A low phase and amplitude noise microwave source for vapor cell atomic clocks

A low-noise microwave source plays a key role in high-performance passive atomic clocks. Here, we propose and implement a microwave frequency synthesizer featuring a low phase and amplitude noise. With the help of a high-Q factor surface acoustic wave band-pass filter, we generate a microwave with targeted frequency by frequency multiplication of a low noise local oscillator at a radio frequency with the closest integer. At the frequency offset of 1 Hz, 10 Hz, 100 Hz, 1 kHz, and 10 kHz, the absolute phase noise of the output 3.417 GHz signal is -53.0, -83.3, -107.7, -119.2, and -124.0 dBc/Hz, respectively. After the microwave-to-optical conversion, the expected intermodulation effect contribution to the frequency stability of the coherent population trapping (CPT) atomic clock is 5.95 × 10^-14 at an averaging time of 1 s. Meanwhile, with a feature of low fluctuation of this chain's output microwave power at the level of 1.19 × 10^-5 W at 1 s, its contribution to the frequency stability of the CPT atomic clock is 7.85 × 10^-14 at the 1 s integration time. Our simple and low noise microwave chain is an ideal microwave source for high-performance, compact CPT clocks and could also be applied to cold atom or ion based microwave clocks.