Adapting a cryogenic sapphire oscillator for very long baseline interferometry

Sheperd Doeleman, Mai Tai, Alan E E Rogers, John G. Hartnett, Michael E. Tobar, Nitin Nand

    Research output: Contribution to journalArticlepeer-review

    31 Citations (Scopus)


    Extension of very long baseline interferometry (VLBI) to observing wavelengths shorter than 1.3 mm provides exceptional angular resolution (∼20 μas) 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 (CSOs) have better stability than hydrogen masers on these timescales 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 1.3 mm.

    Original languageEnglish
    Pages (from-to)582-595
    Number of pages14
    JournalPublications of the Astronomical Society of the Pacific
    Issue number903
    Publication statusPublished - May 2011


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