Transversal SAW filters using a BDT and a SPUDT

Conventional transversal SAW filters use two bidirectional transducers (BDTs) to achieve small transition bandwidth and good sideband rejection. In order to maintain good triple transit echo suppression, they suffer from high insertion loss. Nowadays, low-loss transversal SAW filters with good triple transit echo suppression are realizable through the use of two single-phase unidirectional transducers (SPUDTs). However, it is in general difficult to achieve as small transition bandwidth when compared with the conventional transversal SAW filters. A receiving SPUDT, when matched to zero reflectivity, doesn´t reflect signal coming from a transmitting transducer- either BDT or SPUDT. And so theoretically, good triple transit suppression can be achieved with only one appropriately matched SPUDT. This paper describes the transversal SAW filter designs using combination of a BDT and a SPUDT. The BDT offers easier transition bandwidth shaping through apodisation or withdrawal weighting. The SPUDT offers unidirectionality and good triple transit echo suppression under perfect matching. Therefore the features of lower loss, good triple transit suppression and small transition band can be achieved. A simulation model is presented for this technique together with the simulated and practical filter results. Finally, a short discussion of the benefits of this technique is also presented