Fabrication-Tolerant Microstrip Quarter-Wave Stepped-Impedance Resonator Filter

The etching error causes serious frequency drift of a microstrip stepped-impedance resonator (SIR). This paper proposes a novel microstrip quarter-wave SIR structure, which is insensitive to fabrication tolerances. Inserting several ground strips in the low-impedance section of the resonator keeps the impedance ratio almost constant in spite of inaccurate fabrication. As an additional benefit, the resonator size would be miniaturized because the characteristic impedance of the microstrip line in the low-impedance section is mainly controlled by the signal strips and the inserted ground strips in a coplanar manner. The concepts and characteristics of this modified resonator structure are discussed in detail. The filters constructed by the conventional microstrip SIR were also designed and fabricated at the same frequency band to compare with the proposed ones. Measured results show that each proposed filter has compact size, wide stopband, and high tolerance to fabrication errors. Two four-pole cross-coupled filters were designed and fabricated to demonstrate the feasibility.