Centrally Corrugated Cross-Coupled Wide Stopband Folded Bandpass Filter with Spurline

The present article describes the performance of a fourth-order cross-coupled bandpass filter using double-folded microstrip hairpin resonators for the improvement of skirt characteristics and harmonics suppression. The proposed filter is centered at 2.5 GHz with a fractional bandwidth of 4%. At first, the central folded pair of resonators of the unit hairpin-line cell has been modified with periodic trapezoidal corrugations to achieve improved skirt characteristics with the attenuation level of 40 dB and a compact circuit area with a size reduction of 11.36% over the conventional cross-coupled filter with the same specifications. However, the harmonics have not been suppressed significantly by this compact structure. Subsequently, trapezoidal- shaped meander spurlines have been incorporated in each coupled section of the adjacent folded hairpin-line cells for achieving modal phase velocity compensation. As a result, an extended stopband with a rejection level of 34 dB up to 3.04f0 and an overall size reduction of 20.42 % has been achieved.