  Recently, piezoelectric power conversion has shown great benefits from
replacing the bulky and lossy magnetic inductor in a traditional power
converter with a piezoelectric resonator due to its compact size and low loss.
However, the converter performance is ultimately limited by existing resonator
designs, specifically by moderate quality factor (Q), moderate
electromechanical coupling (kt2), and spurious modes near resonance. This work
reports a spurious-free lithium niobate (LiNbO3) thickness-extensional mode
bulk acoustic resonator design, demonstrating Q of 4000 and kt2 of 30% with a
fractional suppressed region of 62%. We first propose a novel grounded ring
structure for spurious-free resonator design, then validate its performance
experimentally. Upon further work, this design could be extended to
applications requiring spurious suppression, such as filters, tunable
oscillators, transformers, etc.
