Design and simulation of a compact ultra-wideband bandpass filter with a notched band using multiple-mode resonator technique

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1 February 2016, 23(1): The Journal of China Universities of Posts and Telecommunications Design and simulation of a compact ultra-wideband bandpass filter with a notched band using multiple-mode resonator technique Yang Hong (), Chen Jing, Yang Haolan, Liu Yunlong, Yang Yuchuan College of Optoelectrical Engineering, Chongqing University of Posts and Telecommunications, Chongqing , China Abstract A novel, compact, and highly selective ultra-wideband (UWB) bandpass filter with a narrow notched band is presented. Apart from having a basic structure of a slotline multiple-mode resonator (MMR) and microstrip feed lines, this novel design introduces a cross-coupling between the input and output feed lines to enhance the filter selectivity. The design strictly follows the theory and verified by electromagnetic (EM) simulation and experiments. In addition, the narrow notched band is introduced by embedding a pair of split ring resonators (SRR) in order to reject any undesired existing radio signals that may interfere with the Federal Communications Commission (FCC)-defined UWB band. By changing the structural parameters of SRR, it can be easily tuned to any desired frequency. This filter can be integrated in UWB communication systems and efficiently improve the interference immunity from undesired signals such as wireless local area network (WLAN). Keywords bandpass filter, ultra-wideband, multiple-mode resonator (MMR), microstrip line, cross-coupling 1 Introduction UWB technology has a long history that dates back more than 100 years ago. In 1998, the US FCC recognized the significance of UWB technology and released the initial report in February 2002, in which the UWB (range of 3.1 GHz ~10.6 GHz) technology was authorized for unlicensed commercial uses [1]. Since then, both academic and industrial research on various UWB devices have gained increasing popularity. New methods and structures had significantly advanced the development of UWB filters [2 11]. A variety of synthetic approaches to UWB bandpass filter design have been reported. Ref. [3] presents a systematic analytical method for the exact synthesis of UWB filtering responses using the isolated cascade connection of high-pass and low-pass sections. However, this type of filter occupies a large circuitry area. A novel UWB bandpass filter was developed by Zhu et al. using a single MMR [4]. The basic concept of this class of UWB Received date: Corresponding author: Yang Hong, yanghong@cqupt.edu.cn DOI: /S (16) filters is to allocate the first three resonant modes of the MMR inside of the chosen UWB passband [5]. This kind of UWB bandpass filter with compact size and simple structure becomes more and more popular in UWB filter design. The implementation of MMRs on various transmission line structures such as microstrip line (MSL) and coplanar waveguide are reported in Refs. [6 7]. UWB communication does not seriously affect the existing radio systems due to its low-power density. On the other hand, the exclusive use of the frequency spectrum such as WLAN, may interfere with the UWB communication in certain frequencies. Ref. [8] analyzed the impact of interference produced by IEEE a system on the UWB system, the results show that the bit error rate (BER) increases from 6.2% to 33% when the interference source power is increased from 10 dbm to 20 dbm. To circumvent this problem, various UWB filters with either a single or multiple narrow notched bands embedded in the ultra-wide passband were recently developed [9 15]. This work attempts to develop a novel, compact, and highly selective UWB bandpass filter for use in wireless

2 Issue 1 Yang Hong, et al. / Design and simulation of a compact ultra-wideband bandpass filter with 87 systems and provide a new approach for introducing notched bands. The filter design includes a stepped impedance MMR implemented on the slotline, and a pair of SRR on both sides of the input feed line. The center frequency of the notched band can be controlled by adjusting the structural parameters of the SRRs. The proposed filter is constructed on a low cost TP-2 substrate with a relative dielectric constant of 10.2 and a thickness of mm. 2 UWB bandpass filter development The filter design process can be divided into two steps: MMR design and input/output coupling structure design. 