[SI-LIST] : Re: Propagation velocity / discontinuous reference plane

Mike Jenkins (jenkins@lsil.com)
Wed, 06 May 1998 11:11:10 -0700

Andrew, Larry,

It looks like I ought to explain my comment on propagation velocity
with mesh ground planes. Perhaps I should not have said that the
propagation velocity changes. It is, as Larry says, a property of
the medium. Rather, the path length changes. (If, you coil up
50 meters of coax and hold the connectors 8 inches apart, it takes
a bit longer than a nanosecond for a signal to traverse the cable.)

Dumb example, I know. But causing the current path -- in the signal
OR the ground conductor -- to be other than straight will cause the
E & H fields and the resulting Poynting vector to vary in direction.
This is a qualitative argument, but the application where I ran
across this effect reported, I recall, about 20 to 30% increased
effective propagation delay based on the signal trace length. For
comparison, if signal and ground current had to take a +/- 45 degree
detour, the effective increase in delay would be 41.4% (ie, sqrt(2)).
(Larry, this was the old TCM work. Don't know if you were at IBM
then or not. They were trying to reduce capacitance of the lines
buried in ceramic.)

I hope that clarifies my previous comment.

Mike

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