In this paper, a miniaturized wideband bandpass filter utilizing a square ring resonator and loaded open-stubs is proposed. One pair of bent open-stubs characterized as perturbations is installed outside the diagonal corners of the ring, and another pair of loaded open-stubs is added inside the ring resonator. By stretching the perturbation stubs more than half-wavelength of the ring, three pairs of degenerated modes in a ring are split for wideband operation. The first two split modes form the dominant passband. Meanwhile, the loading effect introduced by the loaded open-stubs could move the third split mode into the dominant passband. At the same time, an additional transmission zero is generated by the loaded openstub, which improves the skirt selectivity. Due to the applying of the perturbation stubs as long as more than half-wavelength, this kind of wideband microstrip ring resonator filter occupies a smaller size than those conventional ones that based on ring resonator. To verify the mechanism above, a wideband bandpass filter centered at 3.5 GHz is designed, implemented, and fabricated. Measured results of experimental circuit show good agreement with simulated responses.

Abstract-A compact ultra-wideband (UWB) bandpass filter (BPF) with simultaneous narrow notched band and out-of-band performance improvement is presented. The UWB BPF is built up using the hybrid microstrip and coplanar waveguide (CPW) structure. By employing the split-ring resonator (SRR) which is defected on the lower plane, the narrow notched band was introduced. The center and bandwidth of the notched band can be controlled by adjusting the length and width of the SRRs. A novel cross-shape patch is constructed to implement transmission stopband in the upper out-of-band so as to suppress the spurious passband. The novel UWB BPF is designed and fabricated. The measured insertion loss is less than 1.6 dB, the return loss is more than 13 dB and the variation of group delay is less than 0.12 ns. The width of narrow notched band is about 0.15 GHz and the attenuation is 18 dB at the center frequency of 5.76 GHz. The upper stopband is up to 15.2 GHz with rejection greater than 20 dB.

Abstract-A novel microstrip tri-band bandpass filter is proposed and implemented using hybrid resonator with independently controllable center frequencies and good in-between isolation. This hybrid resonator is constructed by a stepped-impedance stub resonator and a single end shorted resonator. The stepped-impedance stub resonators are applied to achieve the first and second passband, while the third passband is implemented by single end shorted resonators. By applying the even-odd mode approach, the resonance frequency ratio between even mode and odd mode inside the stepped-impedance stub resonators is attained. Furthermore, the filter with multi-path coupling structure can generate the transmission zeros at the edge of the passband, which can effectively improve the filter passband selectivity. Finally, a tri-band filter operating at 1.91, 2.73, and 3.45 GHz is designed and fabricated. The measurement results accord well with the full-wave electromagnetic designed responses.

Abstract-In this paper, a compact microstrip wideband bandpass filter (BPF) based on square ring loaded resonator (SRLR) is proposed. The SRLR is formed by loading a pair of bent open-stubs outside the diagonal corners of a square ring, which generates three split degenerated modes. The first two split modes form a dominant wideband passband. By introducing another pair of loaded open-stubs, the third split mode is moved into the passband to achieve an extra bandwidth for the wideband passband. Measured results show that this proposed BPF has a 3 dB fractional bandwidth of 69%, and the insertion loss of the BPF is less than 1.0 dB.