Dual band planar hybrid coupler with enhanced bandwidth using particle swarm optimization technique

Transcription

1 Dual band planar hybrid coupler with enhanced bandwidth using particle swarm optimization technique Mahdi Yousefi a), Mohammad Mosalanejad b), Gholamreza Moradi c), and Abdolali Abdipour d) Wave Propagation and Microwave Research Lab, Electrical Engineering Department, Amirkabir University of Technology, Tehran, Iran. a) b) c) d) Abstract: This paper presents the design procedure, and implementation of a dual band planar quadrature hybrid with enhanced bandwidth. The topology of the circuit is a dual stage quadrature hybrid, which provides much larger flexibility to allocate the desired operating frequencies and necessary bandwidth than other previously published configurations. In this paper, small dual frequency transformers in two sections are used to modify conventional broad band hybrid coupler for having arbitrary dual band operation frequencies. This performance of the proposed coupler is verified by using commercial simulators and measurements. To this aim, by merging compilers and full-wave simulators, particle swarm optimization is used to achieve proper values to have desired goals. Finally, a dual stage quadrature hybrid for dual band (2.4 and 5.8 GHz) wireless local area network systems has been fabricated, aimed to cover the bands corresponding to the standards IEEE802.11n. The measured results validate good dual band performance at 2.4/5.8 GHz with enhanced bandwidths up to 12.5% and 25%, for the lower and upper bandwidth, respectively which satisfies the bandwidth requirement for the this standard. Keywords: dual band hybrid coupler, particle swarm optimization, wireless local area network (WLAN) systems Classification: Microwave and millimeter wave devices, circuits, and systems References [1] S. Kumar, Ch. Tannous, and T. Danshin, A Multisection Broadband Impedance Transforming Branch-Line Hybrid, IEEE Trans. Microw. Theory Tech., vol. 43, no. 11, pp , Nov

2 [2] I. Lin, M. Vincentis, C. Caloz, and T. Itoh, Arbitrary dual-band components using composite right/left-hand transmission lines, IEEE Trans. Microw. Theory Tech., vol. 52, no. 4, pp , April [3] M.-J. Park and B. Lee, Dual-band, cross coupled branch line coupler, IEEE Microw. Wireless Compon. Lett., vol. 15, no. 10, pp , Oct [4] S. Y. Zheng, S. H. Yeung, W. S. Chan, K. F. Man, S. H. Leung, and Q. Xue, Dual-band rectangular patch hybrid coupler, IEEE Trans. Microw. Theory Tech., vol. 56, no. 7, pp , July [5] K.-K. M. Cheng and F.-L. Wong, A novel approach to the design and implementation of dual-band compact planar 90 branch-line coupler, IEEE Trans. Microw. Theory Tech., vol. 52, no. 11, pp , Nov [6] H. Zhang and K. J. Chen, A stub tapped branch-line coupler for dualband operations, IEEE Microw. Wireless Compon. Lett., vol. 17, no. 2, pp , Feb [7] T. Kim, B. Lee, and M.-J. Park, Dual-band branch-line coupler with two center-tapped stubs, Microw. Opt. Technol. Lett., vol. 50, no. 11, pp , Dec [8] J. Kennedy and R. C. Eberhart, Particle swarm optimization, Proc. IEEE Conf. Neural Networks, Piscataway, USA, pp , [9] R. Levy and L. F. Lind, Synthesis of Symmetrical Branch-Guide Directional Couplers, IEEE Trans. Microw. Theory Tech., vol. MTT-16, no. 2, pp , Feb [10] C. Monzon, A Small Dual-Frequency Transformer in Two Sections, IEEE Trans. Microw. Theory Tech., vol. 51, no. 4, pp , April [11] I.-H. Lin, M. DeVincentis, C. Caloz, and T. Itoh, Arbitrary dual-band components using composite right/left-handed transmission lines, IEEE Trans. Microw. Theory Tech., vol. MTT-4, no. 10, pp , Oct [12] C. Collado, A. Grau, and F. De Flaviis, Dual-Band Planar Quadrature Hybrid With Enhanced Bandwidth Response, IEEE Trans. Microw. Theory Tech., vol. 54, no. 1, Jan Introduction The branch-line coupler is one of the fundamental microwave components used in wide range of applications. This component is used as a power combiner/divider with 90 degrees phase shift in output ports. It is used in many applications such as balanced amplifiers, mixers, antenna and feed networks, but the problem with the conventional coupler is that it has limited bandwidth. Furthermore, the use of circuits able to be functional at multiple frequencies is of great interest for the integration of the circuitry in systems that can operate at multiple frequency bands. In the case of branch-line couplers, different techniques have been investigated to extend the bandwidth of the conventional hybrid such as using the dual stage hybrid coupler [1], left-handed transmission lines [2], the cross-coupled branch-line structure [3], the rectangular patches [4], and stub lines at the input ports [5] or at the center of the branches [6, 7]. In addition, particle swarm optimization (PSO) 1031

