Compact cross-coupled coplanar-waveguide bandpass filters without bondwires

Two types of compact cross-coupled coplanar-waveguide (CPW) bandpass filters, which are the dual of each other, are proposed. They are based on the three-line coupled CPW along with lumped-element K- or J-inverters. Due to the symmetrical filter structure, no bondwires or airbridges are required for implementation of the proposed CPW bandpass filters, which greatly simplifies the fabrication process. Their filter sizes are about 50% of the conventional parallel-coupled filter with no spurious passbands at even harmonics. In addition, by introducing the cross-coupling effect, two transmission zeros at the upper and lower stopbands may be created, which improve the filter selectivity. Simple equivalent-circuit models are also established as effective design tools. Specifically, two 4th-order CPW bandpass filters centered at about 2GHz implemented on a FR4 substrate are demonstrated.