2.1 MMR design According to transmission line theory [16 18], a transmission line resonator can be made up by terminating a finite portion of a uniform transmission line with either open-circuits or short-circuits at both ends. Such a uniform impedance resonator (UIR) has several undesired spurious resonant modes that occur at any multiple of the first resonant frequency. These higher-order resonant modes will usually lead to spurious passband. However, by changing the transmission line structure to shift the higher-order resonances toward the first-order resonance, a MMR can be constituted. These multiple resonances could be simultaneously excited and utilized together make up a wide passband covering these multiple resonant frequencies. This work presents a short-circuited MMR on slotline whose geometrical sketch is illustrated in Fig. 1. This short- circuited MMR is composed of three distinctive slotline sections with one high-impedance section in the middle and two identical low-impedance sections at the two sides. Fig. 1 depicts its equivalent transmission line network, in which the two slotline step discontinuities are negligibly small and hence ignored to simplify the following analysis. Fig. 1 Schematic layout and equivalent circuit network of slotline MMR To manifest its MMR characteristics for the UWB filter design, the longitudinal resonant condition for all the resonant modes has to be satisfied. For this purpose, the input impedance Z in at the left short end, looking into the right end, is indicated in its equivalent transmission line network, as shown in Fig. 1, such that ( ky tanθ1 tanθ2 )( ky tanθ1 + tanθ2 ) Zin = jz 2 k tan θ 1 tan θ 2 1+ k tanθ tanθ Y ( )( 2 ) ( Y ) (1) where k Y= Z 1 /Z 2 is the impedance ratio of the middle and end slotline sections in this MMR. Under the resonant condition Z in =0, a set of algebraic equations can be obtained to determine all the frequencies: k tanθ tanθ = 0 (at even-mode frequencies) (2) Y 1 2 k tanθ + tanθ = 0 (at odd-mode frequencies) (3) Y 1 2 As the lengths of these three sections are readily selected as θ 2 = θ 1 = θ, the first three resonant frequencies (i.e., f 1, f 2, and f 3 ) can be derived from the three closed-form formulas: 1 θ ( f1 ) = tan ky (4) π θ ( f2 ) = 2 (5) θ f = π tan k (6) ( ) 1 3 Y As a result, it is easy to find that the lower and higher frequencies, f 1 and f 3, are mainly determined by the impedance ratio k Y, while the central frequency f 2 is somehow affected by the actual lengths of all three sections. k Y can be adjusted by setting the width ratio (w 2 /w 1 ) of low-impedance and high-impedance section. To realize a FCC-defined UWB passband, the slot widths of the middle and end slotline sections should be properly chosen to allocate these three frequencies toward the lower-end, center, and higher-end of such a UWB passband. In this design, the width w 1 is selected as its minimum tolerable value of 0.1 mm in existing fabrication procedure while minimizing unexpected conductor loss. As the width w 2 is gradually raised from 0.1 mm to 1.0 mm, the first three resonances can be observed to move closer to each other due to enlarged k Y. When w 2 /w 1 is chosen as 4.2, the first three resonant frequencies appear at

3 88 The Journal of China Universities of Posts and Telecommunications GHz, 7.02 GHz and 9.52 GHz. If two microstripslotline transitions are properly introduced at the input and output ports, a dominant passband with five transmission poles covering the FCC-defined UWB can be eventually constructed, as will be demonstrated later on. In order to reduce the overall size, the final MMR shape is shown in Fig Input/output coupling structure design This design uses a microstrip-slotline transition structure to achieve both input coupling and output coupling, this kind of microstrip-slotline transition structure is analyzed in detail in Ref. [19]. Fig. 2 shows a microstrip-slotline transition structure. Both theories and experiments show that the coupling will be the strongest when the slotline and MSL are orthogonal to each other (α = 90 ), and the length of the extension at the intersection is a quarter of the wavelength (l = λ/4). At the same time, a thinner dielectric substrate (the closer the MSL to the slotline) will also increase coupling strength [13]. (c) Fig. 3 The schematic layout of the design The proposed filter is constructed on a low cost TP-2 substrate with a relative dielectric constant of 10.2 and a thickness of mm. The fabricated filter is shown in Fig.4. Fig. 2 Microstrip-slotline transition structure 3 Simulation and experimental results In order to enhance the filter selectivity, an extra coupling between the two feed lines is introduced. The final design of the UWB bandpass filter is shown in Fig. 3. Fig. 4 Fabricated filter The simulation and experimental results are shown in Fig. 5. Simulation results exhibit an ultra-wide passband from 3.4 GHz to 11.0 GHz or 105.5% fractional bandwidth at 7.2 GHz, and a transmission zero was introduced by cross coupling at 1.1 GHz. The experimental results are in consistent with simulation results. Insertion loss becomes large in experimental results because of the tolerance in etching fabrication for the narrow slot widths, the loss of the dielectric substrate, and the discontinuity generated during welding sub-miniature-a (SMA) connectors.

