Design of a 9GHz, 7dB Branchline Coupler with 180 Phase Shift at Outputs

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1 Design of a 9GHz, 7dB Branchline Coupler with 180 Phase Shift at Outputs Usman Sammani Sani Lecturer, Department of Electrical Engineering Bayero University, Kano, P.M.B. 3011, Nigeria. usmanssani@live.com Abstract:--Branchline couplers have a number of applications in today s microwave communications systems. This work gives an informative design of a branchline coupler with 180⁰ phase shift between the output ports using the design guide feature of Advanced Design Systems by Agilent. output ports using the design guide feature of Advanced Design Systems by Agilent. Keywords: -- Branchline Coupler, Transmission Lines, Dielectric Material. I. INTRODUCTION Branchline couplers are used in most communication devices that include amplifiers, phase shifters and mixers. It is a device made up of quarter wave length lines, which are four in number [1]. They are widely used in phase shift and power division applications [2]. The branchline coupler has four ports as shown in figure 1. Power applied at port 1, divides to ports 2 and 3 with a phase difference of 90 or 180. Thus port 1 is an input port, both ports 2 and 3 are outputs and are sometimes called through and coupled ports respectively. It is to be noted that port 4 is an isolated port. As a result, no power is obtained from it. Though, any of the ports could be used as an input port due to the coupler s symmetry. The implementation of the design could be in strip line, microstrip or coaxial line. This work gives an informative design of a branchline coupler with 180⁰ phase shift between the 11 Figure 1. Structure of a Branchline Coupler [3]. Branchline Couplers are usually characterized by quantities known as Coupling (C), Directivity (D), Isolation (I) and Insertion loss (L) which are obtained from the following equations [3-5]: (1) (2) (3) (4) This work is focused on the design of the branchline coupler using microstrip. Microstrip consists of a dielectric material sandwiched between a metal trace of width W and a ground plane of thickness, T. The

2 dielectric material has its own properties such as dielectric constant, ε and loss tangent, TanD [6], [7]. II. METHODOLOGY Its structure is shown below: Advanced Design Systems simulation software was used with the aid of its design guide feature. Gold was used for metallization, which has a thickness of 2µm and conductivity of 4.1x10 7 Sm. Dielectric material used is alumina with a thickness of 100µm. The backside of the alumina has a 10μm thick metal ground plane. Dielectric loss tangent (TanD) for alumina is A branchline coupler component and a microstrip circuit component were placed in the ADS design guide schematic capture and parameters were inputted as shown in the screen shot presented as figure 3 below. Figure 2. Microstrip Transmission line [8]. Figure 3. Screen shot of parameters used in coupler design 12

3 The design guide was then able to compute all the coupler s transmission line parameters as shown in figure 4 below Figure 4. Transmission Line Parameters Computed by ADS Design Guide Simulation was then carried out using the circuit in Figure 4. III. RESULTS & DISCUSSIONS The simulation was carried out from 0.5GHz to 18.5GHz using a step of 0.5GHz. As shown Figure 5, the phase at port 2 at 9GHz was 0 and that at port 3 was Thus the aim of achieving 180 phase difference between the output ports was achieved with an error of %, which is within acceptable. Figure 5. Phase of signal at the output ports. 13 Power applied at port 1, divides to ports 2 and 3 with a phase difference of 90 or 180. Thus port 1 is an input port; both ports 2 and 3 are outputs.

4 Figure 7. Forward Transmission at output ports. Figure 6. Reverse Transmission at output ports Figures 6 and 7 reveal the reverse and backward transmissions at the output ports. As it can be seen, both are the same. This must have been due to the symmetry of the branchline coupler in that each port could be used as the input port. Thus the direction doesn t matter. Figure 8. Layout of the designed coupler 14

5 IV. CONCLUSION The paper has presented the design of a coupler using Agilent Advanced Design Systems software. The design was targeted at 180 phase shift between the outputs, which was achieved with an error of %. Results have also shown that forward and backward transmissions at the output ports are the same. This show that any of the ports could be used as an input port, not necessarily port 1. The paper also reveals how design guide feature of the software simplifies the design process. V. REFERENCES [1] Sun L. et al, A Novel Miniaturized Branch-Line Coupler with Equivalent Transmission Lines, Progress In Electromagnetics Research Letters, Vol. 38, 2013, pp [2] Liu Q.et al, Generalized Impedance-Transforming Dual-Band Branch-Line Couplers for Arbitrary Coupling Levels, Progress In Electromagnetics Research B, Vol. 53, 2013, pp [3] Bendali A. et al, Conception of the Coupler with 3 Branches, IJCSITS, Vol. 2, No. 2, 2012, [4] Tutkur E., Wideband Directional Couplers and Power Splitters, Unpublished Master of Science Thesis, Chalmers University of Technology, [5] Grebennikov A., Power Combiners, Impedance Transformers and Directional Couplers: Part III, in High Frequency Electronics, Summit Technical Media, LLC, 2008, pp [6] Singh S. et al, Extended Port Dual Band Planar Branch Line Coupler, International Journal of Emerging Technology and Advanced Engineering, Vol 4, Issue 1, 2014, pp [7] Laverghetta T. S., Microwaves and Wireless Simplified (2 nd ed.), Artech House, [8] Carr J. J., Microwave and Wireless Communication Technology (1 st ed.), Newnes, Usman Sammani Sani graduated from Bayero University, Kano in 2008, where he obtained a bachelor degree of electrical engineering. He then furthered his studies, in which he obtained an MSc in Electronic Communications and Computer Engineering from The University of Nottingham Malaysia Campus in Usman is presently a lecturer in the Department of Electrical Engineering, Bayero University, Kano, Nigeria. His research interests include digital communications, digital circuits design and testing of fabricated electronic components. 15