Optimising slanted-finger interdigital transducer (SFIT) filters

This paper presents an innovative design method for achieving a maximum performance in slanted-finger interdigital transducer (SFIT) filters by optimising certain parameters. This will lead to an improved performance as for example the mechanical reflections will not be only utilised to make the IDTs of the SFIT filter unidirectional but also to improve the filter response. For instance, passband ripple problems caused by triple transit echo may be prevented. Moreover, the skirt steepness may be increased without increasing the number of finger groups. In addition, the chip size can be reduced. This method will be compared with conventional design methods. Simulated and experimental results of some filter examples help to show the performance that can be achieved by this optimisation method. A fast analysis, which is necessary for optimising, has been developed. The method, which is based on a scaling principle to determine all P-matrix elements, is also described.