Slanted finger inter-digital transducers for the design of improved performance small shape factor mid-bandwidth SAW filters

The virtues of slanted finger inter-digital transducers, or SFITs, for the realization of wide band SAW filters are well proven. A combination of low in-band ripples and good stop band attenuation gives SFITs the added edge over conventional IDT techniques. However, when it comes to small shape factor mid-bandwidth filters, conventional IDT design has been the first choice, often leading to large filter chip lengths. What is ignored though is the size and performance advantage that SFITs can offer, in such cases. This paper describes how SFITs can be useful for the realization of high performance small shape factor SAW filters in a compact size. To illustrate the point, a linear-phase SAW filter using SFITs with a 10% fractional bandwidth at a centre frequency of 15.6 MHz and a 30 dB to 3 dB shape factor of 1.23 is designed and fabricated on Y-Z LiNbO₃. A side lobe suppression of 35 dB is achieved. It is also shown how the use of conventional IDT technique for achieving the same specifications leads to a large filter length.