标题： 用于甚长基线干涉测量的超稳定且噪声极低的信号源，采用低温冷却蓝宝石振荡器 显示英文标题

标题： Ultra stable and very low noise signal source using a cryocooled sapphire oscillator for VLBI

摘要： 我们介绍了基于低相位噪声数字分频器和直接数字合成器的新颖频率合成器的设计与实现。 该合成器生成两个低噪声、精确且可调的信号，频率分别为10 MHz和100 MHz。 我们报告了合成器的残余相位噪声和频率稳定性测量结果，并估计了从超稳定低温蓝宝石振荡器产生的接近11.2 GHz的信号作为种子时，合成器可预期的总频率稳定性。 在10 MHz和100 MHz信号上，合成器的残余单边带相位噪声在1 Hz偏移处分别测得为-135 dBc/Hz和-130 dBc/Hz。 在10 MHz和100 MHz信号上，它们的固有频率稳定性贡献分别测得为sigma_y = 9 x 10^-15和sigma_y = 2.2 x 10^-15，在1秒积分时间下。 从低温蓝宝石振荡器得到的10 MHz和100 MHz信号的总分数频率噪声的阿伦偏差，可以估计为sigma_y = 5.2 x 10^-15\tau ^-1 + 3.6 x 10^-15\tau ^-1/2 + 4 x 10^-16 和sigma_y = 2 x 10^-15\tau ^-1/2 + 3 x 10^-16，对于1 s <\tau < 10^4 s。 我们还计算了相干函数（甚长基线干涉测量中的一个性能指标），当使用低温蓝宝石振荡器和氢原子钟时，观测频率为100 GHz、230 GHz和345 GHz的情况。 结果表明，在100 GHz以上的频率下，低温蓝宝石振荡器具有显著优势。 显示英文摘要

摘要： Here we present the design and implementation of a novel frequency synthesizer based on low phase noise digital dividers and a direct digital synthesizer. The synthesis produces two low noise accurate and tunable signals at 10 MHz and 100 MHz. We report on the measured residual phase noise and frequency stability of the synthesizer, and estimate the total frequency stability, which can be expected from the synthesizer seeded with a signal near 11.2 GHz from an ultra-stable cryocooled sapphire oscillator. The synthesizer residual single sideband phase noise, at 1 Hz offset, on 10 MHz and 100 MHz signals, respectively, were measured to be -135 dBc/Hz and -130 dBc/Hz. Their intrinsic frequency stability contributions, on the 10 MHz and 100 MHz signals, respectively, were measured as sigma_y = 9 x 10^-15 and sigma_y = 2.2 x 10^-15, at 1 s integration time. The Allan Deviation of the total fractional frequency noise on the 10 MHz and 100 MHz signals derived from the synthesizer with the cryocooled sapphire oscillator, may be estimated as sigma_y = 5.2 x 10^-15 \tau ^-1 + 3.6 x 10^-15 \tau ^-1/2 + 4 x 10^-16 and sigma_y = 2 x 10^-15 \tau ^-1/2 + 3 x 10^-16, respectively, for 1 s < \tau < 10^4 s. We also calculate the coherence function, (a figure of merit in VLBI) for observation frequencies of 100 GHz, 230 GHz and 345 GHz, when using the cryocooled sapphire oscillator and an hydrogen maser. The results show that the cryocooled sapphire oscillator offers a significant advantage at frequencies above 100 GHz.