EMC Question of the Week: May 13, 2019
If a 25 MHz sine wave with a peak amplitude of 2 volts is input to a spectrum analyzer, the amplitude of the harmonic at 25 MHz recorded by the analyzer should be
- 126 dB(μV)
- 123 dB(μV)
- 66 dB(μV)
- depends on the RBW of the analyzer
Answer
The correct answer is "b". The peak amplitude of the sine wave is 2 volts, which is 126 dB(μV); but spectrum analyzers record the average power detected within the resolution bandwidth (RBW) of the analyzer. All of the power in the sine wave will fall within one resolution bandwidth regardless of the RBW setting. When that average power is converted to a voltage, it will be the RMS voltage of the input signal. The RMS voltage of a 2-volt sine wave is 1.414 volts or 123 dB(μV).
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