An analytical method for phase noise evaluation in a microwave VCO stabilized by an interferometry structure

Abstract

This study proposes a set of mathematical formulations to model a microwave interferometric structure to stabilize a free-running oscillator. The microwave interferometric structure employed a carrier suppression system to lock the voltage-controlled oscillator (VCO) frequency to a high-Q resonator and clean up the phase noise spectrum. Analytical relations were derived by modeling the system's stabilization loop components with an equivalent system transfer function. This model optimized the feedback loop circuits for a reliable frequency-locked structure with suitable spectral purity using well-known linear system methods. Thus, the phase noise of an oscillator's output signal locked to a high-Q cavity resonant frequency could be predicted and optimized. Furthermore, a prototype cavity-stabilized oscillator (CSO) system with commercial components wasimplemented to validate the proposed model. The results demonstrated that approximately 40 dB phase noise reduction at 10 kHz offset frequency was obtained at X-band, exhibiting good agreement with the proposed mathematical model. Moreover, the prototype structure's feedback circuits were designed, and their performance was validated using simulation and measurement data.