Abstract
In this article, the differential inductor, widely used as an inductive element, is modified as a resonator. This differential drive inductor resonator (DDIR) can achieve miniature size and extension of second-order mode based on the different electric field distributions on different modes. The second- and fourth-order bandpass filters (BPFs) are designed and fabricated with DDIRs at the promising 5G frequency band of 700-MHz spectrum. The low-loss platform substrate integrated suspended line (SISL) is adopted. The filters are centered at 0.76 GHz/ 0.77 GHz. The footprint areas are only 0.0023 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.0052\lambda g^{2}$ </tex-math></inline-formula> without using any lumped elements or substrate with high permittivity. The second-order mode is 5.7/3.92 times the working band without introducing any extra method to broaden the stopband. To improve the rejection of stopband, a T-shaped coupling structure is first proposed on SISL to get the desired location of zeros by weakening the electric coupling and enhancing the magnetic coupling. The comparable version of second- and fourth-order BPFs with T-shaped coupling structure are designed and fabricated. The 20-dB roll-off rate has been decreased by 58.8% and 66.1%.
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