Self-equalized linear-phase microstrip bandpass filter based on negative group delay parallel-coupled three-line units

A self-equalized linear-phase microstrip bandpass filter (BPF) utilizing negative group delay (NGD) parallel-coupled three-line (PCTL) units is presented in this letter. The NGD PCTL unit possesses an NGD valley in its frequency response of group delay (GD). By a combination of two or more NGD valleys, the sharp positive GD peaks at passband edges of a conventional coupled-line BPF are counterbalanced, resulting in a linear phase response over the entire passband. A prototype of linear-phase BPF in S-band is designed by replacing two coupled-line units in a three-pole conventional BPF with the proposed NGD PCTL units. Measured passband GD fluctuation of the BPF utilizing NGD PCTL units is 0.5 ns while that of the BPF using conventional coupled-line units is 1.7 ns, a considerable improvement of 70.5% is achieved with no increase in overall time delay. The out-of-band suppression is well retained, and the circuit size is 0.18 lambda g2.