EMC Question of the Week: August 21, 2017

A current probe with a transfer impedance of 10 Ω is connected to a 50-Ω spectrum analyzer. A single narrow-band peak is observed with an amplitude of -57 dBm. What is the rms value of the measured current?

1. 68 μA
2. 57 μA
3. 45 μA
4. 32 μA

Answer

The correct answer is "d". To convert from dBm to the power in milliwatts, we divide by 10 and do an inverse log (i.e. 10^x). The -57 dBm becomes 2x10^-6 millwatts or 2x10^-9 watts. This is a 50-Ω spectrum analyzer, so to get the voltage we multiply the power by 50 and take the square root. This yields a voltage of 320 μV. Dividing the voltage picked up by the probe by the transfer impedance of 10 ohms yields 32 μA.

Note that even though we made a "peak" measurement using the spectrum analyzer (as opposed to quasi-peak or average), spectrum analyzers inherently measure the average power in a continuous signal. Therefore, the value obtained is an rms value of the time-domain current, not a peak value.

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