Uncertainty of Out-of-Band Distortion Measurement With a Spectrum Analyzer

In broadcasting and wireless communications, modulated signals are deformed by nonlinear radio frequency components, of which power amplifiers are the most significant example. The resulting distortion can leak outside the main signal band, and this leak, called out-of-band (OOB) distortion, can be directly measured with a spectrum analyzer (SA). Little attention has been paid to this measurement. In particular, it is often assumed that there is no measurement uncertainty, regardless of the condition of the instrument. However, in this paper, we show that the measurement of OOB distortion depends strongly on the various setting parameters of the SA because of its intricate internal signal processing for detection and visualization. Our analysis reveals that the underlying reason for this dependence is the difference between envelope amplitude statistics detected inside and outside of the main signal band.