PM-AM correlation measurements and analysis

We measure and analyze effects of nonlinear mixing of phase-noise modulation (PM) and amplitude-noise modulation (AM) in an oscillating signal by real-time correlations measured in the cross power spectral density (CPSD). We outline sensitive measurements of PM-AM correlation coefficients by means of a time-averaged CPSD measurement technique. Separate but simultaneous PM and AM measurements using a two-channel cross-correlation spectrum analyzer, quantifies the relevant effects of intermodulation mixing with excellent sensitivity compared to traditional 3IM measurements and the scatter-plot correlation technique. Time-averaged cross-spectrum measurements provide good estimates of correlation coefficients as a function of Fourier frequency (f). We use normalized PM-AM CPSD measurements of a 645 MHz quartz-MEMs oscillator as an example and find that 1/f PM-AM CPSD is exactly correlated, even for very widely differing levels of PM and AM noise, in which individual PSDs of PM and AM differ by >;40 dB (or, greater than a factor of 10,000). We also verify that white-PM noise has uncorrelated PM-AM CPSD.