Tunable acoustic RF-filters: Discussion of requirements and potential physical embodiments

Multi-band / multi-system mobile phones require a complex RF-frontend architecture. Part count has increased to a point where adding switches and whole signal branches for an additional band seems no longer cost effective. Most of the active components can be adopted to work over a sufficiently wide frequency spectrum in order to cover multiple bands within one and the same circuit. This is not true for established RF filters and duplexers which are highly frequency selective but limited to operate at a fixed frequency band. It is highly desirable to create a technology which allows tuning a narrowband filter to a desired center frequency by means of an electrical control input. Concepts for tunable RF-filters — pursuing the ‘holy grail’ — will be discussed and an overview on the status of this matter will be presented. Existing RF-filter technologies are based on high-Q acoustic resonators realized either in Surface Acoustic Wave (SAW) or Bulk-Acoustic-Wave (BAW) technology. The paper discusses how new materials can be implemented to accomplish the goal of tunable acoustic RF filters. System requirements will be discussed and circuit concepts for the implementation will be presented.