Adapting a Cryogenic Sapphire Oscillator for Very Long Baseline Interferometry

Extension of very long baseline interferometry (VLBI) to observingwavelengths shorter than 1.3 mm provides exceptional angular resolution(~20 μas) and access to new spectral regimes for the study ofastrophysical phenomena. To maintain phase coherence across a globalVLBI array at these wavelengths requires that ultrastable frequencyreferences be used for the heterodyne receivers at all participatingtelescopes. Hydrogen masers have traditionally been used as VLBIreferences, but atmospheric turbulence typically limits (sub)millimeterVLBI coherence times to ~1-30 s. Cryogenic sapphire oscillators (CSOs)have better stability than hydrogen masers on these timescales and arepotential alternatives to masers as VLBI references. Here, we describethe design, implementation, and tests of a system to produce a 10 MHzVLBI frequency standard from the microwave (11.2 GHz) output of a CSO.To improve long-term stability of the new reference, the CSO was lockedto the timing signal from the Global Positioning System satellites andcorrected for the oscillator aging. The long-term performance of the CSOwas measured by comparison against a hydrogen maser in the samelaboratory. The superb short-term performance, along with the improvedlong-term performance achieved by conditioning, makes the CSO a suitablereference for VLBI at wavelengths less than 1.3 mm.