Last Week's Question

If a 100 MHz sine wave with a peak amplitude of 1 volt is input to a spectrum analyzer, the amplitude of the harmonic at 100 MHz recorded by the analyzer should be

  1. 120 dB(μV)
  2. 117 dB(μV)
  3. 60 dB(μV)
  4. depends on the RBW of the analyzer

Answer

The correct answer is "b". The peak amplitude of the sine wave is 1 volt, which is 120 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 1-volt sine wave will be 0.707 volts or 117 dB(μV). Spectrum Analyzer

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