Compact wideband balanced bandpass filter with high common-mode suppression based on cascade parallel coupled lines

A novel differential bandpass filter based on the cascaded parallel coupled lines is proposed, which takes the features of wide passband, high common-mode suppression and compact size. Six transmission zeros are created and located at the frequency range of 0-2f₀(f₀ is centre frequency of the passband), which helps realise the wide passband performances. The balanced bandpass filter consists of the four-stage parallel coupled lines, and the even- and odd- mode characteristic impedances can be adjusted to achieve the desired bandwidth of the passband while the positions of four transmission zeros can be conveniently adjusted by changing the impedance ratio of the asymmetrical stepped impedance resonator (SIR) structures in the proposed filter. By applying the differential and common modes analysis method, the two-port bisections of the filter become a bandpass and all-stop filter topologies under differential- and common-mode excitations, respectively. For performance demonstration of the proposed differential filter, a prototype operating at the frequency band of 2.15-3.85 GHz (with fractional bandwidth of 56.7%) was designed and fabricated on a commercial printed circuit board (PCB) substrate. The insertion loss and return losses of the differential-mode filter are <2 dB and >15 dB in the passband and the common-mode suppression is higher than 30 dB. Good agreements between the simulated and measured results have been observed .