Balanced low-loss 2-IDT double mode SAW filter with narrowed passband and improved selectivity

This paper presents the new balanced low-loss DMS filter with the narrowed passband and improved selectivity on 42° YX LiTaO₃. The filter is realized as two-transducer scheme. A narrow passband of the filter is obtained by choosing a definite gap length between the input and output IDTs. Also the both long weighted input IDT and reflectors are used. A shape of the frequency response of the filter is determined by the weighting function of the input IDT, number of the electrodes in output IDT and reflectors, relationship between the electrode pitches in the input IDT, output IDT and reflectors. An optimization of the mentioned parameters with a computer simulation using equivalent circuit model allows to get a specified high selectivity of the filter with a fractional bandwidth of 0.5-0.6% under condition that the insertion losses are low and input and output impedances are close to the specified real values. To decrease the input and output impedances the parallel connection of the two filters in the different acoustic tracks is used. To improve the selectivity of the filter the cascaded connection and phase weighting the input IDT are employed. The 300 MHz sample of the balanced SAW filter has shown an insertion loss of 3 dB, 2-dB bandwidth of 1.5 MHz, stopband attenuation of 70 dB at ±12 MHz offsets from a center frequency in a 75-Ω balanced system. The filter did not require the matching elements and housed in the 9.1×7.1×2 mm SMD package.