Ultralow residual phase noise frequency synthesizer for space gravitational wave detection

A laser interferometric gravitational wave observatory in space requires spaceborne clocks with ultralow phase noise in the millihertz frequency band. Such noise can be suppressed using a sideband multiplication transfer scheme and pilot tone techniques. To meet the requirements of the clock noise suppression technique, ultralow residual phase noise synthesizers are required to generate the microwave (2.4 GHz) for electro-optic modulator modulation and the pilot tone signal (75 MHz). To this end, two different structures of microwave chains have been designed, implemented and compared. The application of low phase noise phase-locked dielectric resonator oscillators (PDROs) and frequency division techniques enabled the development of a frequency synthesis chain with ultralow residual phase noise. The residual phase noise of the 75 MHz pilot signal is measured to be $ 1.06\times {10}^{-3}\;\mathrm{r}\mathrm{a}\mathrm{d}/\sqrt{\mathrm{H}\mathrm{z}} $, $ 8.18\times {10}^{-5}\;\mathrm{r}\mathrm{a}\mathrm{d}/\sqrt{\mathrm{H}\mathrm{z}} $, $ 7.63\times {10}^{-6}\;\mathrm{r}\mathrm{a}\mathrm{d}/\sqrt{\mathrm{H}\mathrm{z}} $, $ 1.30\times {10}^{-6}\;\mathrm{r}\mathrm{a}\mathrm{d}/\sqrt{\mathrm{H}\mathrm{z}} $ and $ 1.53\times {10}^{-7}\;\mathrm{r}\mathrm{a}\mathrm{d}/\sqrt{\mathrm{H}\mathrm{z}} $ at Fourier frequencies of 0.1 mHz, 1 mHz, 10 mHz, 100 mHz, and 1 Hz, respectively. The pilot signal is generated by frequency division of a 2.4 GHz microwave signal, and the residual phase noise of the latter is lower. As a result, the residual phase noise levels of both signals meet the requirements of the "Taiji Program" in the range of 20 mHz to 1 Hz. By further reducing the residual phase noise of power dividers, frequency dividers and other devices, reducing the temperature sensitivity of key devices, and adding temperature control and pilot tone correction technologies, the noise floor of the frequency synthesizer can be further reduced to meet the requirements of the Taiji Project in the entire frequency range (0.1 mHz−1 Hz). The development of this frequency synthesizer lays a solid foundation for the time-frequency system required for China's space gravitational wave detection.