Ultra-low phase noise frequency synthesis chains for high-performance vapor cell atomic clocks

This paper presents the development of a 9.192 GHz microwave frequency synthesis chain (absolute and residual noise characterization) dedicated to be a local oscillator (LO) in a high-performance cesium vapor cell atomic clock based on coherent population trapping (CPT). The key components are an ultra low noise oven controlled quartz crystal oscillator (OCXO), multiplied to 9.2 GHz using a non-linear transmission line. A dielectric resonator oscillator (DRO) is phase-locked onto the microwave signal to generate the 9.192 GHz frequency. The absolute phase noise of the LO has been characterized at the level of -42, -100, -117 dBrad²/Hz and -129 dBrad²/Hz at 1 Hz, 100 Hz, 1 kHz and 10 kHz offset frequencies respectively. With such performances, the expected Dick effect contribution to the atomic clock short term frequency stability is reported to 6.2×10^-14 at 1 s integration time. Main limitations are pointed out.