Photon PCB design question EMC, keep out and ground loops

I’m designing a circular light with a Particle Photon in the middle. I need to make sure it will fall in within the EMC regulations.

I read already interesting about EMC and Ground loops in this forum which already contain good tips from @hfiennes and others:

From this topics I understand that ground loops are a problem, and that you have to keep the Photon free at one side. However I see several board designs (like BrewPi) that have the Photon in the middle.

The problem is that have a giant ground plane all around the Photon (the design is 40cm/16inch in diameter) with one small keep out area in the middle.

  • Does this count as a ground loop or is this only the case with small traces?
  • Is the keep out area (a small square field in the middle) good enough to receive WiFi, or is it better to work with an external antenna?
  • If I use an external antenna do I need to respect the keep auto area? Or is it only important for the onboard SMD antenna?
  • Should I play more with the keep out size. In this topic they needed to make it 2mm wider, however they also seemed to have removed a ground loop.

I need the best design for EMC approval, so if that means that it’s better to use an wire antenna on the uFL connector I prefer that.

I hope you have some tips.

from my experience:
I’d use only the external antenna, don’t cut out any area for the antenna. Do you use the SMT version of the Photon (or even P1)?

We had some issues during EMC testing with the electron. Some of them caused by the circuit (1-Wire), others caused by the onboard charge regulator.
As the Electron is THT only (and the socket I use is quite heigh) it seems to be impossible to comply without external components on the “VIN”-line of the Electron.

That said, follow the basic EMC rules, prefer the SMT version over the THT, use a SMT-through-hole socket if you need to stick on the SMT-module of the photon,…

Let us know if you made already some progress!

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Thanks for sharing your experience! I’ve designed the board for the two versions (Photon with headers and without). The PCB is manufactured (with keep out zone), however I didn’t test it yet. I’ll update this post when I’ll do a test.

What kind of components did you add on the Vin line?


my emc consultant tried several components, the best was a UKW EMI suppression ferrit from Würth:
Order No: 74275043,

As the headers are quite long I looked for a different solution. I found those SMT headers:
MPE garry Female Header SMD 2,54 mm Series 098

The first PCB batch had a lot of issues, together with my emc consultant we identified a lot of improvements such as pulling unused pins to ground, huge vias from bottom to top layer, about 2.000.000 capacitors, ferrites and some esoteric components :joy:

BTW: I should point out that I didn’t test the photon yet, but the board and the application is prepared for it,…

Update 1:
I forgot to mention 1 big thing: We replaced the external LiPo by an 18650 cell (and battery holder). The long wires to the LiPo also emitted signals caused by poor GND and 1-wire communication. We didn’t try LiPo+charging yet,…

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Today we finished the EMC tests. The electron kept alive all required tests even with some caveats in the design. We successfully stopped the uC by applying an electric field of ~24 V/m (!).

Finally we have to do some ESD-tests, but due some design issues I will do the redesign first.