Direct Synthesis and Design of Wideband Bandpass Filter With Composite Series and Shunt Resonators

In this paper, a wideband bandpass filter with composite series and shunt resonators is proposed together with a direct synthesis and design theory. The filter configuration can provide transmission zeros (TZs) on both sides of the passband. Two types of composite shunt resonators are introduced; one provides extra controllable TZs in the far stopband, whereas the other not only produces close-to-band TZs, but also high-order TZs in the far stopband in planar realization. The composite series resonator is realized by a high-impedance transmission line in shunt with a lumped capacitor. By introducing legitimate wideband circuit model approximations for the basic circuit blocks, a mixed lumped/distributed element version of the proposed filter is realized and experimentally verified. Design flexibility of circuit topology and spurious resonance suppression using TZs is discussed. Two design examples of the fifth-order and ninth-order filter prototypes are synthesized, designed, and measured, demonstrating the effectiveness of the synthesis and design theory as well as the superior performance of the proposed filter configuration.