University of Hyogo

Graduate Students Design 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.

The Design Challenge

Directional couplers are one of the key circuit components used in balanced amplifiers, mixers, phase shifters, and other RF/microwave devices. As a quadrature hybrid, a branch-line coupler consisting of four λ/4 lines is typically used. For microwave applications, however, the performance of this coupler is limited by a relatively narrow 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 coupler occupied a small area because open-circuited coupled-transmission lines as external matching networks were integrated in input/output transmission lines and a broadband characteristic was accomplished with a relative bandwidth of 47.5 percent, based on the equivalent admittance approach for a -20 dB coupler. By tuning the width and gap of the coupled transmission lines, broadband characteristics of the relative bandwidth of over 40 percent were obtained at a center frequency of 4 GHz.

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