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