Electronics > Beginners

PCB Split Power Planes

(1/1)

Melon:
I have a good understanding about ground planes and current return paths, how a slotted ground plane can cause return currents to divert from the optimal path underneath the signal (for high frequencies) and cause a lot of crosstalk and EMI problems. However just now I read about split power planes and I started looking for advice. Everywhere I looked it is said that it is not a good idea to route signals over a split power plane, but I haven't found a good explanation for this. Why is it bad to cross a split power plane? The return current goes through ground, so the path is not disturbed by the slots in other planes, as I have represented in the drawing I attached. Is there a different reason I'm not looking into (such as t-line impedance mismatch)? I'd like a clarification on this topic. Thanks.

chagli:
Hi,
I am quite new in the EMC field.
I think the return path is fine for the desired signal in your case.
But the return path for any RF noise signal superimposed on this signal is bad because it is the same. Let me explain.
This RF noise signal should use the plane just beneath, irrespective if it is ground of power: at RF, the noise can go through decoupling capacitors and change plane easily. In your stackup, the RF noise should return on layer 2 which is split and thus goes through layer 3 which is further away from layer 1 meaning higher inductance, radiated field etc.
The solution here would be to route on layer 4 because the plane just above is not split.
You can see the RF current path in the figure below.
There's also the skin effect which prevents the RF current to cross a plane. The current then has to follow the outer surfaces...
I recommend you "split ground/power planes" section from the great book of Henry W. Ott : "Electromagnetic Compatibility Engineering".

Navigation

[0] Message Index

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod