This paper develops a low-phase noise microwave frequency synthesizer for a cold atom gravimeter. The phase-locked loop technique improves the near-end phase noise at the output frequency of the 100 MHz crystal oscillator. The 6.834 GHz microwave signal required for the ground states transition of the 87Rb atom is obtained by building an ultralow-phase-noise multiplier. The measured absolute phase noise performance of the 6.834 GHz signal is -60 dBc/Hz and -120 dBc/Hz at offset frequencies of 1 Hz and 10 kHz, respectively. The frequency resolution is as small as 1.42×10 -6 Hz. Meanwhile, we investigated the impact of the microwave source’s phase noise on the cold atomic gravimeter’s measurement resolution. The microwave frequency synthesizer is compact and easily expandable to other quantum precision measurement fields such as atomic clocks and atomic interferometers.
Objective Microwaves are commonly used in quantum precision measurements, and their frequency, power and phase must be precisely controlled. Atomic clocks, for example, are typically closed-loop locked using frequency hopping or phase tuning. Other experiments, such as measuring electric fields with Rydberg atoms, necessitate driving and adjusting the microwave antenna’s power. For example, in the cold atomic gravimeter, the doppler shift caused by gravity during the free fall of atoms needs to be compensated by scanning the frequency of the Raman laser and the resolution of the frequency tuning will affect the measurement precision. On the other hand, the performance of the microwave source could affect the measurement accuracy of the experiment and the stability of the atomic clock due to the Dick effect. Because of its ultra-low-phase noise performance, a photo-generated microwave has been widely used in quantum precision measurement. However, due to its complex structure, it is difficult to integrate. This paper develops a miniaturized vehicle-mounted frequency synthesizer with low-phase noise and ultra-high frequency resolution.
Methods Based on the phase-locked loop (PLL) technique, the phase noise of the 100 MHz ultra-low-phase noise crystal oscillator is improved. [Fig. 1(a)]. Next, the 100 MHz signal enters the comb spectrum generator to obtain the 6.7 GHz frequency after filtering. Finally, the 6.7 GHz signal is mixed with the direct digital synthesizer (DDS) to obtain the required 6.834 GHz for the experiment [Fig. 1(b)]. Besides, we developed a control system to tune the DDS (Fig. 2), microwave switch and attenuator to manipulate the output frequency and power of the frequency synthesizer.
Results and Discussions We adjusted the bandwidth of the phase lock loops(PLL) to 31 Hz. After PLL, the absolute phase noise of the 100 MHz signals has been improved to -96 dBc/Hz at the offset frequency of 1 Hz (Fig. 3). Mixing the output frequencies of channel 1 and channel 2 [Fig. 4(a)] to test the frequency sweep. The test results show that the signal phase remains continuous during the sweeping process [Fig. 4(b)]. The absolute phase noise performances of the 6.834 GHz are shown in Fig. 5. At offset frequencies of 1 Hz and 10 kHz, the phase noise is -60 dBc/Hz and -120 dBc/Hz, respectively. The measured results show that microwave frequency synthesis can meet the atom gravimeter’s experimental requirements.
Conclusions This paper builds a miniaturized, transportable microwave source with low-phase noise and ultra-high frequency resolution for a cold atomic gravimeter. The 100 MHz crystal is locked to a 10 MHz crystal by controlling the PLL bandwidth to improve its near-end phase noise from -73 dBc/Hz to -96 dBc/Hz at the offset frequency of 1 Hz. The reference frequency of AD9852 is overclocked to 400 MHz, to extend its frequency output range up to 160 MHz. The achieved frequency resolution is as small as 1.42×10 -6 Hz. The developed control system can programme the frequency and power of the microwave source to meet the needs of most quantum precision measurement experiments. The evaluation results show that the microwave frequency synthesizer can satisfy the cold atomic gravimeter’s requirements of a μGal level measurement precision. Furthermore, the PLL and frequency multiplier scheme can be easily extended to other target frequencies without loss of generality.
