Design of longitudinally coupled SAW resonator filters with Chebyshev characteristics

This paper presents a design method for longitudinally coupled SAW resonator filters with Chebyshev characteristics. First, an equivalent circuit representation of the n-coupled-resonator filter is given by acoustically cascading the distributed-parameter equivalent circuits for IDT´s and grating reflectors, based on the coupling-of-mode (COM) theory. Then, this original circuit is converted to a more refined coupled-resonator type circuit by applying a sequence of equivalent transformations to the original circuit without using any approximations. Finally this circuit is approximated by a lumped-parameter circuit. This enables us to design bandpass filters with Chebyshev characteristics. Some 4-coupled resonator filters on 36°Y-X LiTaO₃ substrates are designed. Filters with a specific bandwidth less than 0.2% can be successfully designed