Reflective array method for analysis and design of weighted DART transducers and filters

The DART (Distributed Acoustic Reflection Transducer) is well established as a primary technology for SAW I.F. filters, particularly for mobile phones. The transducer incorporates internal reflections, with reflection centres displaced almost ideally from transduction centres. Good directivity is obtainable, and filters based on this technology can have simultaneously low insertion loss and good multiple-transit suppression. Weighting can be applied to both transduction and reflection distributions in order to control the response, giving for example low sidelobes and sharp skirts. The Reflective Array Model (RAM) was earlier shown to be effective for analysis of uniform DART´s. Here, it is shown that the RAM can also be applied when the DART has withdrawal-weighted transduction and reflection distributions. A DART such as this can be treated as a sequence of elementary units, some giving transduction and some giving reflection, and these can be cascaded to give the transducer P-matrix. The DART can also be analysed with the transduction and reflection functions taken as analogue functions, to be withdrawal-weighted later. This is convenient for design purposes because many designs can be assessed without the need for withdrawal weighting. In recent filter designs the triple-transit signal is deliberately exploited, resonant SPUDT´s (R-SPUDT´s) are used, and the design is carefully optimised. These factors have enabled the physical size to be reduce substantially, and important factor for mobile phone applications