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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Environmental sensitivity of the superconducting cavity maser oscillator

R.T. Wang, G.J. Dick

Published Jan 2, 2003• 0 citations
Publisher

Abstract

The superconducting cavity maser oscillator (SCMO) is tested for environmental sensitivity in the controlled environment of a frequency standards test facility (FSTF). Measured sensitivities to temperature, magnetic field, pressure and humidity are used to predict SCMO performance in various laboratory environments. A comparison to SCMO frequency stability tests in these same environments shows good results, and indicates a need to reduce sensitivities to temperature and magnetic field. Characterization provides continual evaluation of SCMO for the next generation of frequency standards for NASA's deep space network.>

DOI
10.1109/freq.1992.270017

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The superconducting cavity maser oscillator (SCMO) is tested for environmental sensitivity in the controlled environment of a frequency standards test facility (FSTF). Measured sensitivities to temperature, magnetic field, pressure and humidity are used to predict SCMO performance in various laboratory environments. A comparison to SCMO frequency stability tests in these same environments shows good results, and indicates a need to reduce sensitivities to temperature and magnetic field. Characterization provides continual evaluation of SCMO for the next generation of frequency standards for NASA's deep space network.>
Unprecedented stability of frequency in superconducting, triple-cavity ruby maser oscillator achieved by incorporation of amplitude-control subsystem. New design enhances ultra-stable measurements of time by reducing fluctuations to 2 X 10 to negative 19th power routinely, and to as little as 10 to negative 20th power in exceptional cases. Currents induced in superconducting pickup coil by changes in magnetic field in ruby. Currents from coil fed to superconducting quantum-interference device (SQUID) magnetometer, output used to generate control signal for electronically variable attenuator. Attenuator varies pump-signal amplitude in response to magnetic-field fluctuations in ruby. Very high feedback-loop gain used for sensitivity of control and adequate compensation of fluctuations.
Measurements on the superconducting cavity maser (SCM) oscillator that show frequency stability of parts in 10/sup 5/ for times from 1 s to 10000 s are reported. A phase noise of approximately -80 dB/f/sup 3/ was measured. This short- and midterm performance is believed to be better than that of any known microwave oscillator. In particular, stability at a measuring time of 1 s is ten times better than that of a hydrogen maser, and phase noise is more than 20 dB below that of the best multiplied quartz crystal oscillators.>
The frequency stability obtained with an ion-trap-based frequency standard where /sup 199/Hg/sup +/ ions are confined in a hybrid RF/DC linear trap is described. The 40.5-GHz clock transition is measured to be as narrow as 17 mHz representing a quality factor of 2.4*10/sup 12/. A hydrogen maser interrogation of a 37-mHz Hg/sup +/ resonance is used to measure 1*10/sup -15/ stability of the ion resonance at 10000-s averaging time. In another measurement the frequency of the superconducting cavity maser oscillator is steered to follow the Hg/sup +/ clock transition, again providing frequency stability of 1*10/sup -15/ at 10000-s.>
The frequency stability of an atomic standard based on /sup 199/Hg/sup +/ ions confined in a hybrid RF/DC linear trap is described. The 40.5-GHz clock transition has been measured to be 17 mHz wide, representing a quality factor greater than 2*10/sup 12/. A 160-mHz line is used to steer the output of a 5-MHz crystal oscillator to obtain a stability of 2*10/sup -15/ for 24000-s averaging times. In a separate measurement, a 37-mHz line is used to steer the output of the superconducting cavity maser oscillator to reach 1*10/sup -15/ stability at 10000 s.>
Many physics experiments call on improved resolution to better define the experimental results, thus improving tests of theories. Modern microwave technology combined with high-Q resonators can achieve frequency readout and control with resolutions up to a part in 10(exp 18). When the physical quantity in question in the experiment can be converted to a frequency or a change in frequency, a high-stability microwave oscillator can be applied to obtain state-of-the-art precision. In this work we describe the overall physical concepts and the required experimental procedures for optimizing a high-resolution frequency measurement system that employs a high-Q superconducting microwave cavity and a low-noise frequency synthesizer. The basic approach is to resolve the resonant frequencies of a high-Q (Q > 10(exp 10)) cavity to extremely high precision (one part in 10(exp 17)- 10(exp 18)). Techniques for locking the synthesizer frequency to a resonant frequency of the superconducting cavity to form an ultra-stable oscillator are described. We have recently set up an ultra-high-vacuum high-temperature annealing system to process superconducting niobium cavities, and have been able to consistently achieve Q > 10(exp 9). We have integrated high-Q superconducting cavities with a low-noise microwave synthesizer in a phase-locked-loop to verify the frequency stability of the system. Effects that disturb the cavity resonant frequency (such as the temperature fluctuations and mechanical vibrations) and methods to mitigate those effects are also considered. Applicability of these techniques to experiments will be discussed, and our latest experimental progress in achieving high-resolution frequency measurements using the superconducting-cavity-stabilized-oscillator will be presented.
A cavity consisting of a superconducting lead film on a sapphire substrate has been fabricated to obtain the enhanced frequency stability possible with this configuration. The cavity exhibits a quality value Q exceeding 2×109 in its TE011 mode with a resonant frequency of 2.689 GHz. Methods of fabrication and testing of the cavity are presented in this article. Since the interface between the film and substrate is exposed to the full value of the resonant magnetic field, our experiment is the most sensitive test to date for enhanced losses at the interface itself. We find no evidence of such losses. In fact, the measured values of the surface resistance match very well predictions for rf losses based on the BCS theory.