Adapting a Cryogenic Sapphire Oscillator for Very Long Baseline Interferometry

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.

24 pages, 15 figures

References in corpus (10)

• Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre
• Tests of relativity using a microwave resonator
• Methods for detecting flaring structures in Sagittarius A* with high frequency VLBI
• Improved Test of Lorentz Invariance in Electrodynamics
• Cryogenic sapphire oscillator with exceptionally high long-term frequency stability
• Is there a Supermassive Black Hole at the Center of the Milky Way?
• ELISA: a cryocooled 10 GHz oscillator with 10-15 frequency stability
• Detecting Changing Polarization Structures in Sagittarius A* with High Frequency VLBI
• Long Term Operation and Performance of Cryogenic Sapphire Oscillators
• Creating traveling waves from standing waves from the gyrotropic paramagnetic properties of Fe${}^{3+}$ ions in a high-Q whispering gallery mode sapphire resonator

Cited by in corpus (16)

• First M87 Event Horizon Telescope Results. II. Array and Instrumentation
• 1.5 $\mu$m lasers with sub 10 mHz linewidth
• Universal Interferometric Signatures of a Black Hole's Photon Ring
• An ultrastable silicon cavity in a continuously operating closed-cycle cryostat at 4 K
• Gravitational Lensing Size Scales for Quasars
• Ultra-stable laser with average fractional frequency drift rate below $5\times 10^{-19}/\mathrm{s}$
• The ALMA Phasing System: A Beamforming Capability for Ultra-High-Resolution Science at (Sub)Millimeter Wavelengths
• First Very Long Baseline Interferometry Detections at 870μm
• The Impact of Frequency Standards on Coherence in VLBI at the Highest Frequencies
• A microrod optical-frequency reference in the ambient environment
• A Characterization of the ALMA Phasing System at 345 GHz
• Frequency Stability Measurement of Cryogenic Sapphire Oscillators with a Multichannel Tracking DDS and the Two-Sample Covariance
• Tests of Sapphire Crystals Produced with Different Growth Processes for Ultra-stable Microwave Oscillators
• Experimental and theoretical study of dynamic polarizabilities in the $5S_{1/2}$-$5D_{5/2}$ clock transition in rubidium-87 and determination of E1 matrix elements
• Characterization of the individual short-term frequency stability of Cryogenic Sapphire Oscillators at the 1e-16 level
• Comparison of ultra-stable radio frequency signals synthesized from independent secondary microwave frequency standards