Unidirectional interdigital transducers and resonators using grating SAW substrates and application to low loss wide band filters

Surface acoustic wave (SAW) filters employing a conventional interdigital transducer (IDT) show inherent minimum insertion loss of 6 dB and a strong passband ripple for a low loss filter. In order to avoid these flaws, many types of unidirectional interdigital transducers are proposed. These unidirectional transducers have small transducer efficiency. Therefore it is difficult to obtain wide band and low loss filters. In this paper we describe a new single phase unidirectional transducer (UDT) with about λ/4 electrode width of the best transducer efficiency. The new UDTs are fabricated on the very thin dielectric grating or the very shallow groove gratings SAW substrates. The electromechanical couplings (k²) of electrodes on SiO₂ decrease rapidly for film thickness. Therefore the good properties of UDT is obtained for the very thin thickness of SiO₂ (about 0.4 μm at 100 MHz and 1GHz) also the very thin grooves have a large reflection coefficients of SAW and good UDTs are obtained. The calculation results show wide band low loss filters with various k² materials. Also we show new SAW resonators using the admittance and susceptance changes.