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Harmonic Suppression in a Folded Hairpin-Line Cross-Coupled Bandpass Filter by using Spur-Line

The design of a compact bandpass filter using cross-coupled half-wavelength open-loop microstrip hairpin resonators with improved stopband skirt characteristics and spurious harmonic suppression is presented in this article. The proposed filter has a fractional bandwidth of 5% subject to the design frequency of 2.5 GHz for WLAN applications. At first, a fourth-order compact hairpin-line bandpass filter has been designed by arranging the doubly folded hairpin-line cells symmetrically in a cross-coupled configuration. Accordingly, a size reduction of 11.36% over folded hairpin-line filter has been achieved with an improvement in the skirt characteristics by placing two deep transmission zeros at the edges of the passband. However, the benefits of cross-coupled configuration have been overshadowed by the presence of spurious harmonics with an attenuation level of 10 dB. Subsequently, spur-line has been employed in each coupled section of adjacent folded hairpin-line cells for achieving modal phase velocity compensation. As a result, an extended stopband with a rejection level of 35 dB up to 3.16f0 and overall size reduction of 20.46% has been achieved.