RE: [SI-LIST] : Regarding plane splits

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Mon Jan 01 2001 - 11:36:22 PST


> > So what you say is that if my 2 split voltage planes are well decoupled
> to GND by being
> > adjacent to a GND plane, then I've got essentially a continuous return
> path for the AC
> > current. Its path will be from the 1st sub-plane to the GND underneath
> and then back to
> > the 2nd sub-plane. Do I understand it correctly?
 
I think you understand correctly what had been suggested. However, whether
this holds true or not, is another matter.

I think it is fair to say that, if you go high enough in frequency, it does
not work anymore as a good, "continuous" return path.

As you go higher in frequency, the return path tends to "hug" the signal
trace. The fact that it must spread out in order to find enough capacitance
(whether intrinsic or added capacitors) to make those two plane jumps, means
there is some additional inductance as well as path length in the return
path, which would not have been there had the first plane been continuous.

If you think of what happens to the TEM mode electric and magnetic fields in
the dielectric, I think you have to conclude that something messy happens to
them at the interface. How much this affects the signal integrity of the
waveforms, is something you need to explore, through simulations and/or
experiments.

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