Ural Federal University

Ural Federal University Students Realize RF Miniaturized Branch-Line Coupler Using NI AWR Software

Company Profile

The Ural Federal University is one of the leading educational institutions in the Ural region of Russia and acts as a research and innovation center of the region, cooperating closely with the Russian Academy of Sciences. Training of students is carried out in three main directions and 67 academic majors. Within the 90 years of its history, Ural Federal University has had more than 270,000 graduates, many of whom have become successful in research, culture, and politics. One of the most famous graduates of the university is the first president of Russia, Boris Yeltsin, for whom the university is named.

The Design Challenge

The aim of the design project was to reduce the size of a 3 dB branch-line coupler as much as possible without losing its performance in isolation, imbalance, and operating bandwidth. Under the supervision of Dr. Yuri Mitelman, a group of students, including Victor A. Chechetkin, PhD Student and Denis A. Letavin, MSc student, were challenged to design a miniaturized branch-line coupler based on low-pass microstrip filters instead of conventional transmission lines in order to achieve a compact circuit layout in microstrip implementation. The design of the couplers was to be performed using fifth, seventh, and ninth order low-pass filters (LPFs). Prototypes with these proposed structures, operating at 2 GHz, were designed and implemented using microstrip technology. The design procedure became more complicated for higher order LPFs because of the increasing number of elements in the filters and the decreased unoccupied area inside the coupler, which was due to the larger number of gaps between the elements. This required additional coupling estimations and more iterations of miniaturization (moving the microstrip capacities so they would occupy mostly the internal area of the coupler). The goals of the design project were to apply the LPFs in order to reduce the physical size of the branch-line coupler, find the optimal filter order, and calculate the S-parameters of the designed devices.

The Solution

The students used the schematic simulator in NI AWR Design Environment™ to obtain the equivalent model of the structure in order to choose the initial parameters for the miniaturized design. The integrated environment enabled them to easily make a model with schematic elements and then move it to the AXIEM 3D planar EM simulator, where the simulation went very quickly. 

The proposed miniaturized designs were obtained by replacing the quarter-wavelength microstrip lines with low-pass filters of different orders. The substrate chosen was FR-4, 1 mm height, with ε = 4.2 and tg δ = 0.02. 

All of the designs showed a significant reduction in the size of the coupler. The most compact and simple design was realized with the fifth order, as its size was 13.5 mm × 14.5 mm = 195.5 mm2, which is 68.5 percent smaller than a conventional coupler.

Why NI AWR Design Environment

Dr. Mitelman and his students chose NI AWR Design Environment because it was easy for the students to learn and they could start using it immediately without being experienced with EM and schematic simulators. The AXIEM planar EM simulation tool provided easy transfer of the schematic model to the EM structure and the built-in TX-LINE application to calculate microstrip lines and powerful filter wizard were pivotal in the success of this design effort. 

Note: The design discussed in this success story was also presented at the Loughborough Design and Propagation Conference in the UK. It can be downloaded by IEEE members on IEEE Xplore.