EMC Question of the Week: March 11, 2024

small powered speaker with cord

A device-under-test consists of a 10-cm by 10-cm circuit board in a plastic enclosure with an attached power cord. A DC-to-DC converter on the board operating at 1 MHz is responsible for a radiated emissions failure at 89 MHz.  The emissions are definitely due to

  1. fields emanating from the converter
  2. common-mode current on the power cord
  3. differential-mode current on the power cord
  4. any of the above


The best answer is “b.” At 89 MHz, the free-space wavelength is more than 3 meters. A quarter-wavelength is more than 80 cm. The only antenna capable of radiating well enough to exceed an emission limit is the power cord (in this case, being driven relative to the 10-cm by 10-cm board). The source of the emissions may have been the converter, but the converter components and circuitry are much too small to be a significant radiated emission source at 89 MHz. Likewise, the close proximity of wires in the power cord prevents the differential mode currents from radiating significantly. 

In order for significant switching noise to appear in a radiated emissions measurement, it must be coupled from the inverter circuitry to something large enough to act as the transmitting antenna. This coupling may be conducted or field-coupled to components or traces that carry the noise away from the immediate vicinity of the converter. 

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