Compact bandpass filters based on microstrip and coplanar waveguide resonators

A compact bandpass filter using both microstrip and coplanar waveguide (CPW) quarter-wavelength resonators is proposed. By combining the merits of two-layer layout and stepped-impedance resonators, a filter with various degree of couplings and reduced size may be realized. Moreover, due to the higher spurious resonant frequencies of stepped-impedance CPW resonators, the filter with wide stopband may be achieved. In this study, a compact fourth-order filter composed of two microstrip and two CPW quarter-wavelength resonators is demonstrated and fabricated on the FR4 substrate.