  Virtually all commercial instruments for the measurement of the oscillator PM
noise make use of the Cross Spectrum method (arXiv:1004.5539 [physics.ins-det],
2010). High sensitivity is achieved by correlation and averaging on two equal
channels which measure the same input, and reject the background of the
instrument. We show that a systematic error is always present if the thermal
energy of the input power splitter is not accounted for. Such error can result
in noise under estimation up to a few dB in the lowest-noise quartz
oscillators, and in a complete nonsense in the case of cryogenic oscillators.
As another alarming fact, the presence of metamaterial components in the
oscillator results in unpredictable behavior and large errors, even in well
controlled experimental conditions. We observed a spread of 40 dB in the phase
noise spectra of an oscillator, just replacing the output filter.
