Cryogenically cooled sapphire-rutile dielectric resonators for ultrahigh-frequency stable oscillators for terrestrial and space applications

The highest short-term frequency-stable microwave resonator oscillators uti lize liquid-helium-cooled sapphire dielectric resonators. The temperature c oefficient of frequency of such resonators is very small due to residual pa ramagnetic impurities canceling the temperature coefficient of permittivity (TCP), At higher temperatures, which are accessible in space or with liqui d nitrogen, the effect is too weak, and if extra impurities are added, the loss introduced is too great. An alternative technique involves using two l ow-loss dielectric materials with TCP of opposite sign. Following this appr oach, a sapphire-rutile resonator was designed with mode frequency-temperat ure turning points between 50-80 K, with Q-factors of order 10(7). Previous designs used thin disks of rutile fixed to the ends of the sapphire cylind er. Due to the high permittivity of rutile, such resonators have a high den sity of spurious modes. By placing rings at the end faces instead of disks, the majority of the spurious modes are raised above the operation frequenc y and the requirement for thin disks is removed. Finite-element analysis ha s been applied and compares well with experiment. The application to the de sign of high stability "fly-wheel" oscillators for atomic frequency standar ds is discussed.