Comparative performances of 460 MHz low-loss SAW ring and dual-track image impedance connected filters

Abstract

In this paper we compare two types of 460 MHz low-loss SAW filters on 128/spl deg/YX LiNbO/sub 3/. The first construction is a dual-track image impedance connected (IIC) filter. The second construction is a ring filter using reflective multistrip couplers (RMSCs). Both filters are developed for self-matching condition when filter specified real input/output impedances are achieved by IDT static capacitance compensation with radiation susceptance. At 50 ohm the dual-track IIC filter had insertion loss of 1.5 dB, 1 dB bandwidth of 2.1 MHz and stopband attenuation about 60 dB at /spl plusmn/60 MHz offset from the center frequency. At 50 ohm the ring filter has shown insertion loss of 1 dB, 1 dB bandwidth of 6 MHz, stopband attenuation over 50 dB at /spl plusmn/60 MHz offset from the center frequency. The ring filter with phase weighted IDTs has shown insertion losses of about 2 dB, stopband attenuation over 50 dB at /spl plusmn/20 MHz offset from the center frequency. The phase weighted dual-track IIC filter provided insertion loss of 3.5 dB. The cascade of two unweighted ring filters has shown stopband attenuation about 90 dB and insertion losses below 3 dB. When the cascaded dual-track IIC filters are used, such stopband attenuation is provided but with greater insertion losses of 3-4 dB.