  This article deals with the erratic and inconsistent phase-noise spectra
often seen in low-noise oscillators, whose floor is of the order of $-180$
dBc/Hz or less. Such oscillators are generally measured with two-channel
instruments based on averaging two simultaneous and statistically independent
measures. Our new method consists of inserting a dissipative attenuator between
the oscillator under test and the phase-noise analyzer. The thermal noise of
the attenuator introduces a controlled amount of phase noise. We compare the
phase noise floor to the theoretical expectation with different values of the
attenuation in small steps. The analysis reveals a negative bias
(underestimation of phase noise) due to the thermal energy of the internal
power splitter at the instrument input, and an uncertainty due to crosstalk
between the two channels. In not-so-rare unfortunate cases, the bias results in
a negative phase-noise spectrum, which is an obvious nonsense. Similar results
are observed separately in three labs with instruments from the two major
brands. We give experimental evidence, full theory, and suggestions to mitigate
the problem. Our multiple-attenuators method provides quantitative information
about the correlation phenomena inside the instrument.
