Compact Microwave and Millimeter-Wave Bandpass Filters Using LTCC-Based Hybrid Lumped and Distributed Resonators

A novel class of low-temperature cofired ceramic (LTCC) bandpass filters (BPFs) is proposed in this paper, featuring of compact size and good filtering responses in both microwave and millimeter-wave (mm-wave) bands. By effectively merging the lumped elements and distributed effects, a modified LTCC-based hybrid lumped and distributed resonator (HLDR) is introduced. In detail, the lumped part of a LTCC interdigital capacitor and a high-impedance line is used to reduce circuit size, loss, and cross-talks. The distributed part, saying, the mutual couplings between adjacent HLDRs and to the ground, is used to improve the design flexibility and filtering performance for HLDR filters. With mutual couplings, the coupling matrix method can be applied. Furthermore, the frequency-variant mixed couplings between HLDRs are investigated, and coupling matrix method is improved by introducing a new coupling mechanism. Multiple transmission zeroes (TZs) are firstly realized with second-order in-line topology, but without adding cross-couplings or bandstop structures. To validate above design strategies, one S-band and one Ka-band second-order HLDR BPFs are simulated and measured, respectively. Very compact size, low loss, high-selectivity, and wide upper stopband are observed, especially in mm-wave range.