4 Issue 1 Yang Hong, et al. / Design and simulation of a compact ultra-wideband bandpass filter with 89 Fitter with a pair of SRRs Fig. 5 Simulation and experimental results of the filter 4 UWB bandpass filter with a notched band Many efforts have been carried out to produce a UWB bandpass filter with a notched band [9 15]. These filters were realized via multiple methods, such as an embedded open-circuited stub [9], using defected ground structures and electromagnetic bandgap structures [10], using asymmetric parallel coupled lines [11] and embedding L-shaped slots in the stepped impedance resonator, etc. All of these methods are effective approaches to reject undesired radio signals. However, they are still embarrassed by the large electrical size [9], no compatibility with monolithic microwave integrated circuits [10] and low tuning capability [10 11]. Furthermore, these methods may hinder the overall performance of the initial UWB filter [14]. In this work, a pair of SRRs is embedded to insert a narrow notched band into the passband of the UWB filter. The shape of the SRRs is shown in Fig. 6. They are placed on both sides of the input feed line as shown in Fig. 6. The center frequency of notched band approximates to the resonant frequency of the SRR. Thus the notched band can be allocated at any desired frequency by adjusting the structural parameters of the SRR. Fig. 6 Schematic view and filter with a pair of SRRs Fig. 7 shows the resonant frequency of the SRR for varying T a, where a larger T a moves the notched band to a lower frequency; same thing would happen by increasing T l as shown in Fig. 7. This is because of the change in overall electrical length of the SRR caused by changing variables. Thus, the inserted notch can be tuned within a wide range. Fig. 7 Resonant frequency of the SRR for varying parameters Schematic view of SRRs The center frequency of the notched band is chosen to be 5.8 GHz, the final design parameters of the SRRs are shown in Table 1. The slot width between SRR and input feed line is fixed at 0.1mm. The simulation results of the filter are shown in Fig. 8. Loading the initial bandpass

5 90 The Journal of China Universities of Posts and Telecommunications 2016 filter (BPF) with another two SRRs as depicted in Fig. 6 will introduce two notched bands in the passband of the filter. Each of the notched bands can be adjusted separately. Table 1 Design parameters of the SRRs Parameter Value/mm T h 1.5 T l 3.8 T a 0.3 T d 0.3 T g 0.15 Fig. 8 Simulation results of the UWB bandpass filter with notched band 5 Conclusions In this paper, a compact UWB bandpass filter has been developed and presented. And a new approach of using SRRs to inserting notched bands has also been introduced. Using this technique, the initial UWB bandpass filter is able to block any unwanted existing radio signals that may interfere with UWB communication system without changing the overall performance of the initial filter. The structural parameters of the SRRs are used to control the frequency of the inserted notch. The proposed filter can be integrated in UWB communication systems and efficiently improve the interference immunity from undesired signals such as WLAN systems. This work can provide some reference for the development of UWB bandpass filter. Acknowledgements This work was supported by the Natural Science Foundation of CQ CSTC (CSTC2010DD2412), Chongqing