Tri-/Quad-/Quint-Band Bandpass Filters Based on Transversal Embedded Asymmetrical Square-Ring Resonator

This paper presents a series design method for tri-/quad-/and quint band bandpass filter (BPF) with multiple transmission zeros (TZs) based on an embedded asymmetrical square-ring resonator (ASRR). Firstly, we proposed a series of tri-/quad-/and quint-mode asymmetrical square-ring resonators which resonant modes are demonstrated by employing the multi-path matrix coupling analysis method. Further, we nested two identical ASRRs to construct multiple passbands and introduced multiple transmission zeros between frequency bands based on transversal signal interference concepts. Finally, tri-/quad-/quint-band BPFs are designed according to the proposed design principle using the proposed typical basic structure ASRR. The design, fabrication, and measurement of the three BPFs are conducted to validate their feasibility. The measured results consistently match the theoretical predictions, demonstrating superior performance, such as independent and controllable frequency band characteristics and abundant transmission zeros, while having promising potential to further expand frequency band designs.