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What do you notice here?
Action list:
For some PCB designers, this might just look like a common PCB design.
And fair enough, I was there once too.
But for those striving to achieve the best EMI performance, they will immediately notice one big problem.
The signal traces in this motherboard PCB are not matched with the Return Reference Plane (RRP); instead, they are routed over multiple power planes divided into different sections.
As a consequence, they are also routed over the splits that separate the copper fills.
The issue is that when signals reach these splits, they encounter what is called an impedance discontinuity, which simply means the impedance changes.
But if we recall, impedance is defined as the ratio between voltage and current.
So, if the impedance changes, this ratio changes as well.
Which simply means our signal is now in trouble.
In terms of EMI, the worst issue arises from the excitation of the space between the planes, called the cavity, and from the enlargement of the return current loop, which is now forced to find alternative paths rather than staying under the trace.
This is one of the reasons why I’m not a fan of using power planes as a return path. There's more to it, but I'll leave that for another time.
My recommendation when choosing the stackup:
For each signal layer, make sure you have a Return Reference Plane (RRP) adjacent to it. And if you want to save on RRPs, take advantage of the skin effect by using one RRP for two signal layers, one on each side of the RRP.
I hope this helps you avoid EMI,
Dario
fresuelectronics.com
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