Balanced low-loss longitudinally-coupled double-mode resonator saw filters with impedance conversion

This paper presents the new wideband balanced low-loss SAW filter with impedance conversion from 50-100 Omega to 200-300 Omega in a wide fractional bandwidth of 2-6%. The two types of the longitudinally-coupled double-mode resonator filters on 42deg YX LiTaO₃, 64deg YX and 41deg YX LiNbO₃ are used for this purpose. The first type of the filters is realized as a two-transducer scheme. The second type of the filters is built as the three-transducer scheme. The impedance conversion in the two-transducer filters is provided by a sectional output transducer. The impedance conversion in the three-transducer filters is attained by a series connection of the side transducers. The optimization of a SAW filter is provided with a computer simulation using an equivalent circuit model. The 240-361 MHz samples of the balanced SAW filters have shown an insertion loss of 2-3 dB, stopband attenuation of 20-50 dB. The filters were housed in the 5x5x1.35 mm SMD packages.