A design strategy of a bandpass filter (BPF) with sharp roll-off and improved stopband performances is presented in this paper. The proposed filter prototype consists of a couple of parallel-coupled stepped-impedance resonators (SIRs) and two short-circuited coupled lines (SCCLs). The theoretical analysis and numerical simulations on the parallel-coupled SIRs and coupling structure are carried out. Furthermore, a Butterworth BPF is studied and designed first. By loading the SCCL at the I/O ports of the Chebyshev BPF, four transmission zeros can be introduced beside the passband, which improves the selectivity and stopband performances significantly. For demonstration, a BPF prototype with the center frequency of 2.02 GHz and 3-dB fractional bandwidth of 5.2% is designed, fabricated, and measured. Moreover, the measured insertion loss is less than 1.5 dB and the rejection levels are larger than 40 dB in both lower and upper stopbands. Thus, the measurement results verify the feasibility of the design. K E Y W O R D S bandpass filter, high selectivity, short-circuited coupled line (SCCL), transmission zeros 1 | INTRODUCTIONWith the rapid development of wireless communication systems, filters with sharp roll-off, wide stopband, and deep suppression are highly desired. 1,2 Many different filter design methods have been proposed, such as using multimode resonators, 3 substrate-integrated waveguide, 4 and filter synthesis. 5 Because the parallel coupled line filter has the advantages of simple structure, easy design and implementation, reliable performance, and easy integration, it has been widely used in microwave system. 6,7 However, the selectivity and stopband performance are not so good, because there are no transmission zeros (TZs) besides the passband. Filters with high selectivity are in great demand because of the scarcity of the electromagnetic spectrum resources. Common methods for improving the filter frequency selectivity mainly include increasing the filter order and introducing finite frequency TZs. 8,9 Increasing the order of the filter results in increased insertion loss and circuit size. Therefore, introducing TZs as a better way to enhance filter selectivity has been extensively studied. [10][11][12][13][14][15][16][17] In Reference 10, multiple TZs can be obtained by a compact parallel-coupled transmission line sections which are connected at their both ends. Four TZs are introduced by multiple capacitive cross-couplings in Reference 11, and the locations of the TZs can be adjusted by changing the values of cross-coupled capacitances. In Reference 12, by properly selecting the length of the stubs and their positions, extra TZs are created in the stopband before the spurious passband. Moreover, source-load coupling structure has been employed to introduce TZs. 13,14 In Reference 13, two TZs generated by source-load coupling structure were applied to suppress the first and second spurious harmonics. In addition, two TZs close to the cut-off frequencies which were introduced by the source-load coupling...
