Design of multiple transmission zeros-enabled compact broadband BPFs based on microstrip-to-CPW transition technology

In this paper, we present a miniaturized ultra-wideband (UWB) bandpass filter (BPF) with multiple transmission zeros (TZs), which is based on transition technology of microstrip with short-circuited coplanar waveguide (CPW). The ground plane of the BPF contains a multiple mode resonator (MMR)-based CPW which is capacitively linked through the dielectric to two open-circuited microstrip lines on the top. The MMR is initially designed to allocate its lowest three resonant modes quasi-equally inside the designated UWB spectrum (3.1–10.6 GHz). This is followed by optimization of microstrip lines to provide a good broadband response possessing minimum insertion loss, two TZs at the lower and upper passband edges that improve selectivity and a wide stopband with appreciable attenuation. Later, multiple-folded split ring resonators are coupled to the BPF to inject dual passband TZs. The predicted theory in simulation is verified against measured result and is found to be in good agreement. The prototype covers a substrate area of only 14.6 × 9.2 mm2.