Miniaturized Planar Ferrite Junction Circulator in the Form of Substrate-Integrated Waveguide

Transcription

1 Miniaturized Planar Ferrite Junction Circulator in the Form of Substrate-Integrated Waveguide Wenquan Che, 1 Xiao Jing Ji, 1 Edward K. N. Yung 2 1 Department of Electrical Engineering, Nanjing University of Science and Technology, Nanjing, China 2 Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong Received 11 July 2006; accepted 21 January 2007 ABSTRACT: The theoretical analysis and engineering implementation of the planar substrate-integrated waveguide (SIW) ferrite junction circulator have been proposed in this article. The ferrite junction circulator is implemented in the form of SIW, taking the features of low profile, small volume and easy integration with other planar circuits. The design strategies of the device have been introduced, including the design consideration of the microstrip transition. One C-band prototype of SIW ferrite junction circulator has been fabricated and measured. The experimental results indicate the bandwidth is about 33% at 215 db isolation and the maximum isolation is near 40 db. However, the insertion loss is a little big, owing to the imperfect dielectric material and fabrication inaccuracy. The SIW ferrite junction circulator and the microstrip transition are integrated into a same substrate, resulting in a very compact planar ferrite junction circulator and indicating potential applications in integrated communication and radar systems. VC 2007 Wiley Periodicals, Inc. Int J RF and Microwave CAE 18: 8 13, Keywords: waveguide ferrite junction circulator; substrate-integrated waveguide (SIW); linearly tapered microstrip lines I. INTRODUCTION It is well known that the ferrite circulators are one kind of very important microwave circuits that are used widely in many radar and communication systems [1 3]. The waveguide junction ferrite circulator takes the features of high power handling capacity and high quality factor, etc. However, its implementation in planar form seems to be difficult due to its 3D geometry. This difficulty was resolved a few years ago by a substrate-integrated waveguide (SIW). It is an artificial waveguide structure fabricated in a Correspondence to: W. Che; yeeren_che@yahoo.com.cn. DOI /mmce Published online 13 September 2007 in Wiley InterScience ( planar substrate with periodic metallized via-hole cylinders as sidewalls, and solid top- and bottom-walls of metallization. For implementation in multilayer circuits, such as in LTCC or multilayer PCB structures, the dielectric substrate of SIW is usually thin. The thin substrate may lower the power-handling capability a little; however, the SIW still preserves much of the properties of a conventional rectangular waveguide (RW) [4, 5]. A large number of microwave circuits based on SIW have been constructed [6 9], demonstrating the distinguishable advantages over RW structures including low cost, low profile, ease of integration with planar devices, and significant size and cost reduction. In this paper, a substrate-integrated waveguide ferrite junction circulator has been proposed; the design VC 2007 Wiley Periodicals, Inc. 8

2 Planar SIW Ferrite Junction Circulator 9 Figure 1. The geometry of substrate-integrated waveguide (SIW). strategies of the planar ferrite junction circulator have been introduced. A microstrip transition has also been designed for the convenience of measurement, which has been constructed into the same substrate with the SIW ferrite junction circulator, resulting in a very compact ferrite junction circulator and potential applications in integrated radar and communication systems. II. DESIGN STRATEGIES OF C-BAND SIW FERRITE JUNCTION CIRCULATOR In the design of a ferrite circulator, the main considerations are the parameters of the ferrite and the biasing magnetic field. By adjusting the parameters of the ferrite post and biasing magnetic field, the eigenvalues of the scattering matrix will change, and a circulator will be formed accordingly. The design strategies of one SIW Y-junction ferrite circulator will be introduced below. The input/output ports of the ferrite junction circulator are all SIWs. In the next subsection, the dimensions of the SIW ports of the proposed ferrite circulators would be determined first, followed by the parameters choice of the ferrite post of the circulator. radius. The design frequency of the SIW is 6 GHz. According to the design principles of SIW described in [4], the parameters of the SIW in our work are determined and listed below, the dielectric constant of substrate e r ¼ 10.2, the loss tangent of the dielectric tgd ¼ 0.02, the substrate height h ¼ 3mm,a ¼ 12.8 mm, W ¼ 1.5 mm, and R ¼ 0. 4 mm. The simulated S-parameters of the SIW are illustrated in Figure 2. Obviously, the SIW has nearly the same high-pass performances as the rectangular waveguide, the insertion loss is less than 0.5 db, preserving most of the characteristics of conventional rectangular waveguide. A waveguide Y junction can thus be implemented with three SIW sections, resulting in the foundation of the waveguide Y junction ferrite circulator. B. The Choice of Ferrite Material The proposed SIW junction ferrite circulator is one kind of low-field components, and the normalized saturation magnetization P is thus commonly chosen between 0.4 and 0.8. In addition, the temperature stability of the ferrite material is important for the circulator. If the ferrite material is sensitive to temperature variation, the performance of the circulator will deteriorate. The temperature compensation of the ferrite circulator is thus very critical and usually required for a wide temperature range (usually 508C to 808C). Lithium ferrite with good temperature characteristics is chosen accordingly. The saturated magnetization of the ferrite 4pM S ¼ 1650 Gs, the dielectric constant e f ¼ 14 and the normalized saturated magnetization P ¼ A. The Design of SIW The schematic top view of SIW is illustrated in Figure 1, which is constructed in substrate via two-row parallel metallized cylinders. a is the width of SIW, W is the cylinder spacing, and R is the cylinder Figure 2. SIW. The simulated propagation characteristics of

