Compact Ultra-Wideband Bandpass Filters Using Composite Microstrip–Coplanar-Waveguide Structure

Compact ultra-wideband bandpass filters are proposed based on the composite microstrip–coplanar-waveguide (CPW) structure. In this study, 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. To further improve the selectivity, two microstrip shorted stubs are added to implement a five-pole ultra-wideband bandpass filter with 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 than those of the published wideband filters. The proposed ultra-wideband filters have the merits of compact size, flat group delay, good insertion/return loss, and good selectivity. Agreement between simulated and measured responses of these filters is demonstrated.