Low Loss and Wide Band Filters Using New Dispersive Interdigital Transducers with Floating Electrodes

In order to obtain the high performance of Surface Acoustic Wave (SAW) devices, it is very important for the interdigital transducer (IDT) to be made the unidirectional ones (UIDT). Especially the mobile communication and UWB communication systems require the high efficient unidirectional IDT for the phase linear, wide band, low-loss filters at GHz-ranges. Dispersive interdigital transducers (DIDT) have sharp cut-off and flat wide band frequency characteristics. Unfortunately, the conventional dispersive interdigital transducers have the unidirectionality only to the down-chirp direction (DUDIDT). Therefore the non-dispersive filter with low insertion loss cannot be obtained. If the up-chirp unidirectional dispersive IDT (UUDIDT) is obtained, the phase linear and the very low loss filters are obtained by combining DUDIDT and UUDIDT. In this paper, DUDIDT and UUDIDT were investigated by using floating electrodes with the open and shorted electrodes inserted between the inderdigital transducers. The large directivity of DUDIDT is obtained by inserting the open floating electrode between the negative and positive electrodes. The UUDIDT is obtained by inserting the open and short floating electrodes between the divided negative electrode, and the negative and positive electrodes. The theoretical results showed the large directivity around 12dB in the floating electrode type DUDIDT compared with the directivity of 6dB in the only DUIDT. The short and open floating electrode type UUDIDT showed the up-ward directivity of 6dB compared with the down-ward directivity of 6dB in the conventional DUDIDT. The filters with non-dispersive, phase linear, flat band of wide bandwidth of 5%-30%, sharp cut off and low insertion loss of below 2dB are obtained. The experimental results show almost the good agreement with the theoretical ones.