Ultra-wideband bandpass filter using hybrid structure of CPW and microstrip

Abstract

A novel ultra-wideband BPF using hybrid CPW and microstrip structure is proposed in this paper. The middle microstrip is replaced by an interdigital coupled-line to generate a transmission zero at low frequency. As a result, the selectivity is significantly improved. Moreover, the coupling short stubs(CSS) are used to further improve its selectivity and out-of-band performances. The measured results show that the proposed filter possesses satisfactory out-of-band and in-band performances such as sharp roll-off, compact size(0.49 λg×0.47 λg), low insertion loss, and wide upper stopband. The proposed filter has wide fractional bandwidth of 123%(2.75～11.55 GHz), and a return loss of better than 15 dB is achieved. Besides, the measured results of fabricated filter agree well with the simulation.

Keywords

• ultra-wideband
• bandpass filter
• CPW-microstrip transition
• coupling short stubs
• wide stopband