GROUNDED: Your PCB, EMC/EMI, and SI Nuggets

In this simulation, you can see what would happen to the signal when it transitions from one layer to another in a printed circuit board stackup.

Two simulations are shared here:

- In the case on the left, the signal transitions from the top layer to the bottom layer with a single via. No other vias are placed—simply the signal via used for the transition.

- In the case on the right, the signal transitions from the top to the bottom layer in the same way, using one via, but with two additional vias placed adjacent to the signal via and connected to the Return Reference Plane (RRP).

You can see that the first case is designed like many typical PCB designs out there. The layout is designed with the concept of electrons flowing through the copper, rather than EM-fields in the space around it.

As a consequence, when the signal transitions from top to bottom, the fields, which were contained during horizontal propagation due to the presence of the RRP, are now left uncontained during the vertical propagation of the signal. This allows the EM fields to spread through the layers of the PCB stackup, generating noise.

In the second case on the right, however, when the signal transitions from the top to the bottom layer, it also finds its return and reference path, which keeps the field contained for the entire propagation. In this case, the fields do not spread through the layers in the PCB stackup, thereby avoiding the spread of noise.

Why is this important?

Consider at least two problems:

- The noise spreading in the stackup contaminates other signals (crosstalk).

- The same noise reaches the cables and uses them as antennas to propagate outside of the PCB.

Both are definitely things we want to avoid, and both can affect the EMI performance of the board.

Solution?

Think not only about how the signal (or rather, the EM fields) propagates horizontally but also how it propagates vertically.

Then we can build a structure that can contain and channel these EM fields where we want them to go.

This helps us avoid EMI issues and failed EMC tests as a consequence.

See?

I told you EMI was not black magic.

-Dario

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