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

larry smith (ldsmith@hsmpka.eng.sun.com)
Wed, 6 May 1998 08:49:01 -0700 (PDT)

Andrew - This is more opinion than hard theory, but I believe Mike's
comments about propagation velocity on 45 degree lines apply at
very high frequencies where the wavelength is on the same scale as
the mesh (or other structure).

For most of the work that we do, structures are much smaller than
a wavelength. For example, one wavelength in FR4 at 1GHz is 3 inches,
much larger than the size of typical mesh grids or vias.

The velocity of propagation is determined not at all by the conducting
material, but is a fundamental property of the medium outside of the
conductor. The propagation velocity in FR4 is just c/sqrt(eR). At
frequencies where the wavelength is much bigger than the structure,
all fields in the dielectric (E and B) will be superposition's of the
fields associated with the current tracks. I am sure somebody will
tell me if I am wrong, but I think those fields will propagate through
the structure at c/sqrt(eR) as expected. The fact that there is
electric current flowing at 45 degrees to the average electric and
magnetic fields is just an interesting phenomenon, but should not
influence the propagation velocity of the fields. Remember that
the drift velocity of electrons (or holes in a semiconductor) is
much less than the speed of light, and is unrelated to the speed
of light.

Now, when the structure size is similar to a wavelength, the
field propagation will more closely match the direction of current
flow. But with the dimensions and frequencies that we deal with
in signal integrity, I do not believe this is a concern.

regards,
Larry

> From: Preston Andrew MMUk <andrew.preston@micromass.co.uk>
> To: "'Signal Integrity Mailing List'" <si-list@silab.eng.sun.com>
> Subject: [SI-LIST] : Propagation velocity / discontinuous reference plane
> Date: Wed, 6 May 1998 15:29:25 +0100
> X-Priority: 3
>
> 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.