EMC Question of the Week: March 4, 2024

Current probe around a wire

A current probe with a 10-Ω transfer impedance is connected to a 50-Ω EMI test receiver in peak detection mode. A narrow band peak is observed with an amplitude of 54 dB(μV). What is the rms value of the measured current?

  1. 540 μA
  2. 50 μA
  3. 35 μA
  4. 2 μA

Answer

The best answer is “b.” 54 dB(μV) is 10(54/20) = 500 μV. Dividing by the 10-Ω transfer impedance yields 50 μA. 

Note that this is already an rms value. Spectrum analyzers and EMI test receivers always display the rms amplitude (even in peak detection mode). The word "peak" refers to the highest rms value detected. It is not the peak value in the time domain.

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