Magic shield plates helps naked A20-OLinuXino-LIME2 board to pass CE EMC requirements.


A20-OLinuXino-LIME and A20-OLinuXino-LIME2 are choice for many industrial applications, when the board is inside metal box like BOX-LIME:


it has no problems with shielding the radio emissions, but when just board is used as component, our customers has to shield it by their own in their designs. We had number of request to do official CE EMC certifications for A20-OLinuXino-LIME2 as this would make their life easier, and we decided to do this after we already get some experience with the certification of USB-ISO and ESP32-POE-ISO.

Initially the naked board failed on EMC measurements as seen here from the test results.


The major problem is the Gigabit Ethernet PHY. You can see spikes at 125 Mhz, 325Mhz etc. Disabling the PHY removes all noise and everything is in safe levels. So we knew who is ther trouble maker.

The major noise emitter are the Transmit and Receive clock lines. We put 33 ohm resistors in series R119 and R123 in A20-OLinuXino-LIME2 schematic , but obviously this was not enough. To further fade the line harmonics we replaced the resistors with 0402 Ferite Beads with 600 ohm impedance at 100 Mhz. This lowered the emissions, but still not well enough to be at safe levels.

So we had to make small Faraday cage on top:


and bottom of the board:


this effectively removed all noises and the A20-OLinuXino-LIME2 pass CE requirements:





We are preparing now to implement the Ferite Beads on PHY Tx and Rx lines in all our other boards production assembly, and will offer the shield plates as kit they are good for A20-OLinuXino-LIME and A20-OLinuXino-LIME2 and we will prepare same for A20-OLinuXino-MICRO and A64-OLinuXino.


13 Comments (+add yours?)

  1. diego
    Aug 29, 2019 @ 20:10:40

    Another proof that annoying emc tests are not only to annoy us, but also to improve the design. I always liked the days with the emc specialists.


    • Zupo Llask
      Aug 29, 2019 @ 22:28:46


      What instrument did you choose to measure / test EMF?



    • OLIMEX Ltd
      Aug 30, 2019 @ 07:49:50

      Agree, we learned a lot around these tests. This will help us to build better boards in future.


      • diego
        Aug 31, 2019 @ 13:19:39

        It’s very hard to get the mindset of the emc experts anit used to be a challange following their thoughts but it was in general a big fun and a great opportunity to learn. I had the great opportunity working with an expert on troubleshooting emc issues on an Airbus A319, where cabin systems triggered the emergency frequencies on the VHF radio. It turned out a video oscillator spilt into CAN wiring. Years ago, bit still a good basis for clever emc design.

        I’m happy to see you guys profited as much too.

  2. hjoe
    Sep 01, 2019 @ 14:34:12

    No comment, one question: can the CANbus signals CAN_TX AND CAN_RX be used on any of the connectors. Of course, a transseicer outside has to be connected to use the CANbus


  3. LIME-fan
    Sep 04, 2019 @ 13:39:34

    Great work!


  4. Martine Jackson
    Sep 05, 2019 @ 11:30:24

    Hello, what about your dev board Olimex RT5350F Dev board – is it possible to add a shield here due to the internal antenna? How would you recommend we get this to pass the EMC tests? Thank you.


  5. Lutz
    Sep 26, 2019 @ 01:41:59

    Have u checked max. network cable length after adding 600ohm fbead?


  6. Lutz
    Sep 26, 2019 @ 01:44:26

    and by the way, its more common to just add 15pf on rx/tx lines between ethernet ic and magnetics, you schould try that.


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