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

Preston Andrew MMUk (andrew.preston@micromass.co.uk)
Wed, 6 May 1998 15:29:25 +0100

Whilst trawling through the si-list archive, I came across the following
throw-away comment (Mike Jenkins, "+3.3,5-board stackup problem", 7 July
1997):

> Regarding mesh power planes, one caveat: If the plane uses a diagonal
> mesh (i.e., lines at 45 degrees to signal lines), then the ground
> current can't follow the signal path directly. This can substantially
> reduce the velocity of propagation.

The reasoning seems to suggest that whenever a return current is unable
to follow the signal path closely, then the propagation delay will
increase. This will occur in many circumstances, such as a power-plane
split, a poorly placed bypass cap, or even when there is no appropriate
return plane available.

Can anyone provide any more theory on this phenomenon?

Andrew Preston,
Micromass Ltd.