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The combination of very low loss dielectric single crystal materials (such as sapphire) with a HTS shielding enclosure has led to the achievement of high Q resonators operated in the temperature range 40 K-70 K which show great promise for frequency standard applications. A number of problems remain to be solved. We have already reported how high frequency stability with minimum temperature control may be achieved by means of composite dielectric pucks, the components having opposite signs for their temperature coefficients of permittivity. A second requirement for a frequency standard is that it should possess a specified frequency. Here we report implementation of electronic tuning of a sapphire dielectric puck resonator by using a SrTiO/sub 3/ (STO) tuning element situated in the evanescent field region outside the sapphire puck. In addition the same structure may be used when the sapphire and STO elements act as weakly coupled resonators so that the resonant frequency becomes a very sensitive function of temperature, allowing the possibility of very high resolution thermometry.
Date of Publication: June 1999