3 10 Che, Ji, and Yung Figure 3. Geometry of the ferrite post in the junction. C. The Design of Ferrite Post in the Junction The dimensions of ferrite post depend on the resonance frequency under the condition of demagnetization. In case of transverse magnetization, the operation modes in the ferrite are the surface wave modes, i.e., HE and EH modes. It is difficult to calculate the resonance frequency of this kind of mode. However, it has been known that the resonance frequency of TM mode can be used approximately without causing much deviation [1]. In this way, the TM 111 mode is customarily regarded as the operation mode in the ferrite junction circulator. The cross-section of the ferrite rod in the junction is illustrated as Figure 3. R f, h f are the radius and height of the ferrite post, respectively, P 1 is the height of the junction. The dimensions of the ferrite post are required to satisfy the formula below [1]: Where K 0 R f ¼ p 1 ffiffiffiffi e f sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pr 2 f þð1:84þ 2 h f ð1þ The field pattern of SIW ferrite junction circu- Figure 5. lator. k is the operation wavelength, e f is the relative dielectric constant of the ferrite. In engineering, the empirical value of h f /b 1 is found to be Based on above approximation, the commercial HFSS is used to optimize the geometry, the design frequency is 6 GHz, the resultant dimensions are found as R f ¼ 4.35 mm, h f ¼ 2.4 mm. The simulation model of C-band SIW ferrite junction circulator is shown in Figure 4, the input and output ports are all SIW structures. The field pattern of SIW ferrite junction circulator illustrated in Figure 5 indicates obvious circulation performance of the junction circulator. The simulated S parameters are shown in Figure 6. The center frequency is given at 6 GHz; about 29.2% bandwidth at 15 db return loss can be observed, while the insertion loss is less than 0.8 db; the maximum isolation is close to 30 db. K 0 ¼ 2p=k Figure 4. The simulation model of C-band SIW ferrite junction circulator. [Color figure can be viewed in the online issue, which is available at wiley.com] Figure 6. Simulated results of C-band SIW ferrite junction circulator.

