A miniaturized SAW reflector filter for IF stage of CDMA cellular phone applications

A SAW reflector filter for IF stage of CDMA cellular phone applications has been developed. Because the CDMA systems use the spread spectrum technique, a bandwidth of the modulated signal becomes wide. It is about 1.25 MHz or wider. In addition, an excellent passband flatness and a very steep cutoff characteristic are required for the IF stage filter. The SAW reflector filter consists of two pairs of input IDTs, output IDTs and reflectors. This filter is miniaturized by folding the SAW propagation path using the reflectors. The reflectors are weighted for filter characteristics. Although the concept of this filter was already reported, we have studied improved design technique for reflector weighting and transducer structures. The EWC-SPUDT is introduced to the input IDTs for low insertion loss, and the double split finger structures are introduced to the output IDTs for suppressing acoustic reflection. The filter samples show excellent performances. The size of the filters are 13.5 mm ×6.5 mm ×1.8 mm, which is 30% smaller than a conventional transversal filter.