Date: Wed May 10 2000 - 10:06:11 PDT
Re: Your copper tape suggestion.
Yes, it does work. I have been preaching this technique since 1989 to all my
clients in the form of a "chassis ground ring" on every layer of the PCB with
vias at varying spaces (to prevent high harmonic resonances along the edge)
to create a reasonably low inductance field termination structure for fields
emanating from all inboard locations on the PCB. I use this technique, along
with several others to achieve typical reductions of >20 dB in radiated
emissions. 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.
The setback approach (20-H is a bit arbitrary) has its place in small, high
packaging density boards that can't afford the real estate for a separate
Michael L. Conn
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