4-layer - which layer for ground, power?

depends on your definition of small. with EMI specs now up around

2-3GHz, small just got a whole lot smaller. I have seen plenty of boards with a grid of about 5-10mm, using perhaps 0.5-1mm traces. and all of the small bits add up, when you trace the path(s). By the time the grid gets small enough that multiple paths cancel out the effects of increasing distance, theres more copper than gap, so might as well pour a plane.

all of these things may be true on any give board. It is certainly true that a "ground plane" is no panacaea, and that some semblance of understanding is required.

I like to do an emc audit on my schematics, prior to layout. sort out nets as victims or culprits, and rank in order of severity. That way I know what to pay real close attention too, and what to ignore.

Then post-layout, do it again. what circuit have I actually built, where are things going to go wrong etc. Chop out lumps of ground plane, re-route traces or whatever.

then get the pcb made, and measure crosstalk, impedance etc. of all the sensitive areas before sticking components on and really testing (if its worth bothering. I have done on some layouts, but dont on most).

various rules-of-thumb help too. I always place bypass caps, supply and

0V vias around a chip first, then look at laying out the rest of the parts. always using a 0V plane helps too.

the biggest bang-for-buck is having a nice low-Z 0V (and perhaps power) plane, that isnt chopped to f*ck with traces. I havent done huge logic boards, but the dI/dts would no doubt mandate power planes too.

when you look at the effects of shield traces vs increased separation, separation works pretty well, esp. for a long bit of shield trace, and you are right about the PITA of plopping vias everywhere.

OTOH analogue circuitry often likes it.

The rule-of-thumb would, I guess, be "if it doesnt hurt, pour copper planes everywhere"

Cheers Terry