Artifacts and Errors in Cross-Spectrum Phase Noise Measurements : Invited lecture

Inserting an attenuator between the oscillator under test and the phase noise analyzer, one expects that the white phase noise increases monotonically with the attenuation. By contrast, we observe that with some oscillators the white noise has sharp minimum for a given value of the attenuation, which clearly indicates problem. With other oscillators, it increases monotonically with the attenuation, but the values are not consistent with the thermal energy introduced by the attenuator. In both cases artifacts are present, which takes the form of a sharp notch in the spectrum, occurring where the white FM noise crosses the white PM noise. Such anomalous behavior is the tip of the iceberg, and reveals a common misconception in the cross spectrum instruments and in their use. We provide analytical theory, experimental results, and tips to mitigate the problem.The lecture and this extended abstract are based on the results published in Y. Gruson et al. Metrologia 57(5) p. 055010 1-12, DOI 10.1088/1681-7575/ab8d7b [1]. The latter is the reference article for the reader interested in deeper understanding.