Radio-frequency surface acoustic wave filters for mobile communications systems

Abstract

Surface acoustic wave (SAW) filters are small, rugged, crystal stable, and can be fabricated with high reproducibility. SAW filters play a key role in the rapidly growing market of mobile communication systems and are used for channel filtering as well as for RF filtering. For today’s communications systems novel types of SAW filters are being developed in order to meet the stringent requirements with respect to low insertion loss, high relative bandwidths, good stopband rejection properties, and power durability. Most SAW RF filters are fabricated on cuts of LiTaO3 and LiNbO3. To obtain very low loss filters precise knowledge of the wave propagation and loss mechanisms is a prerequisite. Using advanced analysis tools, substrate-cut-metallization combinations can be chosen for minimized propagation losses. To enhance power durability of SAW filters both substrate-metallization combination as well as the design have to be optimized. Two different filter techniques are used for SAW RF filters. The working principles of double mode structure (DMS) filters and ladder-type filters, both using resonances to minimize the lengths of the structures and the required chip size, will be described. As examples, RF filters for GSM and PCN, diplexer for GSM, and duplexers for cordless phones will be presented.