Compct UWB bandpass filter using microstrip and coplanar waveguide back-to-back coupled structure

A compact ultra-wideband (UWB) bandpass filter (BPF) using microstrip and coplanar waveguide (CPW) back-to-back coupled structure is proposed. First, the detached-mode resonator on the CPW layer is formed to allocate three resonant frequencies to meet the concern of UWB operation. Then, microstrip and CPW back-to-back coupled structure is introduced to achieve the UWB limit. To further improve the stopband performance of the proposed UWB BPF, the periodic serration resonator (PSR) on the microstrip layer is employed. The upper stopband bandwidth could be finely tuned by adjusting the coupled area of the PSRs. To verify the feasibility of the mechanism above, a filter is fabricated and tested. The measured results exhibit good performance including a small insertion loss less than 0.52 dB, good return loss below -18 dB in the midband. Specifically, the upper stopband is observed from 11.5 to 17.3 GHz, and a stopband rejection level below -21 dB.