Surface-wave resonators using grooved reflectors

The resonator described in this paper consists of a pair of reflectors, each made up to a periodic array of weakly-reflecting (shallow) grooves etched into the crystal surface, and separated by an appropriate gap on the free surface of a crystal. Theoretical limits on achievable Q, imposed by propagation and diffraction losses, are presented for the substrates of LiNbO3 and ST-cut quartz. Experimental results are presented for devices at 68 MHz and 170 MHz, with Q values ranging from several thousand to over ten thousand. The measured Q's for a given array increase with groove reflectivity (proportional to groove depth). However, significant deviations from theory are observed for groove depths exceeding approximately 2.5% of a wavelength. These deviations are ascribed to increased scattering into bulk waves.