However, you must be cognizant of
what is the electrical length of the edge; in other words, what is the
effective dielectric constant & how much it may increase the electrical
length of that edge must be considered. If it is on a buried ground plane
then the radiation is internal & we have system noise; but, the electrical
length is longer & will work better for lower frequencies. If the identical
geometry is on an outer layer, the electrical length is shorter (air on one
side) so maybe the resonance is above the spectral content available ... but
if not it then radiates out ... EMI. Of course it will always be "long
enough".
A net search for "slot antennas" indirectly offers a few examples of what SI
needs to avoid. As a good-bad example, note that
http://www.research.ibm.com/journal/rd/472/liu.pdf shows an inverted F
antenna. It could be just a notch in the edge of a ground plane. Could
similar geometry be at the edge of a PCB not intending to be an antenna like
the digital ground plane cutout for the retention screw to chassis ground?
Note that a signal trace (aka transmission line) does not need to cross the
discontinuity (split plane) near the middle. In fact it may work better
(depending on desired radiation polarity) when coupled near the end nearest
what might appear to be a RF short ... NOT!
Jeff Walden
EMC/SI & RF simulation, modeling & analysis
_________________________________________
Walden & Associates
Phone: 717-697-5848
Mechanicsburg, PA 17055-5805
email: jwalden@xxxxxxxx
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