EMC Question of the Week: May 6, 2024

heatsink on a circuit board

Suppose the radiated emissions from a product are proportional to the common-mode current on its power cable. Suppose the common-mode current on the cable is proportional to the time-rate-of-change of the voltage (dV/dt) on a power converter heatsink. When the power converter switches at 1 MHz, the radiated emissions at 33 MHz are 40 dB(μV/m). What would we expect the radiated emissions at 33 MHz to be if the same power converter switches at 3 MHz? 

  1. much lower
  2. about the same
  3. 3 dB higher
  4. 10 dB higher


The best answer is “d.” The voltage and the transition time are the same in both cases, so the peak current coupled to the cable in the time domain is the same. However, the coupling occurs at a rate that is three times higher. In the frequency domain, the amplitude of each harmonic increases by a factor of 3 (or approximately 10 dB). 

Note that if the same question had been asked about the harmonic at 31 MHz, the answer would have been "much lower." 31 MHz is a harmonic of 1 MHz, but not a harmonic of 3 MHz. 

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