A novel UWB filter using a new type of microstrip double-ring resonators

An ultra wideband bandpass filter (BPF) with sharp-rejection is proposed by using a new structure of double ring resonators. The double ring resonator generates a couple of attenuation poles, and its operation mechanism is made clear through the even- and odd-mode analysis of the circuit. The stopband of the filter can be significantly widened by cascading the double ring resonators. A two-stage microstrip BPF with a 3-dB fractional bandwidth of 80% is designed, fabricated and tested. The measured frequency response of the filter validates well the theoretical analysis and electromagnetic simulation.