Resolving EMI Common Mode & Normal Mode Noise

Key methods and techniques for resolving and improving common mode and normal mode EMI noise for EMC conformance & conformance test.

EMC / EMI Design Includes:
EMC design techniques     EMC filter design     EMC PCB design     How to resolve normal & common mode EMI / noise    

EMC / EMI topics:     EMC basics     EMI interference basics     EMC standards     CISPR11     CISPR16     CISPR22     FCC 47 part 15     EMC design techniques     EMC compliance test    

One of the major factors with any electronic design is the emission and reception of electromagnetic noise. This can be a major challenge for many electronic development engineers as some of the issues can be a challenge to solve.

One of the key factors is the mode of used for carrying the noise: common mode and normal mode noise.

Each of these modes for the noise transmission requires a difference filter configuration.

EMI noise modes

When looking at EMC and EMI issues, it is necessary to consider the mode or way in which the noise is being carried.

  • Normal mode EMI noise:   Normal mode EMI noise is the first way in which we tend to imagine noise being carried on any cable. The noise currents flow out along one conductor and back along the other. This makes a compete circuit for the noise currents. Typically screening will provide a great amount of immunity against this form of noise as the inner conductor or conductors are screened from pick-up or radiation of the noise.
  • Common mode EMI noise:   Common mode noise can be a little more difficult to imagine. The currents may flow in the same direction on the lines of a cable - they may even flow along the screening as well, making the screening ineffective.

    The currents that flow are caused by a current that has leaked out via the floating electrostatic capacitance of the load retained against the earth and has then returned to the noise source via the earth. The current may be caused by a direct connection between the load and noise source without going through the earth.

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