P5I-3 Improvement of Balance Performance in LSAW Filters Based on 5-IDT Multi-Mode Structure

Single ended - balanced (Se-Bal) filters are widely used in RF Front End stages of mobile phones together with differential low noise amplifiers (LNA) and differential mixers. Such an approach allows improved noise figure (NF); more effective utilization of LNA´s and improved Noise Immunity due to wideband rejection. Balance performance is one of the most important characteristics of the SAW filter in these applications. Typical specifications demand an imbalance better then plusmn1.0 dB in amplitude and plusmn15deg in phase over the working temperature range. Hybrid SAW filters based on 5-IDT longitudinally coupled multi-mode SAW structures (LCMMS) are very attractive candidates for realization of these specifications. These structures fabricated on the 42-LiTaO₃ substrate allow to achieve necessary bandwidth (BW) (up to 4.75%), low insertion loss (IL) (<2.0 dB), steep transition slopes (<0.5% from pass band edge to stop band edge) and rejection level (RL). We define IL of the filter as maximum IL in the pass band over specified temperature range (usually from -30degC to plusmn85degC) and some process variation. In this work we propose a classification of 5-IDT structures (4 acoustically different structures and 4 different realizations for each of them from an EM-parasitics standpoint). We show how they compare from the viewpoint of Balance performance, IL, BW and RL. Then we propose decomposition of an arbitrary imbalance into simple components (shape, inclination, level and fang) and provide original practical recommendations and approaches for improving each of these components.