Compact ultra-wideband bandpass filters using microstrip-coplanar-waveguide simplified structure

A novel compact ultra-wideband bandpass filter (BPF) is proposed on the back-to-back composite microstrip-to-coplanar waveguide (CPW) structure. In this study, the CPW is composed by the defected ground structure (DGS). The microstrip-CPW transitions and the CPW shorted stubs are adopted as quasi-lumped-circuit elements for realizing a three-pole high-pass filter prototype. By introducing a cross-coupled capacitance between input and output ports of this high-pass filter and suitably designing the transition stretch stubs, a compact three-pole ultra-wideband bandpass filter is implemented with two transmission zeros located close to the passband edges as well as good out-of-band response. Being developed from the quasi-lumped elements, and not from the transmission lines, the proposed ultra-wideband filters have sizes more compact. The proposed ultra-wideband filters have the merits of compact size, flat group delay, good insertion/return loss, and good selectivity. Simulated responses of these filters are demonstrated.