Selectively Tuning the In-Plane Orientation of Resonators in SAW Filters for Flatter Out-of-Band Characteristics

In this work, the spurious cancellation technique is proposed to realize flatter out-of-band characteristics of ladder-type filters. Based on the Y42-cut LiTaO3/SiO2/poly-Si/Si substrate, shear horizontal surface acoustic wave (SH-SAW) resonators with different in-plane orientation θ were fabricated and measured. The frequency differences between the spurious and main modes increase with θ. By selectively tuning the θ of the resonators, a frequency shift of the main mode and an overlapping of at least one spurious mode can be achieved simultaneously. At the same time, the tilted interdigital transducers (IDTs) is used to increase the Q value and weaken the spurious modes. SH-SAW filters with traditional and proposed designs were fabricated, measured, and compared. Filter with traditional design shows the insertion loss (IL) of spurious modes is only 24.1 dB. By selectively tuning the in-plane orientation θ of the resonators in a SAW filter, the IL of spurious modes is enhanced to 32.8 dB. After adopting the tilted IDTs, the IL further increased to 35.7 dB. Therefore, the proposed spurious cancellation technique is very promising for realizing mass-produced SAW filters with flatter OoB characteristics.