LOW-PASS FILTER USING A HYBRID EBG STRUCTURE

LOW-PASS FILTER USING A HYBRID EBG STRUCTUREM.F.Karim,1A.Q.Liu,1A.Alphones,1and X.J.Zhang 21School of Electrical &Electronic Engineering Nanyang Technological University Nanyang Avenue,Singapore 6397982Intel Corporation Yan’an RoadShanghai 200336,P.R.China Received 28September 2004ABSTRACT:This paper presents an innovative design of a hybrid electromagnetic bandgap (EBG)structure based on nonuniform period and dimensions.A high rejection bandwidth is achieved when the non-uniform period and the dimensions follow the Kaiser distribution.The EBG structures are fabricated on the transmission line and the ground plane,where the offset is one-quarter of the EBG period.The measure-ment results show a low-pass filter response up to the C-band with in-sertion loss below 0.25dB and rejection bandwidth above 12GHz.This type of nonuniform EBG structure is more compact than the conven-tional uniform structure.©2005Wiley Periodicals,Inc.Microwave Opt Technol Lett 45:95–98,2005;Published online in Wiley InterScience ().DOI 10.1002/mop.20734Key words:electromagnetic bandgap (EBG);periodic structures;low-pass filter (LPF);nonuniform period 1.INTRODUCTIONElectromagnetic bandgap (EBG)is a periodic structure within which certain bands of electromagnetic propagations are prohib-ited.EBG structures can be implemented on microstrip lines by drilling periodic holes through the substrate or by etching in the ground plane.Microstrip-line structures are more attractive be-cause of their promising applications in microwave integrated circuits (MICs)and monolithic microwave integrated circuits (MMICs).EBG periodic structures have been successfully applied to broadband amplifiers for efficiency enhancement [1],spurious suppression in band-pass filters [2],and optimization of the radi-ation pattern in microstrip antennas [3].Recently,some research-ers have focused on obtaining wide-frequency stop-bands,while others have concentrated on achieving a compact design.A cas-caded connection of various EBG structures for a wide-rejection-frequency bandwidth has been proposed [4],but the design has the shortcomings of large size and restricted applications in microstrip circuits.On the other hand,a more compact EBG structure using chirping and tapering techniques has been investigated [5].Simi-larly,a Chebyshev tapered circular EBG,proposed in [6],has shown vast improvement in pass-band performance.A double-sided EBG structure was investigated in [7],which yielded less rejection bandwidth in the stop-band.However,to date there is scant work on filters with low insertion loss in the pass band and wide-rejection bandwidth for realizing a low-pass filter (LPF)response.In this paper,a new low-pass filter using a hybrid EBG struc-ture is presented.The Kaiser distribution used in [8],which has shown a steep roll-off behavior,is used for obtaining a high-rejection bandwidth in the stop band.A hybrid EBG of square-lattice shape is fabricated on the ground plane and the signal line of the microstrip structure.The measurement results of the hybrid EBG structure are discussed and the comparison with a conven-tional stepped impedance LPF is analyzed.2.DESIGN OF THE HYBRID EBG STRUCTUREThe hybrid EBG structures are etched on taconic substrate with a dielectric constant of 2.43and thickness of 0.76mm.The design parameters that are critical to achieve the desired stop band are the period a and the filling factor d /a .For low-pass filter applications with wide-rejection bandwidth,EBG structures should not be equidistant (that is,the period is not constant).The idea basically evolved from the linear change of the perturbation period applied in linearly chirped fiber Bragg gratings (LCFBGs)in the optical domain [9],as well as in chirped corrugated wave guides in the microwave regime.This technique can achieve a wide bandwidth,as compared to structures with constant period.The Kaiser distri-bution polynomials can be applied to the period and the dimension,and can be expressed asT ͩz lͪϭI 0ͩ␣ͱ1Ϫͩ2z l ͪ2ͪI 0͑␣͒,(1)where I 0is the modified Bessel function of the first kind,z /l is the normalized longitudinal position in the circuit,l is the device length,and ␣ϭ4is the optimum value of the Kaiser window as in [8].The proposed hybrid structure in which the dimension and the period on the transmission line and as well as on ground planeareFigure 1Hybrid EBG structure:(a)top view;(b)bottomviewFigure 2Simulation results for different offset valuesperturbed simultaneously is shown in Figure 1.The perturbation on the transmission line is basically a variation of the impedance according to the distribution.The structure on the signal line and the ground plane follow the same pattern of the period and the dimension.The period and the dimension between the two cen-tered EBGs are a 0and d 0,respectively.According to Eq.