  Extension of very long baseline interferometry (VLBI) to observing
wavelengths shorter than 1.3mm provides exceptional angular resolution (~20
micro arcsec) and access to new spectral regimes for the study of astrophysical
phenomena. To maintain phase coherence across a global VLBI array at these
wavelengths requires that ultrastable frequency references be used for the
heterodyne receivers at all participating telescopes. Hydrogen masers have
traditionally been used as VLBI references, but atmospheric turbulence
typically limits (sub) millimeter VLBI coherence times to ~1-30 s. Cryogenic
Sapphire Oscillators (CSO) have better stability than Hydrogen masers on these
time scale and are potential alternatives to masers as VLBI references. Here,
We describe the design, implementation and tests of a system to produce a 10
MHz VLBI frequency standard from the microwave (11.2 GHz) output of a CSO. To
improve long-term stability of the new reference, the CSO was locked to the
timing signal from the Global Positioning System satellites and corrected for
the oscillator aging. The long-term performance of the CSO was measured by
comparison against a hydrogen maser in the same laboratory. The superb
short-term performance, along with the improved long-term performance achieved
by conditioning, makes the CSO a suitable reference for VLBI at wavelengths
less than 1.3mm.
