High performance SAW IF-filters for digital mobile communication systems using new floating-electrode-type SPUDTs

In this paper, we will describe a low-loss SAW IF-filter on ST-quartz with extremely low group-delay ripple for digital mobile communication systems using new single phase unidirectional transducer (SPUDT) structure and the weighting technique. The original floating electrodes type SPUDT (FEUDT) structure consisting of both open and shorted floating electrodes cannot be applied effectively to substrates with a small electromechanical coupling coefficient K² such as a quartz and LiTaO₃ substrate. Therefore we have newly developed the modified FEUDT structure suitable for substrates with a small K². And we have adopted the distance weighting technique reported for the case of a conventional bidirectional transducer as a weighting technique of SPUDT. We have obtained a mass-productive low-cost PHS (Personal Handy Phone System in Japan) filter with frequency of 243.95 MHz, minimum insertion loss of 3.5 dB, 3 dB bandwidth of >300 kHz, group-delay deviation of <0.3 μs in passband, operational temperature range of 100°C and the same impedance in input and output