Hi Susy,

That was clear enough to understand,
and I DO have your presentation!

thanks for the tips and the reference.

Jack



On 1/4/07, [log in to unmask] <[log in to unmask]> wrote:
>
> 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
>

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