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 provide experimental evidence, full theory and suggestions to mitigate the problem, and a first attempt to assess the uncertainty. Our multiple-attenuators method provides quantitative information about the correlation phenomena inside the instrument.

中文翻译：

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交叉频谱相位噪声测量中的伪影和误差

本文讨论了低噪声振荡器中常见的不稳定和不一致的相位噪声频谱，其本底为 -180 dBc/Hz 或更低的数量级。此类振荡器通常使用双通道仪器进行测量，该仪器基于对两个同时发生且统计上独立的测量进行平均。我们的新方法包括在被测振荡器和相位噪声分析仪之间插入一个耗散衰减器。衰减器的热噪声引入了受控量的相位噪声。我们将相位噪声基底与具有不同衰减值的小步长的理论预期进行比较。分析揭示了由于仪器输入端内部功率分配器的热能导致的负偏置（相位噪声的低估），以及由于两个通道之间的串扰导致的不确定性。在不太罕见的不幸情况下，偏置会导致负相位噪声频谱，这显然是无稽之谈。在三个实验室中分别使用来自两个主要品牌的仪器观察到类似的结果。我们提供实验证据、完整的理论和建议来缓解这个问题，并首次尝试评估不确定性。我们的多衰减器方法提供有关仪器内部相关现象的定量信息。