Frequency performances of a miniature optically pumped cesium beam frequency standard

The optically pumped cesium beam clock named Cs IV is operated with a new short Ramsey cavity satisfying strict requirements on the microwave leakage level. The most relevant characteristics of the device are presented. Cs IV is presently driven by standard electronics coming from a HP 5061 B clock that provides a sinusoidal modulation of the interrogation microwave signal and a microwave power stability of about 1% at a temperature of 20/spl plusmn/1/spl deg/C. The short- and medium-term frequency stability measurement gives /spl sigma//sub y/(1 day)=2/spl times/10/sup -14/: this value holds up to 3 days. The accuracy evaluation results in an uncertainty of 10/sup -12/, and the repeatability is evaluated to 3/spl times/10/sup -13/. It appears that the flicker floor is beginning at 2/spl times/10/sup -14/ and is mainly due to both the power fluctuations of the free running microwave interrogating signal and the fluctuations of the external static magnetic field. The accuracy is limited by the lack of knowledge of the end-to-end cavity phase shift.