EMC Question of the Week: July 20, 2020

Three plots showing two narrow-band signals measured with three different resolution bandwidths

A spectrum analyzer with a 9 kHz resolution bandwidth detects a narrow band signal at 1.01 MHz with an amplitude of 40 dB(μV) and another narrowband signal at 1.03 MHz with an amplitude of 35 dB(μV). What peak amplitude will be displayed if the resolution bandwidth is changed to 100 kHz?

  1. 40 dB(μV)
  2. 41 dB(μV)
  3. 53 dB(μV)
  4. 75 dB(μV)


The correct answer is "b". A 100 kHz bandwidth will not resolve the individual peaks. So the value displayed will represent the sum of the power in the two signals. The voltage will be the root mean square sum of the voltage in each signal. 40 dB(μV) is 100 μV and 35 dB(μV) is 56 μV. The root means square sum of those amplitudes is 115 μV, which is 41 dB(μV) .

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