Design and Analysis of Surface Acoustic Wave Filters

Abstract

The generation and detection of surface acoustic waves (SAWs) using interdigital transducers (IDTs) on a piezoelectric surface have been used to produce many high performance Band Pass Filters. This thesis focuses on the design and simulation of various SAW Band Pass Filters. IDTs can be fabricated on many piezoelectric substrates. The effect of substrate properties – electromechanical coupling coefficient and SAW velocity, on filter frequency response is analyzed. The IDT design properties comprise film thickness ratio, metallization ratio, acoustic aperture, and number of finger pairs. The behavior of electrical equivalent circuit of an IDT that consists of impedance parameters – radiation conductance, radiation susceptance, and capacitance, is simulated and analyzed for different piezoelectric materials. Different IDT designs offer different propagation environment to SAWs. The IDT designs based on electrode spacing – uniform and non-uniform, direction of SAW propagation – bidirectional and unidirectional, acoustic aperture – apodized and unapodized, and electrode configuration – solid electrode and split electrode, are studied. The effect of IDT design on filter performance is assessed. The design of linear phase SAW filter using fourier transform, and effect of truncation on filter specifications – amplitude ripple, side lobe rejection ratio, insertion loss, and transition bandwidth is thoroughly depicted and analyzed. The cosine window function technique is used to improve filter performance. The second order effects – bulk wave interference, diffraction, impedance matching, electromagnetic feedthrough, triple transit interference, and harmonics that corrupt the filter performance are elaborated. The effect of metallization ratio on higher harmonic suppression is studied. Design of advanced SAW band pass filters – comb filters and resulting frequency response is also explored.