EMC Question of the Week: March 12, 2018
If two narrow band signals (one at 30.01 MHz and one at 30.08 MHz) both have an rms amplitude of 1.0 mV, what peak amplitude will be displayed on a 50-Ω spectrum analyzer with a 120 kHz resolution bandwidth?
- 1.0 mV or -47 dBm
- 1.4 mV or -44 dBm
- 2.0 mV or -41 dBm
- 2.8 mV or -38 dBm
Answer
The correct answer is "b." The two narrowband signals are only separated by 70 kHz, therefore they will both be captured simultaneously by an analyzer with a 120 kHz resolution bandwidth. The voltage displayed will correspond to the sum of the signal powers within the bandwidth, which will be V = SQRT[(1 mV)2+(1 mV)2] = 1.4 mV.
By default, most spectrum analyzers will display the voltage in dB(μV) or dBm. 1.4 mV is 63 dB(μV) or on a 50-Ω spectrum analyzer, -44 dBm.
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