Balanced lattice filter with acoustically interacting resonators

In balanced impedance element filters (IEFs) which are symmetrically driven, it is often the case that at least two resonators (impedance elements) can be found which have identical impedance and are situated symmetrically in the filter network. These impedance elements are often not connected directly together, but have equal voltage and currents with the same phase. Normally, such elements are physically realised as two independent SAW resonators, not interacting either electrically or acoustically. We describe here a 1200 MHz bandpass filter in which such two impedance elements, situated in electrically symmetric conditions can be realised as a part of a single SAW resonator. The resonator comprises a long transducer and two reflectors on the ends of the transducer. The said transducer is sub-divided into 2 parts which are not connected electrically but placed synchronously in a way that they generate a common in-phase surface acoustic wave for the whole resonator. The proposed technical solution has clear advantages: a) The transverse dimensions of the filter can be significantly reduced. Normally two resonators are placed in separate acoustic channels, while in the current arrangement they are in the same channel. b) The surface area occupied by the resonators is reduced, because 2 reflectors have been eliminated. c) The ripple of the admittance (and thus the ripples in the filter passband) are reduced due to the use of a longer transducer structure. d) The use of a longer transducer also results in wider achievable passbands. e) The losses are lower than in the case of 2 separate resonators. Experimental results or 1200 MHz filter are presented which confirm the advantages of this approach for lattice filter design.