A miniaturized multilayer tunable super wideband (SWB) bandpass filter (BPF) is presented based on a microstrip structure. A pair of transmission line is coupled with the aid of three defected ground structures (DGS) at ground to improve the coupling and provide ultra wide band pass response. One of the transmission line is placed at the top plane of the upper layer, and the other transmission line is at bottom plane of the lower layer with defected microstrip structures (DMS) to improve the return loss. Bandwidth can be tuned by properly selecting the resonator size. Circuit model for the microstrip resonator and mathematical analysis are given and studied. Finally, the proposed vertical connection with slotline structures and a three pole UWB filter is designed, simulated, fabricated, and the results are well vindicated by an exemplary circuit centered at 6.5 GHz with the measured fractional bandwidth (FBW) of 135%. The filter exhibits a constant group delay of 0.3 ns in the pass band and the size of the resonator is 13.67 mm×17.58 mm×3.2 mm.
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