GHz filters with third harmonic unidirectional transducers

Abstract

High insertion loss is a major disadvantage of harmonic SAW (surface acoustic wave) and STW (surface transverse wave) filters. It is shown that harmonic unidirectional transducers can considerably improve the filter performance. Two harmonic GHz filters with remarkably low insertion loss have been demonstrated. Rayleigh wave filters on a high coupling material tend to have lower insertion loss than STW filters on quartz, but are limited to somewhat lower frequencies. The insertion loss of the Rayleigh wave filter is about 5 dB higher than the insertion loss of the best fundamental device reported so far, but the production of a harmonic filter is easier, resulting in better yield and lower cost. A simple model for calculating the input impedance of an STW transducer is presented. In many cases it predicts the impedance curve accurately enough.