A cryogenic open-cavity sapphire reference oscillator with low spurious mode density

In this paper, we describe the implementation of a microwave cryogenic sapphire oscillator (CSO) at the Laboratoire de Physique et Metrologie des Oscillateurs. In our realization we solved the problem of the spurious modes by operating the sapphire resonator in an open cavity. The CSO compared to a hydrogen maser demonstrates a frequency stability better than 3/spl times/10/sup -14/ at short term. Its long-term frequency instability of the order of 3/spl times/10/sup -12/ / day is limited by a random walk process. A first attempt to use this reference oscillator to characterize other signal sources is presented.