(1),the period and the dimension are varied correspondingly and are denoted as a 1,a 2,a 3and d 1,d 2,d 3(see Fig.1).Full-wave electromagnetic simulation is performed using com-mercial software,Zeland IE-3D.When the EBG structures on the ground plane and the signal line are aligned,the simulation results reveal no stop-band phenomena,as shown in Figure 2.The current distribution for this structure,shown in Figure 3,clearly exhibits that at 10GHz the transmission power can pass through the circuit with less attenuation.The results of the extensive simulation process indicate that there should be an offset between the two structures.The offset distance is varied,and with an offset of 1/8a 0and a 0,the ripples in the pass-band and stop-band regions are significant,and even the rejection bandwidth becomes less than 15dB.The optimum result is obtained at 2.63mm,that is,one-quarter of the EBG period.The current distribution at the same 10GHz shows that the transmission can pass through the first few EBG structures,and after that it is totally suppressed,as shown in Figure 4.The hybrid structure generates an LPF response up to theTABLE 1Parameters of the Hybrid EBG Structure Polynomial Values Periods [mm]Dimensions [mm]x 01a 010.52d 0 5.26x 10.93a 19.78d 1 4.9x 20.75a 27.90d 2 3.95x 30.50a 35.26d 32.63Figure 3Current distributions of the hybrid EBG at 10GHz with no offset:(a)whole circuit;(b)enlargedviewFigure 4Current distributions of the hybrid EBG at 10GHz with an offset of one-quarter period:(a)whole circuit;(b)enlarged viewC-band with minimum insertion loss and wide-rejection band-width.The design parameters for the hybrid EBG structure are listed in Table 1.A stepped-impedance LPF is designed for the performance comparison with the hybrid EBG structure and its parameters are presented in Table 2.3.MEASUREMENT RESULTS AND DISCUSSIONSThe fabricated structure of hybrid EBG is shown in Figure 5.This fabricated structure is an optimum design,as the transmission-line EBG is offset from the ground-plane EBG by one-quarter of the period.The pass-band region has an insertion loss of 0.25dB up to the C-band range,as shown in Figure 6.The maximum attenuation is around 40.1dB at 10.9GHz and the 20-dB rejection bandwidth is from 8to 20GHz.It is observed that up to 40GHz,the rejection behavior is maintained below 20dB.A low-pass stepped-impedance filter is designed for the performance comparison with hybrid EBG,as shown in Figure 7.The order of the LPF is nine with 0.2-dB equal-ripples response characteristics.The hybrid EBG structure and the stepped-impedance filter have the same low-pass response with 0.2-dB insertion loss,as shown in Figure 8.The spurious-transmission peaks occur around 14GHz for the stepped-impedance filter and its rejection bandwidth becomes less than 5GHz.This clearly shows that the hybrid EBG LPF has an enhanced performance,as compared to that of the conventional stepped-impedance filter design in terms of higher rejection bandwidth.The measurement and simulation results of two different filters are summarized in Table 3.4.CONCLUSIONA low-pass filter using a hybrid EBG structure based on the nonuniform period and dimension has been investigated.Thislow-pass filter has been designed,simulated,and fabricated with an insertion loss of 0.25dB in the pass-band up to the C-band.The 20-dB rejection bandwidth is more than 12GHz and the ripple in the reflection characteristic is around 2dB.This hybrid EBG structure,which has been validated and qualitatively compared with the conventional stepped-impedance LPF,may find potential application in wideband rejectionfilters.Figure 7Stepped impedance low-passfilterFigure 8Comparison of the hybrid EBG and stepped-impedance filterTABLE 2Parameters of the Stepped-Impedance FilterParametersl 1l 2l 3w 1w 2w 3Dimension [mm]3.55.635.60.622.2Figure 5Fabricated hybrid EBG structure:(a)top view;(b)bottomview Figure 6Measurement and simulation results of the hybrid structureREFERENCES1.V.Radistic,Y.Qian,and I.Itoh,Broadband power amplifiers using dielectric photonic bandgap structures,IEEE Microwave Wireless Compon Lett 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thesis,Public University of Navarre,Spain,2001.9.K.O.Hill and G.Meltz,Fiber Bragg Grating technology fundamentals and overview,J Lightwave Technol 15(1997),1263–1276.