Municipal Science and Technology Commission of Natural Science Foundation Project (KJ100512), the Research Fund Project of Chongqing University of Posts and Telecommunications (A ). References 1. Revision of part 15 of the commission s rules regarding ultra-wideband transmission system. ET-Docket FCC02-48, Washington,DC,USA: Federal Communications Commission (FCC), 2002: Gómez-García R, Alonso J I. Systematic method for the exact synthesis of ultra-wideband filtering responses using high-pass and low-pass sections. IEEE Transactions on Microwave Theory and Techniques, 2006, 54(10): Tang C W, Chen M G. A microstrip ultra-wideband bandpass filter with cascaded broadband bandpass and bandstop filters. IEEE Transactions on Microwave Theory and Techniques, 2007, 55(11): Zhu L, Sun S, Menzel W. Ultra-wideband (UWB) bandpass filters using multiple-mode resonator. IEEE Microwave and Wireless Components Letters, 2005, 15(11): Zhu L, Bu H Z, Wu K. Aperture compensation technique for innovative design of ultra-wideband microstrip bandpass filter. Proceedings of the IEEE MTT-S International Microwave Symposium Digest (IMS 00): Vol 1, Jun 11 16, 2000, Boston, MA, USA. Piscataway, NJ, USA: IEEE, 2000: Gao J, Zhu L, Menzel W, et al. Short-circuited CPW multiple-mode resonator for ultra-wideband (UWB) bandpass filter. IEEE Microwave and Wireless Components Letters, 2007,16(3): Li R, Zhu L. Compact UWB bandpass filter using stub-loaded multiple-mode resonator. IEEE Microwave and Wireless Components Letters, 2007, 17(1): Liu Y. Research on interference between UWB wireless communications and exiting wireless systems. Master Thesis. Shanghai, China: Donghua University, 2008 (in Chinese) 9. Shaman H, Hong J S. Ultra-wideband (UWB) bandpass filter with embedded band notch structures. IEEE Microwave and Wireless Components Letters, 2007, 17(3): Liu H, Xu Z Q, Wu B, et al. Compact HMSIW UWB bandpass filter using DGS and EBG technology with two notched-band. Proceedings of the International Workshop on Microwave and Millimeter Wave Circuits and System Technology (MMWWCST 13), Oct 24 25, 2013, Chengdu, China. Piscataway, NJ, USA: IEEE, 2013: Shaman H, Hong J S. Asymmetric parallel-coupled lines for notch implementation in UWB filters. IEEE Microwave and Wireless Components Letters, 2007, 17(7): Luo X, Ma J G, Ma K X, et al. Compact UWB bandpass filter with ultra narrow notched band. IEEE Microwave and Wireless Components Letters, 2010, 20(3): Huang J Q, Chu Q X, Liu C Y. Compact UWB filter based on surface-coupled structure with dual notched bands. Progress in Electromagnetics Research, 2010, 106: Pirani S, Nourinia J, Ghobadi C. Band-notched UWB BPF design using parasitic coupled line. IEEE Microwave and Wireless Components Letters, 2010, 20(8): Zhu H, Chu Q X. Ultra-wideband bandpass filter with a notch-band using stub-loaded ring resonator. IEEE Microwave and Wireless Components Letters, 2013, 23(7): Hong J S. Microstrip filters for RF/microwave applications. 2nd ed. New York, NY, USA: John Wiley & Sons, 2012: Pozar D M. Microwave engineering. 4th ed. New York, NY, USA: John Wiley & Sons, 2011: Wadell B C. Transmission line design handbook. Norwood, MA, USA: Artech House, 1991: Antar Y M M, Bhattcharyya A K, Ittipiboon A. Microstripline-slotline transition analysis using the spectral domain technique. IEEE Transactions on Microwave Theory and Techniques, 1992, 40(3): (Editor: Lu Junqiang)