Why doesn't common mode current on an isolated PCB cause EMI problems?

Question

I was reading "Signal and power integrity simplified third edition" by Eric Bogatin and on chapter 11.16 the author states that connecting an external cable to a PCB that contains an unwanted common signal will cause said cable to radiate.

For clarification, this is an image from that same chapter:

The external cable couples to nearby objects (typically the floor) and finds its way back to the ground plane of the PCB, the common signal impedance of the external cable must therefore be high as the conductive return path isn't in close proximity.

Thus far so good, but this got me thinking, won't the same thing happen as well on a pair of PCB traces carrying a common signal without the need of there being an external cable?

Sure, the ground plane is in very close proximity to the PCB traces and therefore dominates the common signal impedance, but I don't see why the PCB traces couldn't couple with nearby objects (outside the PCB) and induce return currents there too. If this is the case, shouldn't it cause EMI problems?

On a video not too long ago from the same author of the book (https://youtu.be/pXSK9KlNsW4?si=bI-oh9YqQfYjtYeV&t=840) starting from minute 14 he claims that common signals on PCBs don't cause EMI problems so long as there isn't an external cable attached.

Again, I don't completely see why this is the case as any nearby coupling will induce a current and cause the system to radiate. It only takes very little common signal current to fail an FCC EMC test.

My guess to all of this is simply that PCB traces are typically far shorter than external cable, as such, its radiating ability is poor and doesn't need to be considered.

Could anyone give me a better picture on this? Thanks

Answer 1

Thus far so good, but this got me thinking, won't the same thing happen as well on a pair of PCB traces carrying a common signal without the need of there being an external cable?

Yes and this does happen but the lengths are usually short enough and they are already capacitively coupled to the ground planes directly beneath the traces. This can be considered as a kind of shield (especially for inner traces). Remember also that high frequency signals will return to the source by the path of least impedance (resistance + capacitance + inductance)

Also notice the shield around the PCB, this will also help return any stray electric fields back to the source (think also of a faraday cage). The problem is if you have an aperture this reduces the shields effectiveness. A cable makes this problem even worse by giving conductance to the RF outside of the shield.

If this is the case, shouldn't it cause EMI problems?

They can if they are improperly grounded, or have holes or gaps in the conductive material. Another issue is skin depth, so the material needs to be thick and conductive enough to be low impedance. The ground must also be low impedance to the case/shield.

Another way to also control radiation is to create filters if possible to prevent RF from conducting into the cable.