Varactor–Graphene-Based Bandpass Filter With Independently Tunable Characteristics of Frequency and Amplitude

This article presents a frequency–amplitude independently tunable bandpass filter (BPF). The filter uses an electrically tunable coaxial substrate integrated waveguide (SIW) resonator, which has a compact size and a constant fractional bandwidth (FBW). The surface-gap coaxial SIW resonator is loaded with eight varactors and a thin graphene sheet at the top and bottom, respectively, which results in possessing independent tuning characteristics of frequency and quality factors. The variable capacitance values of the varactor are used to achieve frequency reconfigurability, and the graphene located on the bottom surface of the resonator is equivalent to a tunable lumped resistor used to control the $Q$ -factor. In addition, detailed steps of BPF design are provided. For validation, a prototype third-order varactor–graphene-loaded multitunable coaxial SIW BPF is fabricated. The measurements show that within the tunable range of the varactors, the proposed filter can be tuned from 0.68 to 1.03 GHz with 5% ± 0.2% 1-dB FBW and the insertion loss of passband can be continuously tuned over 2.68–6.37 dB. The frequency tuning and amplitude tuning functions are not dependent on each other. Over the frequency tuning range, the transmission amplitude at an arbitrary center frequency can be dynamically tuned by controlling the bias voltage applied to the graphene.