Stable Microwave Source Using High Overtone Bulk Resonators

Abstract

The high overtone bulk acoustic resonator (HBAR) provides the basis for stable microwave sources. The HBAR's high Q, closely spaced, periodic resonances provide stabilization for multiple frequency microwave sources. Recently an L-band source with 5 MHz channels has been developed. This HBAR source has phase noise performance equivalent to that of sources based on low frequency quartz crystal stabilization and multiplication, but it requires only a fraction of the hardware.The resonators are fabricated by depositing film transducers on the resonator cr ystaf such as YAG, sapphire, lithlum niobate, or fithium tantalate. The low-loss material of the crystal gives a Q approximately 10 times that of quartz. Resonators have been fabricated at frequencies as high as 10 GHz and have achieved Q's in excess of 65,000 at 1.5 GHz using the compressional mode. Tuning of the resonators has been achieved by deposition of mass loadlng materials such as sificon dioxide. Tuning to within 1 kHz of a standard using an active feedback system during deposition has been accomplished. The resonator is assembled into a rigid housing that minimizes the ef feet of external vibration on the resonator. Vibration sensitivity of 1 x 10 -1l/g has been measured.