Hi Jack....
The ideal choice is to reference the signal to opposite sides of the same plane when it must change directions, but that cannot alwyas be done. Another option as you said, is to reference two different ground planes because the return can move very easily through the vias.
But returning signals on power layers will work too. You should ALWAYS have power and ground layers next to each other at least once in your stackup for medium to high speed boards. If the power and ground are close to each other <.008" then the signal return can couple from one plane to the other and back to the source.
As an example, if you consider the source of the energy to be the cap near an IC, the energy then flows from the power side of the cap to the power pin of the part, then through the part and out the signal pin and out onto a signal layer. Let's say that signal layer references the ground plane, and that ground plane is part of a plane pair. When the signal has to change direction, it moves to the signal layer on the other side of the plane pair and the return now references the power plane. The return on the power plane can capacitively couple to the ground plane because they are so close to each other and then find it's return path back to the source... the ground side of the cap. This does not increase the inductive loop because the return path does not have to separate from the original signal. Remember, returnis happening constantly, not just at the end of the signal.
If the power and gound planes are separated, this will not work and you will be relying on capacitors for return as you mentioned. HOWEVER, the faster the frequency, the more limited the usage of the capacitors (even high frequency ones), and they are virtually useless above 2-300 MHz. So it is better to use the inter-plane capacitance of having the two planes together.
Hope this is clear enough to understand.... it's hard to describe in text what a picture would easily draw.
Susy
>
> From: Jack Olson <[log in to unmask]>
> Date: 2007/01/04 Thu AM 09:50:41 CST
> To: [log in to unmask]
> Subject: [DC] Straddling the Return Path
>
> I have a question about layer stackup and routing.
>
> Most of our designs have a pair of routing layers
> sandwiched between planes, one for vertical and
> the other for horizontal traces.
> Maybe I haven't had enough coffee yet today, but
> I started wondering how the return path energy
> gets from one plane to the other (unless there
> happens to be a decoupling cap nearby?) If both
> planes were GND there wouldn't be a problem,
> since they would be stitched together in many
> places, but one is GND and the other is PWR.
>
> So I started wondering.... Wouldn't it be better to
> have one routing layer ABOVE the GND plane and
> one BENEATH? In this scheme all the routing would
> be straddling the SAME return path and it seems
> logical the it would be less noisy. There would never
> be a discontiuity from the energy trying to get from
> one plane to the other, right?
>
> maybe the gains are not worth the loss in the
> overall scheme, though?
>
> onward thru the fog,
> Jack
>
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