3 is a population based stochastic optimization technique developed by Dr. Eberhart and Dr. Kennedy in 1995 [8], inspired by social behavior of bird flocking or fish schooling. In past several years, PSO has been successfully applied in many research and application areas. It is demonstrated that PSO gets better results in a faster and cheaper way compared with other methods. It has been used for approaches that can be used across a wide range of applications, as well as for specific applications focused on specific requirements. This letter proposes another branch-line coupler structure for the dual band operation based on the dual frequency transformers with two sections in each branch of dual stage broadband hybrid coupler. Furthermore, PSO is applied to the variables of this scheme to achieve dual band operation in desired frequency ranges. Finally, proposed scheme has been fabricated and tested for WLAN systems. The broad-band characteristics of the proposed prototype make it suitable for covering the worldwide frequencies for the n-2009 standard, in the 2.4 GHz band that extends from 2.4 to GHz, and the 5.8 GHz band that extends from 5.15 to 5.85 GHz, respectively. Specifically, the measurements of this prototype show a 12.5% and 25% bandwidth for the lower and upper frequency, respectively, satisfying and exceeding the bandwidth requirements for the above standard. 2 Design procedure Branch line hybrids are extensively used in the realization of a variety of microwave circuits, but this circuit has a big problem and the problem is that it has limited bandwidth. Using dual stage broadband impedance transforming hybrid which is proposed in [1] solves the problem of conventional branch line couplers and increases the bandwidth. In addition, a small dual frequency transformer in two sections which is investigated in [9, 10] results in impedance matching at two arbitrary frequencies. These series sections can be used in general to have dual band components. Fig. 1 (a) expresses two section transformers. According to [10], to have desired input impedance in two different frequencies, following equations should be solved which result in 4 equations in these frequencies. tan(βl a ) tan(βl b ) = Z a(zb 2 R LZ ) Z b (Z R L Za) 2 tan(βl a ) tan(βl b ) = Z a Z b (R L Z ) Z b (ZaR 2 L Zb 2Z ) (1) (2) Using of such dual frequency transformers in each branch of dual stage branch line coupler which has broad bandwidth in comparison with conventional one, leads to a novel dual band quadrature hybrid with increased bandwidth in two upper and lower bands. This novel quadrature hybrid is shown in Fig. 1 (b). Three distinct dual transforms are used in this scheme and the other branches are the same because of symmetry. In order to have symmetry in the proposed scheme, two-section transformers in vertical branches are changed into three-section ones, in which the first and the third sections are the same; therefore all the ports will be identical. The impedances and 1032

