In this paper, a novel terahertz tunable bandstop filter with constant absolute bandwidth is proposed, which consists of graphene-based three-section L resonators. In order to perform bandstop property, the L resonator is used and analyzed in details based on the traditional Z matrix and ABCD matrix. With the introduction of graphene materials, the operating frequency of bandstop filter can be extended to terahertz. Moreover, the tunable performance with constant absolute bandwidth can be achieved by only loading different chemical potentials on a graphene surface. For demonstration, a terahertz tunable bandstop filter prototype is designed and simulated with chemical potentials of 0.1, 0.3, and 1 eV. The simulated results agree well with the anticipation perfectly.
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