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

Chris_Heard@ne.3com.com
Wed, 6 May 1998 21:34:41 -0400

My guess is that electromagnetic waves will travel at the same speed
through the dielectric medium between signal and ground, the only thing
that will change is the characteristic impedance of the transmission line
due to the relative (tiny) increase in distance between signal and ground
currents. Tpd is typically dependant on Er only.

Chris

andrew.preston@micromass.co.uk on 05/06/98 10:29:25 AM

To: si-list@silab.Eng.Sun.COM
cc: (Chris Heard/US/3Com)
Subject: [SI-LIST] : Propagation velocity / discontinuous reference plane

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.