C/N measurement of Spectrum Analyzer

Chapter C/N measurement

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

  • 36 | Guide to Spectrum and Signal AnalysisIntermodulation distortion is even generated in the spectrum analyzer itself and this distortion component is determined by the mixer input level. Consequently, when measuring intermodulation distortion using a spectrum analyzer, it is necessary to take care about the mixer input level. It is possible to determine whether or not the DUT or the spectrum analyzer is generating the distortion by observing whether or not the distortion component changes when the spectrum analyzer input attenuation value is varied.When the spectrum analyzer is generating the distortion, the distortion component changes by 15 dB when the input attenuation is varied by 5 dB. Consequently, in this case, it is necessary to increase the value of the input attenuator to the point where the distortion does not change. In addition, when two signals are input to the DUT, the two signal sources cause mutual interference and hence intermodulation distortion occurs. To distinguish this, confirm whether or not the distortion changes by a factor of 3 relative to the attenuation value when the attenuator in front of the DUT is varied. When the distortion component does not change by a factor of 3, insert an isolator between the signal combiner and the signal sources.C/N measurementThe output signal from equipment such as a signal generator is not a pure sine wave, and as well as harmonic components, it includes noise of amplitude components and frequency components. These are generally called AM noise and FM (phase) noise. Generally, the AM noise is lesser in magnitude in comparison to the FM noise so measurement of FM noise is explained here.The FM noise exists just above and below the carrier wave as shown in Figure 42 and is expressed as the ratio of the single sideband phase noise power to the carrier wave power within a 1 Hz bandwidth for a specified frequency offset from the carrier. When a spectrum analyzer is used, the carrier wave power and the sideband noise can be viewed directly on screen. However, the following points must be noted when using a spectrum analyzer.Figure 42