A Microstrip Linear-Phase BPF Using Dual-Band Negative Group Delay Equalizers

A bandpass filter (BPF) using dual-band negative group delay equalizers (NGDEs) for extending linear-phase bandwidth is reported in this letter. This dual-band NGDE is synthesized based on a coupling-routing-diagram approach and implemented by a parallel-coupled three-line structure. By cascading two NGDEs to the input and output ports of a conventional BPF, the dual-band NGD characteristic associated with NGDEs is applied to counteract the group delay (GD) peaks at the transition bands of the conventional BPF. A microstrip linear-phase BPF prototype in the <tex-math notation="LaTeX">S </tex-math> -band is designed. Compared with the conventional BPF, the GD fluctuation of the equalized BPF is decreased by 75% from 0.4 to 0.1 ns within the passband of 1.50–2.50 GHz, while no obvious degradation of out-of-band suppression is observed and the overall time delay is not increased. The devised BPF achieves a linear-phase characteristic in the whole passband with a compact circuit size of <tex-math notation="LaTeX">0.32~λ g² </tex-math> .