Good EMC results start with a well-chosen stackup.
Is it that simple?
Almost.
I like to think about EMC as the test which measures how well I contain the electromagnetic fields and how well I protect the PC board from external unwanted fields.
I try to keep it simple, to start with.
This is also why the stackup is one of the first things I check when reviewing a PC board layout that is failing EMI tests.
Because it all starts from there.
The EM field is bounded by the conductors, which in this case are the copper layers.
Since we do not want the signal to radiate, we need to have two conductors to guide the signals.
Because the EM fields are in the dielectric space, not in the conductor itself.
This is why the best choice when selecting the stackup is to have a return reference plane adjacent to the signal/power layer.
Notice one absolutely important thing:
I did not call it "GROUND".
Nope.
I called it Return Reference Plane, or for short, RRP.
Why?
Because the name implies three absolutely important things:
1. It has to provide the path for the signal return current to flow back to the source it originated from.
2. It has to provide the reference for the signal voltage (you don't measure voltage with only one lead of the voltmeter).
3. It has to be a plane. With no cuts, splits, or special shapes. This is important because it has to be the path of least impedance for the two requirements above.
Start from here, and watch the harmonic envelope collapse in the spectrum analyzer.
I hope this helps,
Dario
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