Design of a Compact Microstrip Box-Like Bandpass Filter With High-Frequency Selectivity and Excellent Stop-Band Performance

Abstract

This paper presents a compact box-like microstrip bandpass filter. It features high-frequency selectivity and excellent stopband performance. It comprises a half-wavelength stepped-impedance resonator (SIR) and three quarter-wavelength SIRs. The half-wavelength and quarter-wavelength SIRs are hybrid adopted. Therefore, the second-order parasitic passband is suppressed effectively. Simultaneously, through controlling the ratio of the stepped impedance, the third-order resonant frequencies are separated properly, which effectively suppresses the third-order parasitic passband. The filter has a box-like coupling topology, which can introduce one transmission zero, and its position can be located at arbitrary side of the passband. Furthermore, tapped-line is used to accomplish the external coupling, which can introduce two transmission zeros for the so-called zero-feeding structure. The three transmission zeros give the filter a high-frequency selectivity. Additionally, the carefully arranged geometrical layout with box-like coupling topology provides the filter with a miniaturized size. A prototype filter centered at 1.8 GHz, with FBW of 4.0%, is designed, fabricated, and measured. Its structure, operational mechanism, and design steps are discussed in detail. The simulated and measured results are given, and a good agreement between them verifies the effectiveness of the proposed filter and design method.