App note: EMC design guidelines


A great guide (PDF) to electromagnetic compatibility from Murata

International regulations regarding electromagnetic compatibility (EMC) affect many aspects of circuit and system design. However, there are many techniques that can be applied generally to reduce both the emissions from and susceptibility to, electromagnetic interference (EMI).

As a manufacturer of electronic components, Murata Power Solutions is committed to minimizing emissions from its own components and to helping its customers achieve EMC compliance by correct component choice and design. To this end Murata Power Solutions has compiled the following list of general design recommendations.

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    1. Why? Do you have specifics to share? I am very interested in your view.
      Elsewhere I’ve only seen it recommended for crystal circuits, that seems sane enough to me.

  1. As depicted those guard rings are very nice antennas. If there were more vias or if they were much closer to the point they were trying to protect they would be much better. As shown they are inductors not connections to ground for anything other than DC or audio frequencies.

    When in any doubt a solid ground plane (or power plane) above or below an active trace is always better with the exception of op-amps with frequencies in the >100 MHz range.

    1. Ahem, aren’t you over-reacting? The phrase “for anything other than DC or audio frequencies” is a bit too much. :-p Yet most hobbyist through-hole boards work well up to maybe 10MHz, which seems to be more in line with the intent of Murata’s hints, given the drawings of many large through holes. It’s focused more on issues related to Murata’s products which is power-related, and that won’t go above a few MHz, so it’s not a bible on on high-speed signal routing.

      Of course we would do other things for >100MHz (yes, I agree big figures look more impressive). I would read the Xilinx guidelines instead for that.

  2. I will agree that if you are looking at this from the point of view of a hobby it doesn’t matter much what the heck you do it should work well enough in a power application. It will generate more RFI than it should and you do realize that vias are free. ( unless you are making thousands of boards but even then…) Also a four layer board costs only about 60% more than a 2 layer board and much less likely to cause problems.

    Proto boards _work_ at even 40 to 50 MHz just fine for many projects. Poor pcb designs will work but RFI/EMI and even static protection goes to heck in a hurry and if you ever plan to sell your product these are things you have to look for.

    I could be over reacting but I spent many years designing pcbs in air traffic control radar used around the world and there was no room for it might be ok…

  3. Guard rings can also be used in sensitive circuits with high impedance opamps to prevent leakage current along the board surface. Optical sensors with LEDs to measure absorbed light in a liquid sample over a wide temperature range is one example.

  4. KH – Were you referring to the Guard Ring showing in the middle (the loop around the 4 pads) or the guard ring around the board or both?

    They do state “A guard ring around trace layers reduces emission out of the board, only connect to ground at single point and make no other use of the guard ring (i.e. do not use to
    carry ground return from a circuit).” which seems to go against the grain of what KH is saying.

    I found this interesting –
    “Avoid overlapping power planes, keep separate over common ground (reduces system noise and power coupling, see
    figure 10).”

    I would have thought the exact opposite would have been true.

    RF can have many amazing and ‘unexpected’ twists, even for somebody formally trained in the subject. Often curing things one way can upset something else in another way.

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