Adapting a Cryogenic Sapphire Oscillator for Very Long Baseline Interferometry

TL;DR

This paper presents the development and testing of a cryogenic sapphire oscillator-based frequency standard, optimized for very long baseline interferometry at sub-millimeter wavelengths, offering improved phase coherence over traditional hydrogen masers.

Contribution

The authors designed and implemented a CSO-based 10 MHz frequency standard, locked it to GPS signals, and demonstrated its suitability for VLBI at wavelengths below 1.3mm, surpassing hydrogen masers in stability.

Findings

CSO provides superior short-term stability compared to hydrogen masers.

Locking the CSO to GPS improves long-term stability.

The system is suitable for high-resolution VLBI observations at sub-millimeter wavelengths.

Abstract

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.