Tunable microstrip bandpass filters with two transmission zeros

This paper presents an analytical technique for positioning the locations of two transmission zeros for a microstrip bandpass filter. In addition, this paper discusses how the tapping positions of asymmetric feed lines affect the coupling between resonators. The bandpass filter uses two resonators with two transmission zeros to obtain lower insertion loss than a conventional cross-coupled microstrip filter. Also, by using four resonators, the bandpass filter has better out-of-band rejection than the conventional cross-coupled microstrip filter. The filter designs using four cascaded resonators provide a sharp passband that can be tuned using a piezoelectric transducer to change the effective dielectric constant. The filters demonstrate high selectivity, compact size, and can be used in many wireless communication systems.