Design of UWB bandpass filter with dual notched bands

. RESEARCH PAPER. SCIENCE CHINA Information Sciences June 212 Vol. 55 No. 6: 1436 144 doi: 1.17/s11432-12-4554-2 Design of UWB bandpass filter with dual notched bands CHU QingXin & TIAN XuKun School of Electronic and Information Engineering, South China University of Technology, Guangzhou 5164, China, Received January 2, 211; accepted September 3, 211; published online March 14, 212 Abstract An ultra-wideband (UWB) bandpass filter (BPF) with dual sharply rejected notch-bands to eliminate the interference from coexisting radio systems is presented in this paper. The characteristics of the proposed notched structure are investigated using simulations to demonstrate the design flexibility of the notchband performance. After integrating the proposed notched structure into the basic UWB BPF based on a stepped-impedance stub loaded resonator (SISLR), a UWB BPF with dual sharply rejected notched bands at 5.2 and 5.8 GHz is constructed. Finally, the designed filter is built and tested. The simulated and measured results are found to be in good agreement to verify the proposed design. Keywords bandpass filter, bandstop units, notched bands, ultra-wideband Citation Chu Q X, Tian X K. Design of UWB bandpass filter with dual notched bands. Sci China Inf Sci, 212, 55: 1436 144, doi: 1.17/s11432-12-4554-2 1 Introduction Since the release of the frequency band covering 3.1 1.6 GHz by the Federal Communication Commission (FCC) in early 22 [1], the ultra-wideband (UWB) radio system has received a great deal of attention. As one of the key passive components in support of high-performance UWB systems, UWB bandpass filters (BPFs) have been investigated extensively [2 6]. Because existing undesired radio signals such as wireless local-area network (WLAN) signals around 5. GHz, e.g. 5.15 5.35 and 5.725 5.825 GHz bands, may interfere with the UWB range defined by the FCC, a narrow notched band is necessary to reject these signals. Recently, considerable efforts have been made to develop UWB BPFs with single or multiple notched bands using various structures [7 1]. In [7], an embedded open stub was presented that generated a notched band, which may be of a relatively large size. Asymmetrical parallel-coupledline structures were developed in [8,9] for single or double notched band realizations, but they are not convenient for independent control of the notched frequencies and bandwidth. In [1], a notch-band implementation based on a multilayer broadside coupling structure was reported, but requires complicated fabrication technology. In this paper, the characteristics of the notched structure are investigated further for a dual notched band implementation in a UWB BPF. The notched bands are generated by four non-uniform bandstop Corresponding author (email: qxchu@scut.edu.cn) c Science China Press and Springer-Verlag Berlin Heidelberg 212 info.scichina.com www.springerlink.com

Chu Q X, et al. Sci China Inf Sci June 212 Vol. 55 No. 6 1437 2.35 L Unit 1 1.55 4.4 7.6 7.99 6.9.3 1.56 d.3.5 3.21 1 2 L 4.4 Unit 2 1.55 d.3 Aperture back on the ground 3 4.5 L=1.55 mm, L =2.5 mm L=2.5 mm, L =2.5 mm L=2.55 mm, L =2.5 mm 4.8 5.1 5.4 5.7 6. 6.3 6.6 6.9 2.35 Figure 1 Configuration of the notched structure (mm). Figure 2 Simulated response of the notched structure with different L and L by setting d = d =.44 mm. units added to the coupled lines with all dimensions of the basic UWB BPF unchanged. The bandstop units are constructed using folded stepped-impedance resonators (SIRs) with tunable resonant frequencies which can be set to any desired notched frequencies. Coupling between the bandstop units and the feed lines determines the bandwidth of the corresponding notched bands independently. A UWB BPF with dual sharply rejected notched bands at 5.2 and 5.8 GHz is then constructed. The simulated and measured results are found to be in good agreement to verify the proposed design. 2 Characteristics of the notched structure As shown in Figure 1, the notched structure adopted in this design is constructed by adding bandstop units to the aperture-backed parallel-coupled lines. The bandstop units are formed using folded SIRs, which are electromagnetically coupled to the main feed lines to form a narrow bandstop filter. The notched frequency can be determined by the resonant frequency of each folded-sir-shaped bandstop unit, as proposed and analyzed in [11]. As depicted in Figure 2, a single notched band is introduced when the two bandstop units have the same dimensions. However, it should be noted that two bandstop units with different resonant frequencies would lead to dual narrow notched bands, which could be controlled independently by adjusting the structural parameters of the corresponding bandstop units. As seen in Figure 2, by setting L =2.5 mm, the notched frequency introduced by Unit 1 varies with different length L, while the other notched band introduced by Unit 2 is not affected. Because the notched structure can provide flexible control of the notched frequencies as desired, it can find further application in multiple notched band filter design. The bandwidths of the notched bands also depend on the coupling between the bandstop units and the parallel-coupled lines, as illustrated in Figure 3 (a) and (b). The figures show that a larger coupling gap results in weaker coupling, and a narrower bandwidth is obtained for the notched structure. There is another important characteristic in bandwidth control that the bandwidth of each notched band can be adjusted independently by the corresponding coupling gaps. As shown in Figures 3 (a) and (b), the coupling between bandstop Unit 1 and the parallel-coupled lines only affects the second notched band while the coupling between bandstop Unit 2 and the parallel-coupled lines would only affect the first notched band. 3 UWB BPF with dual notched bands Because most WLAN systems in 5. GHz frequency bands are designed to operate in bands from 5.15 to 5.35 GHz (IEEE 82.11a lower band) and 5.725 to 5.825 GHz (IEEE 82.11a upper band), it is possible

