University of Hyogo Graduate Students Design a Unique Broadband Branch-Line Coupler

Success Story University of Hyogo Graduate Students Design a Unique Broadband Branch-Line Coupler Company Profile The University of Hyogo is a public university located in Chuo-ku, Kobe City, Japan. The university was established in 2004 by integrating three universities that were run by the Hyogo Prefecture government: Kobe University of Commerce, Himeji Institute of Technology, and the College of Nursing Art and Science, Hyogo. The School of Engineering has its origin in the Himeji Institute of Technology and through education and research under the concept “Learn to Create,” aims to foster skilled engineers who can truly serve the well-being and safety of humankind. Application: The Design Challenge Microwave Components Couplers Directional couplers are one of the key circuit components used in balanced amplifiers, Software: mixers, phase shifters, and other RF/microwave devices. As a quadrature hybrid, a NI AWR Design Environment branch-line coupler consisting of four λ/4 lines is typically used. For microwave Microwave Office applications, however, the performance of this coupler is limited by a relatively narrow AXIEM bandwidth of about 10 percent. For a branch-line coupler with loose coupling, the coupling factor C (= 20 log10 |S41|) is less than -10 dB, requiring high-impedance lines. Consequently, the minimum coupling factor of the device is limited by the highest impedance of a pair of branch lines that can be manufactured by general printed circuit board (PCB) technologies. The Solution The University of Hyogo student design team solved the minimum coupling factor limitation by using NI AWR Design Environment, specifically Microwave Office circuit simulator, to design a pair of broadband branch-line couplers with loose couplings utilizing open/short-circuited coupled-transmission lines to replace the branch-line coupler. ‘‘ The availability of and The coupler occupied a small area because open-circuited coupled-transmission lines as familiarity with NI AWR external matching networks were integrated in input/output transmission lines and a Design Environment was broadband characteristic was accomplished with a relative bandwidth of 47.5 percent, key in the success of this based on the equivalent admittance approach for a -20 dB coupler. By tuning the width design. In particular, AXIEM and gap of the coupled transmission lines, broadband characteristics of the relative enabled us to analyze with a bandwidth of over 40 percent were obtained at a center frequency of 4 GHz. very fine mesh such as 5 or 10 microns.’’ – Tadashi Kawai Associate Professor The Technical University of Hyogo u-hyogo.ac.jp/english Fabricated coupler with C=-20 dB. ni.com/awr Figure 1 shows the circuit configuration of the branch-line coupler with open/short-circuited coupled-transmission lines. These were replaced with shunt branches of the conventional branch-line coupler and open-circuited ones were utilized as external matching networks. In this figure, characteristic admittances and impedances of the lines are normalized by those of the input/output port. The branch-line coupler was then designed with a coupling factor of -20 dB utilizing the proposed circuit structure. Figure 2 shows loci of the equivalent admittance with some reference planes. As shown in the figure, the locus of the equivalent admittance through the whole circuit passes through the matching point at three frequencies and a large portion of the locus is included within the circle of 20 dB return loss. This led the team to believe that broadband characteristics could be obtained through the proposed circuit construction. For the branch-line coupler with open/short-circuited coupled-transmission lines, a flat power balance and a 90° phase difference was obtained over a wide frequency range. The relative bandwidth in Figure 2 is 47.5 percent for 20 dB return loss and directivity. This value is about four times more than that of a conventional branch-line coupler without matching networks. j j1/2 j2 A j1/3 Matching Network Z3e,Z3o, θ #1 #3 RETURN LOSS Y1 = 20dB RETURN LOSS = 20dB θ fn = 1.5 B B′ 0.2 0.5 1.0 2.0 1.0 Y2e,Y2o θ fn = 0.5 #2 #4 A′ -j1/3 Yeq -j1/2 -j2 Yeq' Yeq''' -j Figure 1: Branch-line coupler with open/short-circuit. Figure 2: Loci of equivalent admittances with some reference planes. For the branch-line couplers with coupling factors of -15 and -30 dB, respectively, as is the case in the -20 dB coupler, very flat coupling, 20 dB return loss, and directivity can be obtained over wide frequency ranges. To validate the proposed design procedure, the students simulated the designed branch-line coupler using AXIEM 3D electromagnetic (EM) simulator. S11(cal) S31(cal) S11(sim) S31(sim) S21(cal) S41(cal) S21(sim) S41(sim) A substrate with a dielectric constant of 4.5 and a thickness of 0.508 mm was 0 assumed. Figure 3 shows the simulation results of the -20 dB coupler designed at a center frequency of 4 GHz. -10 -20 Why NI AWR Design Environment ] B d [ | -30 The design team chose NI AWR Design Environment because of its availability j i and their familiarity with the product through the AWR University Program, |S -40 which donates free software to qualifying teaching universities. AXIEM enabled the designers to analyze with a very fine mesh such as 5 or 10 microns. -50 For more information about this application, the original paper that is the -60 2.533.544.555.5 inspiration for this success story can be accessed by IEEE members at Frequency[GHz] ieeexplore.ieee.org/document/6986376. Figure 3: Simulation results (S-parameters) for -20 dB branch-line coupler. ©2019 National Instruments Corporation. All rights reserved. AWR, AWR Design Environment, AXIEM, Microwave Office, National Instruments, NI, and ni.com are trademarks of National Instruments. Other product and company names listed are trademarks or trade names of their respective companies. SS-M-HYG-2019.5.11.

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