Millimeter-wave coplanar-waveguide parallel-coupled bandpass filters with lumped-element K-inverters

Compact coplanar-waveguide (CPW) bandpass filters are proposed, by introducing additional lumped inductor and capacitors to realize K-inverters in the conventional CPW parallel-coupled filter structure for achieving an equivalent to the quarter-wavelength resonator filters. As a result, the filter size may be reduced by about half. In addition, by introducing the cross-coupled effect, two transmission zeros at upper and lower stopbands may be created. Simple equivalent-circuit models are also established as effective design tools. Specifically, two CPW bandpass filters centered at about 40GHz with compact sizes and good selectivity are implemented and carefully examined.