Sensitivity of the Cross-Spectrum Method for Measurement of Oscillator Phase Noise

Abstract:
Low oscillator phase noise is crucial for good performance of nearly all communication systems. Probably the best way to measure the phase noise of an oscillator is the crossspectrum method. Basically, the phase noise of a device-under-test oscillator is measured using two identical measurement circuits synchronized by a phase-locked loop. Beside the theoretical background of the cross-spectrum approach and the discussion of the most common estimators, we present our practical realization of a cross-spectrum measurement setup for the determination of the power spectral density of an oscillator’s phase noise at a carrier frequency of 16.384 MHz. A main focus during the design of the system is to achieve maximum measurement sensitivity. Here, the impact of the ADC noise floor on the system’s sensitivity is identified to be the crucial aspect. Furthermore, we present an approach to measure the phase noise at frequencies very close to the carrier frequency where the result is severely disrupted by the error transfer function of the PLL in the measurement circuit.