Controlled Impedance Trace-reference plane Placement

Question

I am new to high-speed PCB design. I was studying controlled impedance traces from the internet with the help of various documents. Everywhere it says controlled impedance traces need a reference plane directly below it. I know that the reference plane is used to return the current path(Please correct me if I am wrong).

My question is why controlled impedance traces need a reference plane immediately below them. What will happen if we put a signal layer below the layer which contains a controlled impedance trace.

Answer 1

If you have this stackup - controlled impedance track, signal track, reference plane, where the controlled impedance has been calculated assuming the signal track isn't there, then the signal track will do two things

1. Reduce the impedance of the top track in its vicinity and
2. Couple signals to/from the top track

If it's just one signal track, then the reduction in impedance will only be over a short length and may be negligible. If the top track crosses the signal track at right angles, then the coupling of signals will be fairly small, and you may get away with it.

If you have a whole layer of signal tracks, then the impedance change will be significant. The top track will couple all the signal traces to all the others.

However, this pontificating what the effect would be if you were to run a signal layer between a controlled impedance layer and its reference plane is really a waste of time, because it's something you don't do. You never need to do it. If you feel the need to do it, re-order your stack up so the signal layer is below the reference plane. Or if you do it, you haven't got controlled impedance traces. .