EMC Question of the Week: September 9, 2024

During an immunity test, the field strength measured inside a test chamber is 160 dB(μV/m). After the test equipment is powered down, the measured field strength due to the EUT is 40 dB(μV/m). So, powering down the test equipment reduced the measured field strength by,   

1. 160 dB(μV/m)
2. 120 dB(μV/m)
3. -120 dB(μV/m)
4. 40 dB(μV/m)

Answer

The best answer is “a.” During the test, the field strength is 160 dB(μV/m), which is 100 V/m. After powering down, the field strength is 40 dB(μV/m), which is 0.00010 V/m. That is about a 100 V/m difference [i.e., 160 dB(μV/m)].

$$The ratio of the two field strengths is 160 dB(μV/m) - 40 dB(μV/m) = 120 dB. So, turning off the test equipment reduces the measured field strength by 120 dB. However, 120 dB is not the same as 120 dB(μV/m). 120 dB expresses a ratio of two signal amplitudes. 120 dB(μV/m) is a field strength. In fact, 120 dB(μV/m) is 1 V/m and, by any interpretation, is not the correct answer to this question.

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