A High Performance Bandpass Filter with a Wide and Deep Stopband by Using Square Stepped Impedance Resonators

In this paper, a microstrip bandpass filter with a wide and deep stopband by using the modified stepped-impedance resonator (SIR) was designed and implemented. The modified SIR is formed by properly folding the conventional SIR to be a square open-loop SIR. By analyzing the admittance of the even and odd mode, the transmission zero of the modified SIR can be easily found, thus having a wide and deep stopband simultaneously. Bandpass filter (BPF) can have high performances with a wide and deep stopband as well as good selectivity by using the proposed SIR, compared to the conventional SIR. A filter example centered at f 0 of 1.26 GHz has been implemented to verify the design concept. The results showed that the |S 21| was greater than 65 dB at the spurious response of 2f 0 and a wide stopband with a rejection level over 50 dB is extended from 1.5f 0 to 3.5f 0. Moreover, experimental results have a good agreement with the simulated results.

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