[SI-LIST] Re: : May 9th Presentation: "Radiation from Edge Effects in Printe...

Charles:

I haven't found the excerpt you provided from May of 2000 (repeated here for 
convenience)...

> ... Because the slot created along the edge of any surface plane (or
> fill) may be inadvertently excited to radiate, mounting provisions for
> "resonance spoiler" capacitors should be placed along the slot. These
> capacitor provisions allow "doctoring" of the structure if emission 
problems
> are found and avoid another board spin.

...but, the above comment can apply to the slots created by moats around 
functional circuits located within the body of a PCB, and the slots created 
by the addition of chassis-ground rings around the periphery of a PCB. I 
suspect from your other comments that the latter case is of concern. With 
this interpretation in mind, I suggest you review the following in your 
design.

Signal ground planes should extend AT LEAST three to five dielectric 
thicknesses (i.e., the distance between the power and signal ground planes) 
beyond the power plane edge. Propagations (i.e., noise from any source) on 
the power planes will then be strongly coupled/terminated to the adjacent 
ground planes, thereby minimizing radiation.

With the above in place and at your option, picket fences of 
staggered-spacing vias (to minimize possible narrowband ladder resonances) 
between ground planes can enclose the noisy power planes, thereby forming an 
approximation to an on-board Faraday cage which helps confine leakage of 
emissions from the power plane edges.

Assuming chassis ground rings around the board periphery, include "spoiler" 
chip capacitor mounting provisions (perhaps three per side/edge) that if 
installed would allow a capacitor bridge from the ground plane(s) edge to the 
chassis ring structure. Using a suitable near-field probe and a spectrum 
analyzer, check for high field strengths along the board edges ("high" is 
relative of course). If emission peaks are detected, selectively add one 
capacitor at a time in the vicinity of the maximum E-fields and recheck the 
emissions. Look for frequency shifts as well, as you may simply have changed 
the resonant point for the emissions. The primary intent of the capacitor 
provisions is to detune and lower the Q of any slot resonances that may 
arise, which in turn would lower the radiated emissions. Selection of a 
capacitor value that is self-resonant (when mounted) at the detected 
frequency of concern sometimes helps.

As an elaboration to the above, if your near-field probe is a magnetic field 
detector, and strong fields are detected near the center of a board edge, you 
should add capacitors half way to the corners on each side of the detected 
point. 

I hope this commentary is of help.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
Cell: (408)821-9843

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