Artifacts and Errors in Cross-Spectrum Phase Noise Measurements
This seminar is intended to give a warning about: (i) the internal processing inside the phase-noise analyzers, (ii) the oscillators whose white phase noise floor seems too low, chiefly the 100-MHz OCXOs, and (iii) the need to introduce in the domain of phase-noise measurements the basic concepts of uncertainty found in the International Vocabulary of Metrology (VIM).
The measurement of low-noise quartz oscillators, or of other low noise oscillators exhibiting a noise floor of the order of -180 dBc/Hz or less, relies on the cross-spectrum method. The measurement may take long time, from hours to 1-2 days, for the number of averaged cross spectra to be sufficient to reject the background of the noise analyzer.
That said, something anomalous is often seen in a region approximately one decade wide, located where higher-slope noise joins the white floor or the 1/f noise. The plot may be quite thick and irregular, and a dip may appear, where the phase noise looks lower than the white noise floor. Such anomalies reveal something worse. The white region of the PM noise spectrum may be affected by gross errors and, in the not-so-rare worst case, is a total nonsense. We address the problem through a simple experiment, where we insert a dissipative attenuator between the oscillator and the PN analyzer. Surprisingly, in some cases the attenuation results in a lower noise floor. Such erratic behavior is reproducible, having been observed separately in three labs with instruments from the two major brands.
We provide the experimental evidence, the full theory, and suggestions to mitigate the problem.
Enrico Rubiola is a professor at the Université de Franche Comté and a researcher with the Department of Time and Frequency of the CNRS FEMTO-ST Institute, Besançon, France, and an associated researcher at INRiM, the Italian institute of primary metrology in Torino. Formerly, he was a professor at the Université Henri Poincaré, Nancy, France, and an assistant professor at the Politecnico di Torino. He was also a guest professor at the Università degli Studi di Parma, Italy, and a guest scientist at the NASA/CALTECH Jet Propulsion Laboratory