Advanced Design and Analytical Modeling of Multiband Bandpass Filters Using Interconnected Multimode Resonators and Multi-level Impedance Structures

This paper presents a compact topology for implementing multiband bandpass filters. The design uses interconnected multimode resonators (MMRs) and multi-level impedance structures to achieve a specific frequency response. This approach simplifies the design of quadruple bandpass filters for 4G and 5G applications. Since line widths cannot be adjusted post-construction, resonance positions require tuning. To evaluate the filter design process, a prototype incorporating MMRs was designed, manufactured, and analyzed, demonstrating close alignment between analytical predictions and experimental measurements, despite no simulation or optimization being performed during the design phase, except for result verification.Additionally, a design criterion is established to facilitate the rapid and reliable synthesis of multiband responses by varying only a few geometrical parameters of the MMRs. A simulation of this structure was conducted using CST software to confirm the proposed theory's accuracy. A reverse-biased varactor diode, which functions as a capacitor with specific admittance, is utilized to provide the necessary tuning capability. The paper also highlights the impact of the varactor diode's admittance on resonance location adjustments. To validate the design, the authors present a fabricated prototype of the proposed filter, which features quarter bands at 1.8, 2.1, 2.7, and 3.4 GHz, achieving an attenuations greater than -15 dB. The quarter band filter is primarily used in wireless telecommunications networks. Due to their specialized design, these filters can process multiple frequency bands simultaneously, enhancing communication quality and increasing network capacity in crowded and interference-prone environments.