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

Weston Beal ([email protected])
Wed, 6 May 98 13:22 CDT

> From silab.Eng.Sun.COM!owner-si-list Wed May 6 11:33:05 1998
> Date: Wed, 6 May 1998 08:49:01 -0700 (PDT)
> From: larry smith <[email protected]>
> Reply-To: larry smith <[email protected]>
> Subject: Re: [SI-LIST] : Propagation velocity / discontinuous reference plane
> To: [email protected], [email protected]
>
....
>
> 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).

But this applies to a well behaved plane wave or TEM propagation, I think.
When you break up the ground plane, your wave is no longer strictly TEM.
It may be close enough to assume TEM, though, like we often do. If you
look at velocity as 1/sqrt(L*C) then gridding a ground plane should increase
L and decrease C. Which one changes more with the gridding effect would
dictate the change in velocity. Again, at the dimension that we work with,
this should be a very slight change. I'm still working on the bigger problems.

> 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

But this could change the average propagation path of the wave in the
dielectric, and change the measured velocity from one end of a trace
to the other. Still very small effect.

> 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.
>
....
>
> regards,
> Larry
>

Regards,
Weston

>
>
>
> > From: Preston Andrew MMUk <[email protected].uk>
> > To: "'Signal Integrity Mailing List'" <[email protected]>
> > 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.
>
>