Cross-spectrum Measurement of Thermal-noise Limited Oscillators

TL;DR

This paper investigates the impact of anti-correlated thermal noise from power splitters on cross-spectrum measurements of thermal-noise limited oscillators, providing theory, simulations, experiments, and solutions.

Contribution

It reveals a new anti-correlated thermal noise effect from power splitters and analyzes its impact on oscillator noise measurements, offering potential solutions.

Findings

Anti-correlated thermal noise affects measurement accuracy.

Different power splitters influence the thermal noise measurement.

Ferrite-isolators and amplifiers modify the noise measurement results.

Abstract

Cross-spectrum analysis is a commonly-used technique for the detection of phase and amplitude noise of a signal in the presence of interfering noise. It extracts the desired correlated noise from two time series in the presence of uncorrelated interfering noise. Recently, we demonstrated that the phase-inversion (anti-correlation) effect due to AM 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. An oscillator whose thermal noise is primarily set by the 50 ohm source resistance is referred to as a thermally-limited oscillator. 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.