A dual-band bandstop filter (DBBSF) with a dual-mode loop resonator is presented. In order to allow the DBBSF to exhibit high skirt selectivity and to generate a wide rejection bandwidth a capacitive coupled-line section is introduced. By properly controlling the length of the coupled-line section and the cross-coupled capacitor, more than two finite transmission zeros were inserted at the stopband and enhanced the rejection bandwidth.Introduction: A high performance bandstop filter (BSF) is one of the most important components of microwave circuits. To meet the size requirements of modern microwave communications systems, BSFs with low insertion loss, sharp rejection and wide rejection bandwidth are in high demand. A microstrip dual-mode loop resonator BSF has been developed for the aforementioned requirements [1,2]. In recent years, various topologies for microstrip dual-mode loop-resonators (DMLRs) have been reported to elucidate the mechanism of coupling between the two degenerated modes, which is mainly determined by the feed-line structure or different types of perturbation [1][2][3][4][5]. In [3][4][5], dual-mode BSFs have been illustrated to obtain high selectivity, low insertion loss and good stopband attenuation. However, the loopresonator coupled to a thru-line is suitable for the design of narrowband BSFs. In this Letter, in order to achieve size reduction and a wide rejection bandwidth, a dual-mode BSF with a cross-coupled capacitor is proposed and a frequency-selecting coupling structure (FSCS) is implemented [6,7]. The basic FSCS is realised by using a capacitive coupled-line section between microstrip feeding lines. The rejection bandwidth is extended by the FSCS and the design of the DBBSF. Moreover, adjusting the length of the capacitive coupledlines can determine the centre frequency of the first stopband and produce three transmission zeros (TZs) which effectively enhance the rejection bandwidth. The proposed DBBSF achieves the stopband frequency at 2.1 GHz with over 100% fractional bandwidth. More than 20 dB rejection is measured from 1.1 to 3.4 GHz.
A compact band selection filter (BSF) is implemented in a 0.18-μm CMOS technology. The structure consists of a pair of symmetric slow wave anti-coupled line and a shunted transistor. When the transistor is in the off-state, the BSF can work as a low-pass filter. The measured insertion loss is 3 dB, and the return loss is better than 9 dB from DC to 10 GHz. When the transistor is in the on-state, the BSF can work as a bandpass filter. The measured insertion loss is 3 dB, and the return loss is better than 10 dB from 14 to 27 GHz. Keywords: band selection filter, CMOS, lowpass filter, bandpass filter Classification: Microwave and millimeter wave devices, circuits, and systems
References[1] C. Lugo and J. Papapolymerou, "Dual mode reconfigurable filter with asymmetrical transmission zeros and center frequency control," IEEE Microw
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