Very low amplitude ripple SAW filter for infrastructure systems using 41°Y-X lithium niobate: Full FEM/BEM design approach

High data rate communication devices demand high relative bandwidth and therefore wide band filters are necessary. In infrastructure systems like base stations and microwave radio-links, such filtering function was carried out by ceramic filter technology. They were preferred for their low in-band ripple and VSWR necessary for low EVM and amplifier linearity. We have developed a design technique allowing SAW devices to advantageously replace ceramic filters in many systems .One important parameter that determines the performance of a piezoelectric device is the electromechanical coupling coefficient. RF filters designed on widely used 42?Y-X LiTaO₃ are not meeting the stringent specification requested by infrastructure systems. To increase the width of the bandwidth and reduce the in-band ripple, a material with high electromechanical coupling coefficient is needed. This paper describes the search for the optimal working point using a full FEM/BEM approach and proposes a new design technique on 41?Y-X LiTaO₃. This technique is applied to high reliability RF filters. On 41?Y-X LiTaO₃, bandwidths larger than 8% at 3 dB attenuation are achieved leading to very low amplitude ripple. A GSM Rx filter for base station at 1747.5 MHz is demonstrated. Filter bandwidth is 140 MHz leading to ripple in the frequency band 1710 MHz to 1785 MHz being lower than 0.2 dB nominal and 1 dB over temperature and total 6 sigma manufacturing. Insertion loss is 2.5 dB while image rejection is better than 45 dB. Size is 3 ? 3 mm² in a high reliability ceramic cavity package. The close correspondence between simulation and measurement demonstrates the ability of our methodology to design SAW devices rapidly and accurately on virtually any material. The filters designed by this technique on 41?Y-X LiTaO₃ are demonstrating a wide band and very low in-band ripple perfectly matching the infrastructure systems requirement.