Analysis of the Phase Noise for Frequency Controlled Cryogenic Sapphire Oscillators

Cryogenic sapphire oscillators (CSO) typically require frequency stabilization and noise reduction servos to achieve ultra-high frequency stability and ultra-low phase noise. In order to analyze the performance of a 10.8 GHz frequency-locked CSO and determine the system’s limits, an extended phase noise model based on Leeson’s framework is utilized. This model accounts for noise from the locking system and variations in power and temperature. The results align well with predictions, showing that the phase noise can be reduced from −75 dBc/Hz in a free-running state to −95 dBc/Hz in a locked state at an offset frequency of 1 Hz. The analysis revealed that the fractional frequency stability of the frequency-locked CSO is limited to 4×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−15</sup> at 1-second averaging time, primarily due to power fluctuations within the sapphire-loaded cavity. With the insights gained from this phase noise model, it is possible to further improve the performance of CSOs in the future.