CONTROL OF ELECTROMECHANICAL COUPLING IN STACKED CRYSTAL FILTERS

Stacked crystal filters (SCF) and coupled resonator filters (CRF) are electromechanical devices which make use of acoustic coupling between two transducing piezoelectric films to achieve the filter functionality. They rely on the principles of generating and transferring acoustic energy from one piezoelectric film to another at a particular range of frequencies where the structure is resonant. Simplistically, electrical energy is converted into mechanical by the input piezoelectric film, is transferred to the output piezoelectric film, which converts this mechanical energy back to electrical. More generally, all aspects of the electromechanical coupling in the device play important roles in determining filter center frequency, loss, and bandwidth characteristics. This paper presents methods to understand and improve electromechanical coupling by proper device impedance matching, device symmetries, and beneficial use of external components like inductors.