Very Wideband, Compact Microstrip Bandstop Filter Covering S-Band to Ku-Band

This paper reports a wide bandwidth planar bandstop filter with improved RF characteristics. The proposed filter on alumina is realized incorporating tapped open stub along with spurline topology. Further, stepped impedance resonator (SIR) approach has been introduced in the tapped stubs to achieve wider band performance with improved selectivity. The proposed topology effectively controls the transmission poles. Fabrication of this BSF has been carried out on glass substrate showing minimal effect of permittivity variation on bandwidth performance. This validates the applied approach with achievable bandwidth of more than 100% ranging from S- to Ku-band. Close agreement with simulation and practical results have been demonstrated with measured insertion loss of less than 1？dB and attenuation loss better than 30？dB at C-band. 1. Introduction Band stop filters find applications in oscillator and mixers to remove higher-order harmonics and other unwanted spurious signals. Duplexers and switches are comprised of BSF for filtering out unwanted signal along as they can interfere with the desired signals. Conventional methods to implement bandstop filters involve use of shunt stubs or stepped-impedance microstrip lines with large circuit size [1]. To reduce filter area, certain slow-wave structures, such as open-loop resonators, are widely adopted [2]. These traditional BSFs are normally having the narrow stop band response. As demand for wider stop band is gaining popularity alternative structures like photonic band gap (PBG) electronic bandgap (EBG), and the defected ground plane (DGS) are explored to cater the demand. Further to enhance the stop bandwidth, use of four or more cells of above-mentioned topologies is needed. However, this leads to a larger size and more transmission losses in the stop band. Alternatively, EBG and DGS require etching process on the backside ground plane in addition to position calibration using costlier lithographic techniques. Further its turn-around time is high and makes it incompatible to match with other topologies in the overall system configuration. Liu et al. [3] proposed spurline with cross-junction open stubs to have wider bandwidth with small size, but selectivity at one frequency end is compromised to cater for wider bandwidth. All the reported circuits are restricted to lower end of frequency where the effects of losses are not prominent, making it easier to analyze. Wider bandwidth topology at higher frequencies as reported by Hsieh and Wang [4] covers 2.3？GHz to 9.5？GHz range but has limited bandwidth. Wang and