Abstract
The authors present a sapphire oscillator with a long hold-time cryostat and helium recovery system to overcome the inherent difficulty of using this liquid-helium-cooled frequency standard at a remote site. The oscillator has been used as a frequency reference for very long baseline interferometry (VLBI) radio astronomy observations. Data are presented which show that the performance of the oscillator is superior to a rubidium standard in this application. While laboratory data have demonstrated a fractional frequency stability of about 9*10-15 for integration times between 1 and 300 s the VLBI frequency stability measurements in this case are limited by the signal-to-noise ratio in the correlated data.
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