Design and Realization of a Tunable Microstrip Bandstop Filter based on Characteristic-Polynomial Transformation

Abstract

Rapid advances in communication systems have increased the demand for efficient spectral allocation and interference mitigation techniques. Among the available solutions, frequency-agile filters are especially attractive because they allow the tuning of key response characteristics such as center frequency and filter shape. Although substantial research has been dedicated to tunable filters, achieving a comprehensive and practical design remains challenging due to limitations in complexity, implementation, and physical size. Multiple tuning mechanisms have been reported for tunable filters, with mechanical and electronic tuning being among the most common. Electronic tuning can be realized through elements such as p-i-n diodes, varactor diodes, and radio-frequency (RF) microelectromechanical systems (MEMS) switches. These devices can be integrated into microstrip structures, making microstrip technology a practical platform for the implementation of reconfigurable filters. In this regard, tunable bandstop filters with rejection bandwidths broader than those of conventional notch responses remain less commonly reported than their bandpass counterparts, particularly when a synthesis-based design approach is pursued. This makes their study especially relevant for applications requiring increased rejection bandwidth together with reconfigurability. In this work, a narrowband bandstop filter with an enhanced rejection bandwidth is developed from the theoretical foundation of polynomial synthesis. It is shown that by switching the numerators of the characteristic polynomials, a bandstop response can be obtained from an equi-ripple bandpass prototype, resulting in a broader rejection band than that of a typical notch bandstop filter.