1438 Chu Q X, et al. Sci China Inf Sci June 212 Vol. 55 No. 6 (a) (b) 1 2 2 d=.24 mm, d =.44 mm d=.64 mm, d =.44 mm d=.44 mm, d =.44 mm d=.44 mm, d =.44 mm d=.64 mm, d =.44 mm d=.24 mm, d =.44 mm 3 3 4.6 4.8 5. 5.2 5.4 5.6 5.8 6. 6.2 6.4 6.6 6.8 4.6 4.8 5. 5.2 5.4 5.6 5.8 6. 6.2 6.4 6.6 6.8 1 Figure 3 Simulated response of the notch-structure with varied coupling gaps by setting L=2.5 mm, L =2.55 mm. (a) shows varied d; (b)showsvariedd. Figure 4 Configuration of the proposed UWB BPF with dual notched bands (mm). to generate dual notched bands to cover both 5.15 to 5.35 GHz and 5.725 to 5.825 GHz by adopting the structure shown above. Figure 4 shows that the configuration of the basic UWB BPF mainly depends on cascading the aperturebacked interdigital parallel-coupled lines and a SISLR, which was presented and detailed in [12]. To investigate the applications and the characteristics of the notched structure in the UWB BPF, the proposed notched structure has been integrated with a basic UWB BPF. The proposed UWB BPF with dual notched bands can be obtained by applying four bandstop units to the feed lines, with the coupling gaps and all dimensions of the basic UWB BPF remaining unchanged. For this application, the four bandstop units are divided into two groups, where bandstop Units 1 and 2 are designed to generate a notched band at 5.8 GHz while the other two generate the notched band at 5.2 GHz. To obtain a sharp rejection skirt with a sufficient rejection level, non-uniform bandstop units are adopted in this design to allow the loss poles to spread out at different frequencies across the notches [13]. As shown in Figure 5, bandstop Units 1 and 2 (and Units 3 and 4) are designed at different resonant frequencies with slight deviations from the desired central frequency and narrow notched bands with