4 Planar SIW Ferrite Junction Circulator 11 Figure 7. SIW-microstrip tapered transition. where C 0 is the light velocity in vacuum, x is the operation angle frequency, Z t is the distributed series impedance of equivalent transmission line for the linearly tapered microstrip line (LTML), e eff is the effective dielectric constant of an LTML [12]. Considering the impedance matching between the SIW and the microstrip line, the width W t and length L t of the tapered transition can thus be obtained [11], the width W t ¼ 4.2 mm, length L t ¼ 7.6 mm. These parameters can be used as initial values in HFSS simulation for further optimization, and the optimized dimensions of the tapered microstrip line are finally found, W t ¼ 6.6 mm, L t ¼ 7.4 mm. IV. RESULTS AND DISCUSSION III. SIW-MICROSTRIP TAPERED TRANSITION For the sake of measurement or integration of the SIW ferrite junction circulator with other microwave planar circuits, the transitions from SIWs to microstrip lines or other planar transmission lines are necessary [9]. The SIW-microstrip transition is illustrated in Figure 7, and the tapered microstrip is used to excite the TE 10 mode inside SIW. It has been concluded that the SIW can be treated as a conventional rectangular waveguide with equivalent width of its SIW counterpart [4, 5], the equivalent wave impedance Z e of the SIW can thus be determined as Z e ¼ p2 8 h a eff rffiffiffiffiffiffiffiffiffiffiffiffi l r l 0 1 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi e r e a k eff ð2þ where h, e r, l r are the thickness, relative permittivity and permeability of the substrate, respectively, e 0 and l 0 are the permittivity and permeability in vacuum, k is the operation wavelength, a eff is the effective width of SIW given by following formula [10]: a ¼ 2a eff p pw arc ctg 4a eff ln W 4R ð3þ One prototype of SIW ferrite junction circulator has been fabricated and measured to verify the validity of above design strategies [16]; the picture of the prototype is shown in Figure 8. The substrate material used in our prototype is one kind of compound ceramic materials, the expected dielectric constant e r ¼ The measured S-parameters are illustrated in Figure 9, as well as the simulation results. In order to guarantee the temperature stability of the SIW ferrite circulator, the permanent magnets with temperature compensation were used in the experiment. The experiments were carried out at room temperature. The experimental results have shown that a 31.88% bandwidth at 15 db isolation has been obtained; the maximum isolation is near 40 db; the maximum insertion loss is about 2 db within the band of 15 db isolation band. The differences between the simulated and measured data are appreciable, as can be observed from Figure 9. The deviation of the center frequency lies in two aspects. Firstly, the dielectric constant stated by the material supplier is quite different with its practical value, which causes the deviation; secondly, the biasing magnetic field is not as strong as the simulation requirement, which causes the frequency deviation also. The SIW-microstrip tapered transition can be treated as a linearly tapered microstrip line (LTML) [11 15]. The characteristic impedance of a tapered microstrip line can be defined below Z t e eff Z c ¼ C 0 p jx ffiffiffiffiffiffi ð4þ Figure 8. Prototype of SIW ferrite junction circulator.

5 12 Che, Ji, and Yung strategies have been discussed. The measured results of a prototype have shown that 31.88% bandwidth at 15dB isolation loss can be expected; the maximum isolation is near 40 db. However, the measured insertion loss is a little high, about 2 db. It is necessary to select substrate material with good performance and improve the fabrication accuracy of SIW ferrite junction circulator. In addition, the frequency deviation can be observed also, owing to the deviation of the dielectric constant of the substrate, as well as improper magnetic field biasing. In this way, some improvements should be made to achieve better results. On the whole, the proposed SIW ferrite junction circulator not only has the same characteristics of conventional rectangular waveguide ferrite junction circulator, but also has other advantages such as low profile, small volume, and light weight, etc. With a simple taper transition, the SIW ferrite junction circulator can be integrated with planar components easily, demonstrating a high integration and potential applications in integrated communication and radar systems. ACKNOWLEDGMENTS Figure 9. Experimental and simulation results of the proposed SIW circulator. In addition, the measured insertion loss is a little bad, 2 db or so. The loss tangent of the practical dielectric material is bigger than the theoretical index, which causes a bigger insertion loss also. In addition, the fabrication inaccuracy causes a lot of error. In our prototype, the multilayer fabrication technology has been used; the misalignment of three layers is very obvious, which contributes also to the big loss. The theoretical analysis has found that the dielectric loss contributes much of the insertion loss of the circulator. In addition, the fabrication inaccuracy of multilayer structure is also a main contribution factor of the loss. Therefore, in order to improve the loss characteristic of SIW ferrite junction circulator, the best way is to choose the substrate material with good characteristics and improve the fabrication accuracy of the multilayer structure. V. CONCLUSION A compact C-band ferrite junction circulator constructed in SIW has been proposed and the design The authors express their gratitude for financial support to the Natural Science Foundation of Jiangsu Province under grant BK , and the National Science Foundation of China under grant REFERENCES 1. J. Helszajn and F.C. Tan, Design data for radialwaveguide circulator using partial height ferrite resonators, IEEE Trans Microwave Theory Tech 23 (1975), Y.A. Akaiwa, A numerical analysis of waveguide H- plane Y-junction circulators with circular partial height ferrite post, J Inst E Commun Eng Jpn (1978), E.J. Denlinger, Design of partial height ferrite waveguide circulators, IEEE Trans Microwave Theory Tech 22 (1974), Y. Cassivi, L. Perregrini, P. Arcioni, M. Bressan, K. Wu, and G. Conciauro, Dispersion characteristics of substrate integrated rectangular waveguide, IEEE Microwave Wireless Components Lett 12 (2002), W. Che, X. Ji, and K. Wu, Characteristics comparison of substrate-integrated rectangular waveguide and its equivalent rectangular waveguide, Microwave Opt Technol Lett 46 (2005), Y. Cassivi and K. Wu, Low cost microwave oscillator using substrate integrated waveguide cavity, IEEE

