High Selectivity Dual-Wideband Balun Filter Utilizing a Multimode T-line Loaded Middle-Shorted CSRR

Abstract

This brief presents a dual-band balun filter based on a novel slotline multi-mode T-line loaded middle-shorted complementary split-ring resonator (TLMS-CSRR) with wide bandwidth and high selectivity. Coupled with ${{\tau }}$ -shaped and ${{\pi }}$ -shaped slotline stub as input and output microstrip-slotline transition structure respectively, high rejection between two passbands and at harmonic frequencies are realized. With the TLMS-CSRR, the proposed balun filter obtains five transmission poles (TPs) and four transmission zeros (TZs) which broaden the bandwidth and sharpen the roll-off skirt significantly. Besides, a high-Q folded slotline coupled with input feeding line is introduced in ${{\tau }}$ -shaped structure to enhance the quality factor of TZ, resulting in more than 16 dB suppression between passbands. The ${{\pi }}$ -shaped slotline structure enhances harmonic rejection by 15 dB@ up to $8.7{f} _{1}$ (2.3 GHz). The fabricated balun filter demonstrates two passbands operating from 1.9 GHz to 2.7 GHz and 4.8 GHz to 5.6 GHz respectively. The measured magnitude imbalances are 0.4 dB and 0.6 dB while phase imbalances are 2.5° and 3°.