Ultrastable Cryogenic Microwave Oscillators

Abstract

Ultrastable cryogenic microwave oscillators are secondary frequency standards in the microwave domain. The best of these oscillators have demonstrated a short term frequency stability in the range 10−14 to a few times 10−16. The main application for these oscillators is as flywheel oscillators for the next generation of passive atomic frequency standards, and as local oscillators in space telemetry ground stations to clean up the transmitter close in phase noise. Fractional frequency stabilities of passive atomic frequency standards are now approaching 3 × 10−14/√τ where τ is the measurement time, limited only by the number of atoms that are being interrogated. This requires an interrogation oscillator whose short-term stability is of the order of 10−14 or better, which cannot be provided by present-day quartz technology. Ultrastable cryogenic microwave oscillators are based on resonators which have very high electrical Q-factors. The resolution of the resonator’s linewidth is typically limited by electronics noise to about 1 ppm and hence Q-factors in excess of 108 are required. As these are only attained in superconducting cavities or sapphire resonators at low temperatures, use of liquid helium cooling is mandatory, which has so far restricted these oscillators to the research or metrology laboratory. Recently, there has been an effort to dispense with the need for liquid helium and make compact flywheel oscillators for the new generation of primary frequency standards. Work is under way to achieve this goal in space-borne and mobile liquid-nitrogen-cooled systems. The best cryogenic oscillators developed to date are the “whispering gallery” (WG) mode sapphire resonator-oscillators of NASA’s Jet Propulsion Laboratory (JPL) and the University of Western Australia (UWA), as well as Stanford University’s superconducting cavity stabilized oscillator (SCSO). All of these oscillators have demonstrated frequency stabilities in the range of a few times 10−15 to a few times 10−16. In this contribution we review only liquid-helium-cooled secondary frequency standards, such as those just mentioned, which have attained frequency stabilities of 10-14 or better.