6 Planar SIW Ferrite Junction Circulator 13 Microwave Wireless Components Lett 13 (2003), Z.C. Hao, W. Hong, J.X. Chen, X.P. Chen, and K. Wu, Planar diplexer for microwave integrated circuits, IEE Proc Microwaves Antennas Propag 152 (2005), D. Deslandes and K. Wu, Single-substrate integration technique of planar circuits and waveguide filters, IEEE Trans Microwave Theory Tech 51 (2003), D. Deslandes and K. Wu, Integrated microstrip and rectangular waveguide in planar form, IEEE Microwave Wireless Components Lett 11 (2001), W. Che, L. Xu, D. Wang, L. Geng, K. Deng, and Y.L. Chow, Equivalence between substrate-integrated rectangular waveguide (SIRW) short-circuit load and its equivalent rectangular waveguide short-circuit load, Microwave Opt Technol Lett 48 (2006), C.L. Edward, M.L. Edwards, and S. Cheng, A simplified analytic CAD model for linearly tapered microstrip lines including losses, IEEE Trans Microwave Theory Tech 52 (2004), M. Kobayashi, A dispersion formula satisfying recent requirements in microstrip CAD, IEEE Trans Microwave Theory Tech 36 (1988), W. D Orazio, K. Wu, and J. Helszajn, A substrate integrated waveguide degree-2 circulator, IEEE Microwave Wireless Components Lett 35 (2004), M. Kobayashi and R. Terakado, Accurately approximate formula of effective filling fraction for microstrip line with isotropic substrate and its application to the case with anisotropic substrate, IEEE Trans Microwave Theory Tech 27 (1979), P. Rustogi, Linearly tapered transmission line and its application in microwaves, IEEE Trans Microwave Theory Tech 17 (1969), C.E. Fay and R.L. Comstock, Operation of the ferrite junction circulator, IEEE Trans Microwave Theory Tech 13 (1965), BIOGRAPHIES Wenquan Che (M 01) received her B.Sc. from the East China Institute of Science and Technology, Nanjing, China, in 1990, and M.Sc. from Nanjing University of Science and Technology, Nanjing, China, in 1995, and Ph.D. from City University of Hong Kong, in From March 1995 to November 1999, she was with the Nanjing University of Science and Technology, where she conducted teaching and academic research as a lecturer in the areas of EM theory and microwave devices, especially the ferrite devices. In 1999, she was with the City University of Hong Kong, as a research assistant. From March 2002 to Sept. 2002, she was with Poly Grames Research Center, Polytechnique de Montreal, Canada, as a visiting scholar. From July 2005 to Sept and from July 2006 to Sept. 2006, she was with Department of Electronic Engineering, City University of Hong Kong as a research fellow. She is currently an associate professor in Nanjing University of Science & Technology. Her current research interests involve electromagnetic computation, multilayer substrate integrated circuits and antennas, microwave and millimeter-wave devices based on high-frequency smart materials, and planar/co-planar structure circuits and subsystems in millimeter wave frequency. She has authored or coauthored over 40 articles in referred journals. Edward Kai-Ning Yung received his B.Sc. in 1972, M.Sc. in 1974, and Ph.D. in 1977, all from the University of Mississippi. After graduation, Edward worked briefly in the Electromagnetic Laboratory, University of Illinois at Urbana-Champaign. He returned to Hong Kong in 1978 and began his teaching career at the Hong Kong Polytechnique. He joined the newly established City University of Hong Kong in 1984 and was instrumental setting up a new department. He was promoted to full professor in 1989, and in 1994, he was awarded one of the first two personal chairs in the University. He is the founding Director of the Wireless Communications Research Center, formerly known as Telecommunications Research Center. Despite his heavy administrative load, Edward remains active in research in microwave devices and antenna designs for wireless communications. He is the principle investigator of many projects worth tens of million Hong Kong dollars. He is the author of over 300 papers, including 150 in referred journals. Edward is also active in applied research, consultancy, and other technology transfers. He holds one patent. He was the recipient of many awards in applied research, including the Grand Prize in the Texas Instrument Design Championship, and the Silver Medal in the Chinese International Invention Exposition. Xiao Jing Ji received her B.Sc. degree and M.Sc. degree from Department of Electrical Engineering, Nanjing University of Science & Technology, China, in 2004 and 2006, respectively. She is currently an engineer in Nanjing Institute of Electronic Technology, China. Her research interest is millimeter wave ferrite devices in planar form.