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

Abstract

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.

A simple scheme of low phase noise microwave synthesizers based on the sub-sampling phase lock loop

“…Correspondingly, the power consumption and the cost of the synthesizer are considerably decreased. 7,8,10,11 The detailed schematic of the sub-sampling PLL is shown in Fig. 2.…”

“…Low phase noise microwave frequency synthesizers are generally used in the experiments of Raman atomic interferometer, 1 testing local position invariance, 2 precision atomic spectroscopy measurement, 3 atomic spin squeezing, 4 and atomic clock. [5][6][7][8][9][10][11] The phase noise of the microwave synthesizer is one of the key factors in limiting the precision of those measurements.…”

“…Thus, it will still remain a well-adapted solution to develop the ultra-low noise quartz crystal oscillator based microwave frequency synthesizer, and there have been several reports on developing microwave synthesizers for atomic clocks. [7][8][9][10][11][12] In most of these schemes, to phase lock a dielectric resonant oscillator (DRO), the 100 MHz OCXO is usually frequency multiplied to 6.8 GHz (for the Rb atomic clocks) or 9.2 GHz (for the Cs atomic clocks) with a non-linear transmission line (NLTL) or a Step Recovery Diode (SRD), and then mixed with the output signal of the DRO to generate an intermediate frequency signal which is further phase locked to a DDS, whose referenced signal is also provided by the OCXO. In that way the output frequency of the DRO is indirectly phase locked to the 100 MHz of OCXO.…”

“…In this process, our previous work shows that the additional phase noise of the NLTL are greatly affected by its operation parameters such as the input power, the input and output impedances. 10 To suppress the additional phase noise of the NLTL, the architecture of the schemes would be very complex, bulky, and expensive for optimizing the operation parameters of the NLTL. To simplify the structure, B. François et al designed a simple microwave synthesizer with a 1.6 GHz custom-designed frequency multiplication module.…”

Demonstration of a Sub-Sampling Phase Lock Loop Based Microwave Source for Reducing Dick Effect in Atomic Clocks*

“…In this process, our previous work shows that the additional phase noise of the NLTL is greatly affected by its operation parameters such as the input power, the input and output impedances. [10] To suppress the additional phase noise of the NLTL, the architecture of the schemes would be very complex, bulky, and expensive for optimizing the operation parameters of the NLTL. To simplify the structure, François et al designed a simple microwave source with a 1.6 GHz custom-designed frequency multiplication module, but the subsequent frequency division and multiplication are still needed.…”