Synthesis of Wideband High-Selectivity Bandpass Filter Based on Tri-Mode Lumped-Element Resonators in Heterogeneous LTCC Substrate

In this article, we propose a synthesis method for wideband high-selectivity bandpass filters (BPFs) based on the tri-mode lumped-element resonator in heterogeneous low-temperature co-fired ceramic (LTCC) substrates. The tri-mode lumped-element resonator is analyzed to generate three resonances and two controllable transmission zeros without using cross couplings, showing a simple topology. A Remez-like numerical iterative algorithm is adopted to construct the polynomials of the tri-mode resonator in the bandpass domain, which can eliminate the need for complex transmission-function polynomial derivations and high-order iterations, allowing direct calculation of lumped-element values by the polynomials without optimization. Furthermore, the extracted-pole technique is used to expand the filter design. A comprehensive synthesis workflow is proposed for high-order filtering responses by integrating the extracted-pole technique and the tri-mode lumped-element resonator. The resultant filter is capable of achieving the desired responses with a wide bandwidth and multiple transmission zeros in a simple and practically achievable topology. For demonstration, a BPF covering 5G-n77 band (3.3–4.2 GHz) is implemented using a heterogeneous LTCC substrate. The measured result shows a low loss of 0.76dB and the circuit size is only$1.3\times 1.09\times 0.53$mm3(or$0.016\lambda _{{0}} \times 0.014\lambda _{{0}} \times 0.007\lambda _{{0}}$), which is suitable for 5G applications.