Cross-spectrum measurement of thermal-noise limited oscillators

• Published in: Review of scientific instruments Vol. 87; no. 3; p. 034708
• Main Authors: Hati, A, Nelson, C W, Howe, D A
• Format: Journal Article
• Language: English
• Published: United States 01.03.2016
• ISSN: 1089-7623
• DOI: 10.1063/1.4944808

Cross-spectrum analysis is a commonly used technique for the detection of phase and amplitude noise of a signal in the presence of interfering uncorrelated noise. Recently, we demonstrated that the phase-inversion (anti-correlation) effect due to amplitude noise leakage can cause complete or partial collapse of the cross-spectral function. In this paper, we discuss the newly discovered effect of anti-correlated thermal noise that originates from the common-mode power divider (splitter), an essential component in a cross-spectrum noise measurement system. We studied this effect for different power splitters and discuss its influence on the measurement of thermal-noise limited oscillators. We provide theory, simulation and experimental results. In addition, we expand this study to reveal how the presence of ferrite-isolators and amplifiers at the output ports of the power splitters can affect the oscillator noise measurements. Finally, we discuss a possible solution to overcome this problem.