P3H-1 Tunable Solidly Mounted Resonators

Solidly-mounted resonator (SMR) configurations were pioneered by Newell in the mid-1960s. In Newell's designs, a piezoelectric resonator was indirectly attached to a substrate by means of an intervening Bragg stack of plates of low and high acoustic impedance, similar to an anti-reflecting optical coating. By proper arrangement of the stack parameters, it was conjectured that high resonator quality factor (Q) values could be attained. However, devices based on this novel concept proved not to be satisfactorily realizable with fabrication techniques available at that time. More recent advances in microfabrication technologies have now produced practical commercial realizations. In the usual SMR embodiment, only the mechanical impedance transforming property of the Bragg stack is utilized. In [25] was reported initial work on a method to provide the Bragg stack with tunability by making use of piezoelectric layers. The individual strata are then represented as three-port networks, the electrical port supplying an additional degree of freedom for tuning or adjustment. Because of unavoidable manufacturing variability, a means of altering the equivalent electrical length of each layer allows SMR devices based on this modality to be optimized; alternatively, the tuning may be applied in a dynamic fashion, to change the frequency at which minimum power is lost to the mounting substrate. This paper describes some of what may be accomplished by means of piezoelectric Bragg stacks, as well as descriptive equivalent network realizations and requisite operational relations.