Miniaturize Broadband Coplanar Stripline Bandpass Filter Based on Cross-coupled Resonances

This article proposed three miniaturized double stepped impedance broadband filters using a coplanar stripline (CPS) cross-couple resonances and are studied. The proposed filters utilize cross-coupled resonances in a way that the high-impedance transmission line fragments are inserted between two lower-impedance parts, which delivers an increased impedance ratio. The first filter is based on capacitive coupling between two high-impedance resonators and a two-pole cross-coupled system, which is enhanced to the high-impedance line by strip coupling. In the second filter, to improve the selectivity of the lower cut-off of the filter, additional half arc is added to obtain an additional attenuation pole by tri cross-coupled technique. Finally, in the third proposed filter design to enhance the cut-off selectivity of the upper and lower end, a shorting two-arc strip is introduced on both sides of the high impedance line, which exhibits three poles. The filters give a linear phase response at the pass band resulting in a flat group delay. The performances of the three proposed filters are scrutinized and matched in terms of bandwidth, return loss, and pass-band insertion loss. The proposed designed filters show a fractional bandwidth (FBW) of 41%, which is lesser than the first and second filters offering FBW of 55% and 42%, respectively. The return loss minima of the proposed filter and other two filters are -22.4 dB, -15.4 dB and -18.4 dB at central frequency of 4.1, 3.5 and 3.8 GHz, respectively. Moreover, the proposed filter gives a pass-band insertion loss of -0.35 dB, whereas the first and second filters offer the insertion loss of -0.45 dB and -0.44 dB, respectively. The equivalent lumped-element circuits of each band pass filter are derived and presented. The realized filter is designed and fabricated on Rogers 5880 substrate of size
$20 \text{mm }\times 10$
mm, which is smaller than other planar fitters with same cut-off and attenuation characteristics. The proposed filter can be used in various multi-frequency systems owing to its compact size. The measured and simulated findings of the proposed CPS cross-couple resonances broadband bandpass filters are substantially consistent.