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In this letter, a unique type of balanced bandpass filter (BPF) with high selectivity and tunable attenuation is proposed through the comprehensive design of a microstrip balanced BPF and graphene nanoplates. In order to achieve the passband response in differential-mode (DM) excitation and produce a wide and deep stopband in common-mode (CM) excitation, the balanced couple of lines are used in balanced BPF. The graphene nanoplates are loaded on the open ends of λ/4 stepped-impedance resonators to dissipate the electromagnetic field. By raising the bias voltage applied to the graphene nanoplates, the values of the graphene's surface impedance can be adjusted from high to low.The insertion loss of the balanced BPF can be indirectly tuned while maintaining the return loss at a comparatively low level. A prototype of the proposed gain tunable balanced BPF is fabricated and measured. The measured resultsshow that a tuning range of insertion loss from 1.75 to 10.7 dB can be obtained for the DM circuit over the operating bandwidth centered at 1.84 GHz, without interrupting the signal rejection of the CM circuit.

In this paper, a multi-reconfigurable filter which consists of three identical varactor-graphene-loaded coaxial substrate integrated waveguide (SIW) resonators is presented. The center frequency and transmission amplitude of the proposed filter is independently reconfigurable by means of electrically controlling the values of the varactor capacitance and the graphene resistance, respectively. Tuners and their bias circuits are mounted on the surfaces of the SIW resonator to adjust the tuning of resonant frequency and unloaded quality factor, which enable to realize the frequency and amplitude of the presented multi-reconfigurable filter. In addition, the proposed filter applies a cascaded triplet topology to generate a transmission zero at the upper side in order to enhance skirt selectivity and out-of-band rejection. The circular shape of the resonator provides great flexibility and produces an additional compactness of the filter. The proposed multi-reconfigurable coaxial SIW filter is designed and fabricated, with high-Q varactors and graphene thin sheets as frequency and amplitude tuning elements, respectively, exhibiting the following reconfigurable characteristics: 1) a wide tuning range of 0.58-1.01 GHz with a constant 1-dB fractional bandwidth (FBW) of 2±0.2%. 2) a continuous tuning range of amplitude at any center frequency in the achievable frequency band. For instance, the proposed filter can be tuned with an amplitude range of 3.02-19.8 dB by varying the value of the graphene sheet resistance at 1.01 GH. 3) free combination of the arbitrary center frequency and attenuation level. The proposed filter with tunable filtering and attenuating responses has widely potential in reconfigurable wireless communication systems and radar systems due to its high integration and versatility.