Dual-wideband bandpass filters with extended stopband based on coupled-line and coupled three-line resonators

Coupled-line and coupled three-line resonators are pro- posed to design dual-wideband bandpass filters. Compared with the shorted and open stubs shunt at the same locations of the main line, in addition to saving the circuit area, these resonators provide alter- native ways to the design of dual-wideband filters, with larger possible bandwidths and different frequency ratio of the two center passbands. The geometric parameters of the coupled-line and the coupled three- line structures are determined by deriving their equivalent circuits to a shunt open stub in parallel connection with a shunt shorted stub. To extend the upper stopband, a cross-shaped admittance inverter is devised to play the role of the 90-degree transmission line section at the center frequency and to create transmission zeros at the spurious passbands, so that the upper stopband of the filter can be extended. It is a quarter-wave section with two open stubs of unequal lengths shunt at its center. For demonstration, two dual-wideband bandpass filters operating at 900/1575MHz and 900/2000MHz are fabricated and measured. Measured results of the experimental circuits show good agreement with simulated responses.