Compact coplanar waveguide pseudo-elliptic filter at microwave frequencies

Microwave filter with pseudo-elliptic response is realized using coplanar waveguide technology. Based on the quarter-wavelength side-coupled ring topology, the filter presents a second order bandpass response with transmission zeros in its out-of-band frequencies. The inter-ground distance of the coplanar waveguide (CPW) lines is set to be 3.4 mm which is about twice the thickness of the substrate while the inter-ground distance for the coupled-sections is equal to 3.71 mm. Both sides of the ground plane of the CPW are short-circuited to eliminate the odd-mode and to allow only the even-mode propagation along the lines. The measured passband response is centered at 2.4 GHz with a relative bandwidth of 12.5% and 3 dB minimum loss. Explanations on the filter design are given in the paper along with its simulation and measurement results.