In this paper, a compact ultra-wideband bandpass filter using a back to back Coplanar Waveguide (CPW)-to-microstrip transition structure is proposed. Compared with traditional ultra-wideband bandpass filters using hybrid structures, the proposed filter has a sharper selectivity because of two transmission zeros located at the lower and upper edges of the passband, respectively, generated by a modified T-shaped stub. Moreover, to further improve its out-of-band performance open stubs are introduced to produce extra transmission zeros at high frequency. A prototype of the proposed filter is fabricated and measured. The results show that the proposed filter achieves a bandwidth of 133% from 2.4 to 11.9 GHz, and the selectivity (skirt factor) is optimized from 50% to 92% compared to a former ultra-wideband (UWB) bandpass filter (BPF) with a similar structure. Besides, the proposed filter can offer some other advantages such as good return loss, low insertion loss, stable group delay, and compact size (16×7.55mm2). This filter can be a good candidate for UWB applications.
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