In this paper, a dual passband filter with spoof surface plasmon polaritons (SSPPs) and interdigital capacitance structure loaded on a coplanar waveguide (CPW) is proposed. Firstly, the hourglass-shaped SSPP unit-cell structure and the interdigital capacitor structure are introduced on the coplanar waveguide transmission line to obtain high fractional bandwidth and low insertion loss passband characteristics. Then, a dual passband filter is formed by loading the interdigital capacitor loop resonator to excite the trapped waves. The simulation results show that the proposed dual passband filter has excellent upper sideband rejection and dual passband filtering performance. The fractional bandwidths of the two passbands of the design are 46.8% (1.49-2.40 GHz) and 15.1% (2.98-3.63 GHz), respectively, which can achieve more than -40 dB rejection in the range of 4.77-7.48 GHz. The upper and lower cutoff frequencies of the two passbands can be independently regulated by changing the structural parameters of the proposed filter. In order to gain a deeper understanding of the operating principle of the dual passband filter, the corresponding dispersion curves and electric field distribution, LC equivalent circuit analysis are given. Finally, the prototype of the designed filter is fabricated according to the optimized parameter values. The experimental results are in good agreement with the simulation ones, indicating that the proposed dual-passband filter is of great importance in microwave integrated circuit applications.

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In this paper, a compact broadband bandpass filter with wide out-of-band rejection characteristics based on halberd-shaped spoof surface plasmon polariton (SSPP) is proposed. The filtering structure is achieved by etching a periodic halberd-shaped groove at the bottom of the substrate and a microstrip-to-slot line transition with a crescent-shaped patch at the top. Compared with the traditional dumbbell-shaped SSPP, the halberd-shaped SSPP has good slow-wave property, and the designed bandpass filter based on halberd-shaped SSPP can achieve a more compact size. The upper cutoff frequency and lower cutoff frequency of the passband can be adjusted by regulating the SSPP structure and the transition structure from microstrip-to-slot line, respectively. The simulation results show that the center frequency of broadband bandpass filter is 2.85 GHz, with the relative bandwidth of 130%, and the return loss in the passband is better than –10 dB, and the extreme strong out-of-band rejection of –40 dB from 5.6 GHz to 20 GHz. The size of the broadband bandpass filter is compact, only 1.08<i>λ</i>_g×0.39<i>λ</i>_g, where <i>λ</i>_g is the wavelength at the center frequency. In order to verify the effectiveness of the wideband bandpass filter, the traditional printed circuit board technology is used to fabricate the wideband bandpass filter. The measurement results are in good agreement with the simulation results, verifying the feasibility of the design. The proposed broadband bandpass filter shows promising prospects for developing SSPP functional devices and circuits at microwave frequencies.