Routing over a split in the return reference plane is one of the surest recipes for EMI failure.
It took me some time to understand this.
I didn't realize that when a signal crosses a split, it loses its reference potential, and the return current has to find a longer path to return to the source that generated it.
Little did I know that the size of the current loop is directly related to the radiated emissions from differential-mode currents.
What was even more painful was that I didn't understand how the parasitics created by the signal crossing the split would also generate common-mode current, which is even more efficient at radiating than differential-mode currents.
What you see in this image is one of the most common issues when a project fails EMC tests due to EMI.
Make sure your signals are always supported by a solid return reference plane (RRP) adjacent to them.
This will save you from a lot of EMI troubles.
I hope this helps.
Dario
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