Development of low phase noise microwave frequency synthesizers for reducing Dick effect of Cs fountain clocks

The Dick effect is one of the main factors limiting the short-term frequency stability of Cs fountain clocks, which is especially decided by the phase noise of the local oscillator at smaller offset frequency. Here we report on the development of a 9.192 GHz microwave frequency synthesizer with low phase noise to be used as the local oscillator for Cs fountain clocks. The synthesizer is based on frequency multiplication and synthesis from an ultra-low phase noise 5 MHz Oven Controlled Crystal Oscillator (OCXO). The key component of the frequency multiplication is a non-linear transmission-line (NLTL) as a frequency comb generator. Based on the principle of the NLTL, we carefully optimized the input power, the input and output impedances of the NLTL to suppress its excess phase noise. The measured results show that the absolute phase noises of 9.192 GHz signal are -64 dBc/Hz, -83 dBc/Hz, -92 dBc/Hz and -117 dBc/Hz at 1 Hz, 10 Hz, 100 Hz and 1 kHz offset frequencies, respectively. The residual phase noise of the synthesizer is − 82 dBc/Hz at 1 Hz offset frequency. The frequency stability limit due to the absolute phase noise via Dick effect is theoretically estimated to be 7.0 × 10−14τ−1/2.