Low-Loss Microstrip Tri-Band Differential Bandpass Filters Using a Non-Edge-Coupled Structure

In this paper, a novel differential tri-band bandpass filter with a low-loss characteristic and high selectivity is proposed. The low-loss feature is attributed to the non-coupled structure, which circumvents the additional radiation losses from coupling slots. Furthermore, the excellent isolation and significantly enhanced selectivity between passbands are achieved via the inherent transmission zeros among them. Three desirable differential operating passbands can be conveniently allocated by adjusting the impedance ratios of the tri-section stepped impedance resonators. Consequently, the proposed filter design demonstrates a straightforward and efficient design methodology. To validate the feasibility of this approach, a differential tri-band bandpass with passbands at 1.35 GHz, 4.5 GHz, and 7.6 GHz was constructed and experimentally verified. The measured minimum insertion losses were 0.15 dB, 0.5 dB, and 1.2 dB, respectively, indicating high performance. Specifically, the roll-off rates of the lower and upper edges of the three passbands are as follows: for the first passband, 30 dB/GHz and 23 dB/GHz; for the second passband, 27 dB/GHz and 25 dB/GHz; and for the third passband, 24 dB/GHz and 29 dB/GHz. The achieved concordance between simulated and measured results confirms the practicality and viability of this design for advanced communication systems. © 2025, Electromagnetics Academy. All rights reserved.