Date: Thu Apr 20 2000 - 13:21:34 PDT
In most of todays high-speed designs, the PCBs have many layers and multiple
planes. Discussions so far have been simplified to get concepts across. The
interplane stiching [i.e., multiple connections between the edges of multiple
ground planes (and between power planes, too)] is a definite positive as
regards containing potential radiation. Although it depends on the detailed
geometry of the stiching, in general the effective Zo of the planar
transmission line interface(s) is minimized; hence, the mismatch to the free
space impedance is greatest, which leads to less energy transfer and less
radiated emissions. Further, any field fringing from a buried power plane
(i.e., having an extended ground plane both above and below it) will be
captured/intercepted with high efficiency. This same power plane, if in
direct contact with another power plane that has an offending wave hitting
the edge, will conduct away much of the energy and reduce the potential
radiation. This is equivalent to adding a second transmission line to the
"open end" of the one that has the offending wave. Most of the energy will
transfer to the other (passive) planar transmission line and be conducted
back into the PCB. Note however that this effect will couple interplanar
noise generated between one set of planes into another set.
In total, it's generally a good thing to stitch multiple "like" planes
together not only at the edges, but in a grid across the entire PCB as it
reduces common mode impedance. Lowering of common mode impedance helps
reduce multiple sources of interference generation and therefore helps reduce
total emissions from the PCB.
By the way, Chris, did you get my E-mail request a few days ago re: LINUX and
Fasthenry? If not I'll send it again.
Michael L. Conn
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