4 Fig. 1. (a) Small dual section transformer. (b) Layout of novel branch line coupler. (c) Top view of fabricated quadrature hybrid coupler. electrical length of dual stage coupler are calculated using the procedure in [1]. Impedance and electrical length of each section in proposed scheme must be the same with those of dual stage coupler in even and odd mode in 2 different center frequencies. So (1), and (2) are basic equations to achieve these parameters analytically. But, as can be seen in Fig. 1 (b), there are lots of parameters that should be tuned to achieve optimal scattering parameters and proper phase differences for the coupler. Because of sophisticated equations for each section and due to the fact that classical microwave equations are frequency dependent, all variables should be optimized by full wave simulator. This process is inevitable in designing microwave components. In this case, PSO algorithm is used to find and optimize the wide range of this scheme s variables, which is better solution to find wide range of parameters. PSO algorithm models swarm behavior, so if there is a bunch of parameters in a specific problem, the values of these parameters move toward the position in the problem space, so the best answer for the problem is finally reached. It should be emphasized that the algorithm is continuous, i.e. the particle s parameters can be assigned any value in the defined interval. Also, a function that evaluates the position in the solution space is needed, which is called the fitness function. To specify the fitness function more accurately, some conditions are needed. The conditions for the fitness function are defined as follows: S 21 S 31 1dB (3) 4 db S 21, S 31 3dB (4) S 11, S db (5) 1033

5 85 S 21 S (6) These conditions must be satisfied in upper and lower bandwidths of WLAN, which are 2.4 to GHz, and 5.15 to 5.85 GHz. To do this, the conditions are checked in some prototype frequencies which are located in upper and lower bandwidth to assure that the conditions are satisfied all over the WLAN bandwidth. In this case, 6 frequencies are considered as prototype frequencies, 2 center and 4 marginal frequencies in upper and lower bands. To this aim, the PSO codes have been written and a full-wave simulator based on the moment method has been joined to the code. By proper definition of fitness function, appropriate values for coupler parameters would be achieved. 3 Results and discussion A photograph of the implemented circuit is shown in Fig. 1 (c). The circuit is fabricated on Rogers RO4003C with relative dielectric constant er = 3.38 and thickness h = mm. After running PSO algorithm, the physical dimensions of the circuit according to the Fig. 1 (b) are calculated. Table I. contains the value of designed parameters. The simulated and measured S21 and S31 parameters of the coupler are shown in Fig. 2 (a). This indicates that at both lower and upper frequency bands, the S21 and S31 are limited between 3dB and 4 db and their difference are lower than 1 db, which means equal coupled and transmitted signals to the output ports at these frequency bands. In addition, according to Fig. 2 (b) the simulated and measured S11 and S41 indicate that the both input return loss and the isolation are better than 15 db, along both lower and upper frequency bands. According to these results, the experimental bandwidths for the lower and upper bands are respectively 12.5% and 25%, which fulfills the requirements of a dual band hybrid for the WLAN systems. In previous works such as [11, 12] the upper band bandwidth is 5% and 18%, respectively. In comparison, in this work the upper band bandwidth is 25% which is higher than them. Fig. 2 (c) shows the differential phase between the ports 2 and 3, which are through and coupled ports. The results show agreement in measurement and method of moment EM simulation for Table I. Values of designed parameters. 1034

6 Fig. 2. Measured and EM simulated (a) S21 and S31, (b) S11 and S41, (c) Phase difference between coupled ports. the scattering parameters and for the differential phase between the coupled ports of the quadrature hybrid, which is designed for WLAN systems. 4 Conclusion A novel dual band modified branch line coupler with acceptable operation bandwidth by employing small dual frequency transformers in two sections has been proposed. Due to wide range of physical parameters of the circuit, PSO algorithm is applied to find the parameters to optimize the circuit in the best way. Because of simple structure and optimization algorithm, it can be fabricated and designed easily. This type of dual band coupler has been designed and measured to be used in WLAN systems under the standard IEEE802.11n. A minimum of 12.5% and 25% bandwidths were measured for the lower and upper band, respectively. Thus, the proposed circuit satisfies the requirements of this standard. This method can be applied to wide range of microwave components by using full-wave simulation and optimization algorithms. 1035