Reducing EMI with ground plane

Question

I came across this post on how to reduce EMI in a PCB. From this, I learned that there actually exist two types of current. The first is the conductive current (the current that flows through a conductor) and the second is magic (or displacement current). This displacement current is 99.9999999% the reason why electronics is considered black magic :).

In the post they show that it is best to use smaller traces for high frequency is a must! From Maxwell's equations:

The contribution from Maxwell is the second term in the parentheses, this term explains how much displacement current will exist.

Basically having a smaller conductive area, reduces this term and thus the magnetic field created for high-speed signals. Hence reducing unwanted EMI emissions and interference with neighboring tracks.

The post included this picture:

Please see link here

My questions:

1. Is what I explained above correct?

2. I know that adding a ground plane to the bottom layer of the PCB helps with EMI, however, how does it help exactly? I am assuming is reduces the capacitance, but how does it reduce it?

Answer 1

Is what I explained above correct?

I don't think you really need to bring any of Maxwell's equations into this.

It helps both electric field problems and, it helps magnetic field problems. As a large piece of copper, the ground-plane acts like a shield and diverts electric field interference to a safe place rather than hit sensitive nodes.

Magnetically, when a high speed signal is transmitted from one chip to another there will be an associated high-speed current. That's the forward path but, the return path from the receiving chip back to the sending chip is also a high-speed current. Without a ground-plane the forward and return PCB tracks form a loop that will emit EMI. If you are skilled you can stack forward and return tracks on top of each other and minimize the EMI loop size.

_{That's also why we use specialist cables like coax and twisted pair.}

However, with a ground-plane, the return current naturally flows underneath the forward current track and this automatically minimizes the EMI loop size.