Crosstalk calculation for bottom layer tracks

[BecirPeco]

Hello,

I have a question about crosstalk calculation. I have a four layer board with stackup Top-Signal, Inner1-GND, Inner2-PWR, Bottom-Signal. If I want to calculate crosstalk between two traces on bottom using the calculator, what is the substrate height H. Do I sum up all the dielectric thickness (prepreg and core) until GND layer.

[mairomaster]

In your case the PWR plane on Inner-2 is used as a reference for the signals on the bottom layer. Hence, use the distance between Bottom and Layer-2 (normally the same as the one between Top and Layer-1).

[BecirPeco]

Thanks for the answer. So the reference plane is the adjacent plane, regardless if it is GND or PWR.

[mairomaster]

Yes, especially with 4 layer board, Inner1 is quite far away from the Bottom and can't be used as a good reference layer even if you didn't have the power plane on Inner2.

Using power planes as reference layers is a bit trickier, that should be kept in mind. The details about that are not perfectly clear to me either, but I know that is the case. If you have very sensitive signals, I would always prefer to use a good GND reference for them.

[robertferanec]

BecirPeco, as mairomaster mentioned, using PWR as a reference plane may be sometimes tricky. A good power reference plane is a full solid plane which is for example 3V3 (many decoupling capacitors to GND - these capacitors are basically short circuits between GND and 3V3 for high speed signals). Some power planes are not good power planes, if you are not sure, always route the critical signals above a GND plane.

[BecirPeco]

Thanks for your feedback. It seems I will have to go have a look at signals on bottom. Basically, the board is packed with components on top, so on the bottom I ran some long high speed signals. Would it make sense to change stackup, put GND next to bottom layer just for these signals? So it would be top, pwr, gnd, bot (with critical signals).

[robertferanec]

Maybe that could be a good solution.