EMC Question of the Week: August 16, 2021

Testing an EUT on a 30-meter open-field site, an EMI test receiver measures a 29 dB(μV/m) peak at 100 MHz. With the DUT powered off, the measured signal amplitude is 26 dB(μV/m). What is the likely amplitude of the peak at 100 MHz due to the EUT? 

1. 26 dB(μV/m)
2. 18 dB(μV/m)
3. 3 dB(μV/m)
4. 3 dB

Answer

The best answer is “a.” Since the sources are uncorrelated, the power in the two signals adds. The difference in the signal powers is equal to the root-mean-square difference in the field strengths. 29 dB(μV/m) is 28 μV/m. 26 dB(μV/m) is 20 μV/m. The root-mean-square difference is $$\sqrt{{\left(28\right)}^2-{\left(20\right)}^2}\approx 20\text{μV/m}\text{=}\text{26}\text{dB(μV/m).}$$

Note that we could have saved ourselves some calculation by noting that the combined signal is 3 dB higher than the ambient signal. A 3-dB increase in the signal represents a doubling of the power in the signal, so the amplitude of the second source must be the same as the first.

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