Coupled bulk acoustic wave resonator filters: key technology for single-to-balanced RF filters

Coupled Resonator Filters (CRF) are a new type of Bulk Acoustic Wave (BAW) device in which two piezoresonators are stacked on top of each other in a way that a certain degree of acoustic interaction occurs. CRFs feature complete galvanic isolation between input and output and thus enable to offer BAW filters with mode-conversion (single-ended to balanced) as well as impedance transformation. The basic concept behind these devices is reviewed. The size of a CRF BAW filter is excitingly small as compared to SAW filters. First experimental results for CRFs operating at 1.8 GHz which have been manufactured at Infineon Technologies will be presented. Certain manufacturing issues are discussed and measurements of the temperature coefficient TCF are shown.