A flat-time-delay transversely coupled resonator SAW filter comprising parallel connected filter tracks

A transversely coupled resonator SAW filter suitable for an IF-stage of digital cellular phones has been developed. Recently, improvement of pass-band group delay flatness was strongly expected to satisfy low distortion receiving quality for digital phones. For this purpose, it is necessary to derive optimum matching conditions according to the preceding stage and the following stage. Usually they have different impedances. The authors have developed a new transversely coupled resonator SAW filter that takes the matching circuits into account. This filter consists of three filter tracks. Two tracks are connected in parallel with each other, and the other track is connected in series to them. Thus the impedance of the parallel connected tracks is half of the other impedance due to the increased interdigital static capacitance. Furthermore, at the inter-stage side, IDT electrodes of the parallel connected tracks are withdrawn. The inter-stage matching condition is still good. Thus, the flat-time-delay SAW filter according to the matching condition can be designed. The practical filter sample shows an excellent performance