©2005Wiley Periodicals,Inc.ON POTENTIAL APPLICATIONS OFMETAMATERIALS FOR THE DESIGN OF BROADBAND PHASE SHIFTERSIgor S.Nefedov 1,2and Sergei A.Tretyakov 11Radiolaboratory/SMARAD Center of Excellence Helsinki University of Technology P.O.Box 3000FI-02015TKK,Finland 2Institute of Radio Engineering and Electronics Russian Academy of Sciences Zelyonaya 38410019,Saratov,RussiaReceived 13October 2004ABSTRACT:The possible design of phase shifters with reduced fre-quency dispersion using combined sections of forward-wave (FW)and backward-wave (BW)transmission lines is discussed.It is shown that inclusion of backward-wave sections into a transmission line always increases the total dispersion.On the other hand,we show that disper-sion can be reduced by means of lines with positive anomalous disper-sion and provide an example of such line.©2005Wiley Periodicals,Inc.Microwave Opt Technol Lett 45:98–102,2005;Published online in Wiley InterScience ().DOI 10.1002/mop.20735Key words:metamaterials;backward waves;phase shifters;anomalous dispersion1.INTRODUCTIONRecent years have witnessed a growing interest in metamaterials and complex transmission lines supporting propagation of back-ward waves at microwave frequencies.Different terms are used for these metamaterials:left-handed media,Veselago media,back-ward-wave (BW)media,media with negative refractive index or with negative permittivity and permeability,lines with negative dispersion.Several new application ideas have been proposed.Most of them utilize the effect of anomalous refraction at an interface between a forward wave (FW)and a BW medium,surface polaritons on such interfaces,and phase compensation which takes place when a wave propagates across slabs of FW and BW materials.The first idea belongs to V.Veselago,who pub-lished pioneer work on the electrodynamics of media with negative parameters and discussed a design of a planar lens [1].This idea was developed by many authors,who considered wave focusing,including subwavelength imaging [2],wave leakage (a new type of leaky-wave antennas)[3],and so forth.The design of 3D BW media with the use of resonant particles such as split-ring resona-tors [4]is difficult due to high losses in these resonant structures.Two-dimensional structures for the microwave range can be real-ized without any resonant elements by periodically loading 2D transmission lines with lumped-element series capacitors and shunt inductors [5,6].The phase compensation idea was proposed by N.Engheta [7],who suggested a subwavelength cavity resonator formed by two adjacent material slabs that support forward and backward waves.However,as shown in [8],this structure in fact exhibits resonant response only due to the resonant behavior of the filling material.This problem was further explored in detail in [9].The idea of phase compensation was further extended to the design of phase shifters in [10],with a discussion of a broadband phase shifter formed by connected transmission-line sections with forward and backward waves.This design allows us to realize negative phase-shift values in comparatively short sections of transmission lines.A natural question arises:Is it possible to reduce the frequency dispersion of phase shifters using this ap-proach,in addition to achieving negative phase shifts?And if possible,what kind of transmission line or filling metamaterial is required?These are the questions that we address in this paper.2.POSITIVE AND NEGATIVE,NORMAL AND ANOMALOUS KINDS OF DISPERSIONIn this introductory section,we define various dispersion types to be used in the following discussion and explain how these various dispersion laws can be realized in artificial transmission lines.Characterizing dispersion of media or transmission lines with negligible losses that support waves with the propagation constant ␤at frequency ␻,two terms are commonly used:positive disper-sion,when ␤d ␤/d ␻Ͼ0and negative dispersion,when ␤d ␤/d ␻Ͻ0.Positive dispersion means that the line supports anI.S.Nefedov is currently with the Radiolaboratory of TKK.TABLE 3Comparison Results of the Hybrid EBG and Stepped-Impedance FiltersFilterPassband Insertion Loss,S 21[dB]Stop Band RejectionBandwidth 20-dB Isolation BW [GHz]Return Loss,S 11[dB]S 21max [dB]f 0[GHz]1st Null 1st Peak Stepped impedance (simulation)0.24310.654511Hybrid EBG (simulation)0.2441112.752212Hybrid EBG (measurement)0.254010.912.417.613.7。