Your concerns seem a little bit vague to be getting the responses you have
so far. I agree that the capacitance of the planes is a minor issue compared
to noise concerns. You would need a pretty thin dielectric between power and
ground to make much difference anyway, so the stackup is probably flexible.
It would help to know what kind of job the PCB is supposed to perform,
because if you are designing a power supply it might not help to hear things
like "controlled impedance is more important than keeping planes together".
When you talk about noise generators, do you mean you are concerned that
noise will be radiated off the board and affect something else? Or do you
mean you need to keep the noisy stuff away from other things on the same
board? In the latter case, you may want the noisy steuff routed on outer
layers, and keep the good clean stuff between planes. You may want to keep
noisy stuff near the edge of the board, pour ground around it and route the
bad stuff right back to the connector so it never gets near the good planes,
or split the ground/power planes partially to force the noisy stuff back
towards the connector (or wherever the ground is going) so it doesn't mess
up the good portions of the planes, etc. Or you may need to keep the power
and ground together, route all the junky stuff on top and keep the critical
stuff on the back, something like that. But it is hard to get a mental
picture of what you are trying to do based on the information you gave.

Best wishes,
Jack

                -----Original Message-----
                From:   [log in to unmask] [mailto:[log in to unmask]]
                Sent:   Wednesday, October 20, 1999 8:46 AM
                Subject:        Reducing noise

                Hi,  I hope someone can help me out here.  I'm looking for
suggestions on
                ways to quite down traces that we know are going to be noise
generators.  If
                using a 6 layer board...what is the best stack up?  Should
power and ground
                planes always be kept next to each other for optimum
capacitive performance?
                Which layer should these noisy lines be kept on?  Any help?