Modulation Distortion of Spectrum Analyzer

Chapter Modulation Distortion

Guide to Spectrum and Signal Analysis
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Guide to Spectrum and Signal Analysis

  • 24 | Guide to Spectrum and Signal AnalysisModulation DistortionDistortion of an amplitude modulated carrier wave is commonly due to either or both of the following:a) second and subsequent harmonics of the modulation signal and,b) over modulation of the carrier wave. i.e. %M>100%.Measuring modulation distortion can be performed directly from the frequency domain display of a spec trum analyzer. Consider Figure 28.The upper and lower sidebands adjacent to the carrier are the modulation components but the second and subsequent pairs of sidebands are due to the harmonics of the modulation signal. Using a logarithmic scale, the level difference between the first and second sidebands gives the 2nd harmonic distortion for the waveform. In the case of Figure 28 this is 6 dB. This same procedure can be used for 3rd harmonic distortion also.Figure 28