Chu Q X, et al. Sci China Inf Sci June 212 Vol. 55 No. 6 1439 1 2 4.5 5. Bandstop Unit 1 Bandstop Unit 2 Bandstop Unit 3 Bandstop Unit 4 Cascading response 5.5 6. 6.5 Figure 5 Simulated responses of the bandstop units and the notched bands generated by cascading the bandstop units. Figure 6 Photographs of the fabricated filter. 1 2 3 4 6 7 8 9 1 (a) 2 1 1 2 1 1 1 2 3 4 5 6 7 8 9 11112131415 S 11 Simulated Measured 2 3 4 6 1 2 3 4 5 6 7 8 9 1 11 12 13 14 15 S 11 1 2 (b) Simulated Measured 3 4. 4.5 5. 5.5 6. 6.5 7. Figure 7 (a) Simulated and measured responses of the proposed UWB BPF with dual notched bands; (b) enlarged view of the notched response. steep attenuation slopes can be obtained independently for each bandstop unit. As a result, dual sharply rejected notched bands can be generated in the desired frequency band with two loss poles by cascading the four bandstop units. The bandwidth of each notched band can also be tuned independently by adjusting the corresponding coupling gaps. After carrying out the simulation and optimization, the proposed filter with dual notched bands is built on a substrate with a relative dielectric constant of 2.55 and thickness of.8 mm. Figure 6 shows photographs of the fabricated filter. The simulated and measured results of the proposed UWB BPF are in good agreement, as shown in Figure 7(a). The proposed filter exhibits a good measured UWB passband performance from 3. to 1.84 GHz, with dual sharply rejected notched bands at 5.2 and 5.8 GHz. The group delay is flat and small in the passband, except in the notched bands. It can be seen more clearly in Figure 7(b) that the notched band located at 5.25 GHz has a measured 1 db notched band from 5.15 to 5.39 GHz while the other notched band located at 5.775 GHz has a measured 1 db notched band from 5.65 to 5.83 GHz, which can effectively reject signals from WLANs. The overall size of the fabricated filter is approximately 16.4 mm by 18.9 mm. 4 Conclusion A UWB BPF with dual sharply rejected notched bands has been proposed in this paper. Non-uniform bandstop units constructed using folded SIRs were electromagnetically coupled to the parallel-coupled lines to realize dual notched bands with sharp rejection skirts and no significant influence on the passband performance, displaying attractive properties in a notched band implementation. The proposed UWB

144 Chu Q X, et al. Sci China Inf Sci June 212 Vol. 55 No. 6 BPF with dual notched bands was finally built and tested. Both simulated and measured results were found to be in good agreement to verify the proposed design. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. U6354, 68133) and State Key Laboratory of Millimeter Waves (Grant No. K21112). References 1 Federal Communication Commission. Revision of Part 15 of the Commission s Rules Regarding Ultra-wideband Transmission System. First Report and Order, FCC2, 22, 48 2 Hsu C L, Hsu F C, Kuo J T. Microstrip bandpass filters for ultra-wideband (UWB) wireless communications. In: Proceeding of IEEE MTT-S, 25, 679 682 3 Zhu L, Sun S, Menzel W. Ultra-wideband (UWB) bandpass filter using multiple-mode resonator. IEEE Microw Wirel Compon Lett, 25, 15: 796 798 4 Li R, Zhu L. Compact UWB bandpass filter using stub-loaded multiple-mode resonator. IEEE Microw Wirel Compon Lett, 27, 17: 4 42 5 Chu Q X, Li S T. Compact UWB bandpass filter with improved upper-stopband performance. Electron Lett, 28, 44: 742 743 6 Chu Q X, Tian X K. Design of UWB bandpass filter using stepped-impedance stub loaded resonator. IEEE Microw Wirel Compon Lett, 21, 2: 51 53 7 Shanman H, Hong J S. Ultra-wideband (UWB) bandpass filter with embedded band notch structures. IEEE Microw Wirel Compon Lett, 27, 17: 193 195 8 Shanman H, Hong J S. Asymmetric parallel-coupled lines for notch implementation in UWB filters. IEEE Microw Wirel Compon Lett, 27, 17: 516 518 9 Song K J, Xue Q. Compact ultra-wideband (UWB) bandpass filters with multiple notched bands. IEEE Microw Wirel Compon Lett, 21, 2, 447 449 1 Hao Z C, Hong J S. Compact UWB filter with double notch-bands using multilayer LCP technology. IEEE Microw Wirel Compon Lett, 29, 19: 5 52 11 Makimoto M, Yamashita S. Bandpass filters using parallel coupled stripline stepped impedance resonators. IEEE Trans Microw Theory Tech, 198, 54: 1413 1417 12 Chu Q X, Tian X K. Design of a Compact UWB bandpass filter with notched band. In: Proceeding of Asian-Pacific Microwave Conference. Yokohama: 21. 37 4 13 Levy R, Snyder R V, Shin S. Bandstop filters with extended upper passbands. IEEE Trans Microw Theory Tech, 26, 54: 253 2515