Spurious modes suppression in Stacked Crystal Filter

Stacked Crystal Filters (SCFs) are used in Bandpass filters and other applications where frequency control is required. SCFs use the Solidly Mounted Resonator (SMR) structure to enable operations at RF frequencies, typically 1-5 GHz. In the response spectrum of the SCFs, besides the main resonance mode, there are some unwanted additional modes present which affect the filter´s performance away from the primary passband frequency. A single passband is typically a required specification of a band pass filter; therefore the other unwanted modes are to be suppressed as a transmission band can manifest at each unwanted mode. It will turn out that with proper choice of layer materials and thicknesses the unwanted modes can be suppressed, but the level of suppression will be sensitive to the exact layer thicknesses fabricated. A research work is needed to focus on evaluating the materials used in the SCF´s resonating mirror and seek a novel mirror methodology for the suppression of these unwanted modes even over manufacturing variations. In this paper, we will investigate careful control of the acoustic mirror stack above which the SMR stack is fabricated to create the SCF. By choosing appropriate materials and adjusting their respective thicknesses, the spurious modes can be suppressed without degrading the overall primary mode performance of the SCF. Trade-offs between the acoustic mirror properties relating to the filter´s addressable bandwidth, and the desired suppression of lateral modes of one/two stage SCFs will be discussed.