Compact High-Selectivity Wide Stopband Microstrip Cross-Coupled Bandpass Filter With Spurline

The article presents the design of a compact narrowband microstrip cross-coupled bandpass filter with improved selectivity and a wide stopband. The proposed fourth-order quasi-elliptic filter is designed at 2.5 GHz with a fractional bandwidth of 4% suitable for WLAN applications. At first, doubly-folded half-wavelength hairpin lines have been arranged symmetrically in a cross-coupled configuration combining the electric, magnetic, and mixed-coupling. Accordingly, a size reduction of 17% over the folded inline hairpin-line filter with the same specifications has been achieved. Moreover, the selectivity has been improved greatly by the introduction of two deep transmission zeros with an attenuation level of 48 dB at the edges of the passband. However, the presence of the spurious harmonics with an attenuation level of 10 dB limits the performance of the filter related to the stopband rejection. As a remedy, conventional and meander spurlines have been incorporated in each hybrid coupled section of adjacent cross-coupled cells for achieving the modal phase velocity compensation. Accordingly, an extended stopband with a rejection level of 38 dB up to
$4f_{0}$
has been recorded by using a meander spurline and an overall size reduction of 33% has been achieved.