ULTRA-STABLE SAPPHIRE RESONATOR-OSCILLATOR

This paper reports preliminary measurements on the properties of cryog enic sapphire resonators with various concentrations of paramagnetic i mpurities. These resonators have been mounted in both copper and niobi um shields. We have found that we can achieve extrema in the frequency -temperature function in the temperature range 5-18 K due to the compe nsating effects of the dielectric constant temperature dependence and the Curie law dependence of the susceptibility of the paramagnetic imp urities. In sapphire samples with dominant impurities Cr3+ and Fe3+, w e find strong influences on the mode frequency when we are near the el ectron spin resonances (ESR) at 11.45 GHz and 12.04 GHz, respectively, and no extrema occurs when we are above the ESR frequency. In samples where dominant paramagnetic impurity has a high ESR we get an essenti ally frequency independent frequency-temperature compensation. In thes e samples we have observed unloaded resonator Q factors as high as 4 x 10(9). The increased Q factors coupled with reduced shield interactio ns should result in an improvement in frequency stability for an oscil lator based around this resonator as compared to the best results achi eved to date at this laboratory (an Allan Variance of 9 x 10(-15)). We report the preliminary measurement of the tilt sensitivity of our osc illator to be a fractional frequency shift of 6.5 x 10(-12) per degree .