Frequency-temperature compensation techniques for high-Q microwave resonators

Low-noise high-stability resonator oscillators based on high-Q monolithic s apphire "Whispering Gallery" (WG)-mode resonators have become important dev ices for telecommunication, radar and metrological applications. The extrem ely high quality factor of sapphire, of 2 x 10(5) at room temperature, 5 x 10(7) at liquid nitrogen temperature and 5 x 10(9) at liquid helium tempera ture has enabled the lowest phase noise and highly frequency-stable oscilla tors in the microwave regime to be constructed. To create an oscillator wit h exceptional frequency stability, the resonator must have its frequency-te mperature dependence annulled at some temperature, as well as a high qualit y factor. The Temperature Coefficient of Permittivity (TCP) for sapphire is quite large, at 10-100 parts per million/K above 77 K. This mechanism allo ws temperature fluctuations: to transform to resonator frequency fluctuatio ns. A number of research groups worldwide have investigated various methods of compensating the TCP of a sapphire dielectric resonator at different temper atures. The usual electromagnetic technique of annulment involves the use o f paramagnetic impurities contributing an opposite temperature coefficient of the magnetic susceptibility to the TCP. This technique has only been rea lized successfully in liquid helium environments. Near 4K the thermal expan sion and permittivity effects are small and only small quantities of the pa ramagnetic ions are necessary to compensate the mode frequency. Compensatio n is due to impurity ions that were incidentally left over from the manufac turing process. Recently, there has been an effort to dispense with the need for liquid hel ium and make a compact flywheel oscillator for the new generation of primar y frequency standards such as the cesium fountain at the Laboratoire Primai re du Temps ct des Frequences (LPTF), France. To achieve the stability limi t imposed by quantum projection noise requires that the local oscillator st ability is of the order of 10(-14) Currently work is under way to achieve t his goal in space-borne and mobile liquid-nitrogen-cooled systems. The work appears promising and, as at early 2000, the realization of this goal shou ld not be far off. In this contribution we review techniques that cancel the TCP of sapphire a ns other dielectric resonators. Details of the temperature control system r equired to achieve current and target frequency stabilities are discussed.