If you are failing the EMC test in Radiated Emissions, this will help you see why:
The formula you see here describes the radiated emissions due to differential mode currents measured at the far field.
The formula is simplified to highlight the point I wanted to share with you.
For the picky ones: electrically small loop antenna, at 3m distance, at 90-deg angle, at far field.
The key is:
Radiations due to differential mode currents are proportional to the area enclosed by the current loop.
Why is this important for you as a PCB designer?
Because if you design your layout without considering the path the current takes to go from the source to the load and then back to the source (this is where many get tricked)...
...then you may fail the radiated emissions test. Simple math.
The bigger the area, the bigger the radiated electric field.
The biggest issue is that this largely depends on the layout of the board, so you need to fix this at the beginning, NOT AT THE END of your design!
Why do I say this?
Generally speaking:
Because the frequency is often fixed by the application, changing it might decrease performance. I won’t get into that here.
The same goes for the current because the differential mode current is the current required by the system to operate, so changing this is not an option in many cases.
The takeaway: I recommend designing the PCB with this in mind from the start. Don’t wait until the end of the development process to think about EMC.
Otherwise, how are you going to fix this?
Also, this applies to the loop of the cables, so keep this in mind.
I hope this helps,
Dario
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