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An electrical signal needs a return path.
There is no signal that goes only from A to B, it also needs the return path back from B to A.
It is a loop.
And the return path is equally important as the signal path.
If a trace travels over an uncut GND plane, then the "return" current travels rather exactly back atbthe same location in the plane. Not much right or left.
(Electromagnetic field)
If there is a cut in the GND plane ... under the trace...
Then the return current at this place of plane can not travel directly, it needs to use a different, longer path ... causing impedance jumps in the signal line, echoing, bad signal quality...
It's not unusual for an economical and not super fast design to utilize power planes as transmission line ground. It's o.k. if the power plane has low inductance and suffciently distributed bypassing to the primary logic ground. The requirement is usually fulfilled for logic power supplies that should have bypassing capacitors placed near each supply pin. For other planes, e.g. a 12 V planes you possibly don't have sufficient bypassing in the transmission line region. Still no problem for differential signals that don't involve return currents but not so good for single ended signals.
An electrical signal needs a return path.
There is no signal that goes only from A to B, it also needs the return path back from B to A.
It is a loop.
And the return path is equally important as the signal path.
If a trace travels over an uncut GND plane, then the "return" current travels rather exactly back atbthe same location in the plane. Not much right or left.
(Electromagnetic field)
If there is a cut in the GND plane ... under the trace...
Then the return current at this place of plane can not travel directly, it needs to use a different, longer path ... causing impedance jumps in the signal line, echoing, bad signal quality...
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