The splitfinger directional transducer (SFDT): a new single phase unidirectional transducer

We have developed a new type of single phase unidirectional transducer (SPUDT) which we call splitfinger directional transducer (SFDT). This transducer has a very simple geometry. It has a uniform pitch of λ/4 and the electrodes which provide transduction and reflection have a width of λ/8. A reflection cell consists of two λ/8 electrodes, in which one electrode is shorted (connected to ground), the other is floating. The mechanical reflection of such a cell is zero, but the electrical reflections are very strong, depending on the coupling coefficient of the used substrate. Therefore the SFDT can be used on high coupling substrates, such as LiNbO₃ or LiTaO ₃ and the influence of fabrication tolerances to its characteristics is significantly reduced. Reflector weighting can be provided very easily by changing the aperture length of the floating electrodes, having in mind that two short-circuit λ/8 electrodes have zero reflection. We have investigated the characteristics of this new SPUDT theoretically and experimentally. We have developed a test structure to measure the directivity of the transducer and to calculate the reflection coefficient of a reflection cell by means of this result. Furthermore we will present the measured and calculated characteristics of a broadband low-loss filter which uses the new SPUDTs. The filter has a center frequency of 70 MHz, a 3-dB-bandwidth of 1.2 MHz and a minimum insertion loss of 2.5 dB (measured)