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DEVELOPING A CAD TOOL FOR RADIO FREQUENCY AND MICROWAVE ENGINEERING EDUCATION
D. BAIS AND D. A. ROGERS Department of Electrical and Computer Engineering North Dakota State University Fargo, ND 58105-5285

ABSTRACT Use of computer-aided design (CAD) tools in radio frequency (RF) and microwave engineering is common in electrical engineering education. Study in these areas involves advanced electromagnetic theory and development of strong analytical skills for implementing practical devices. Many graphics and non-graphics based CAD tools have been developed to support various areas of RF and microwave engineering. The Computer-Aided Microwave Design System, or CAMDS, was designed and developed as a tool especially to help engineers and engineering students to solve problems in these areas. CAMDS presents the significant advantage of being an interactive program written in MATLAB 7. It is particularly useful since it uses a programming language that is familiar to electrical engineering students. In its present form CAMDS is an educational tool that targets problems ranging from transmission line theory to microwave component design. INTRODUCTION Use of CAD tools has become an essential part of radio frequency (RF) and microwave engineering courses (henceforth referred to as microwave engineering courses). Problem solving both at the graduate and undergraduate levels can require rigorous mathematical analysis in addition to understanding the underlying concepts needed to perform necessary design or analysis tasks. Although it is important to be well acquainted with hand calculations for the designs to understand the method, use of CAD tools is encouraged at intermediate and advanced levels of expertise. On-the-job applications require engineers to make use of more advanced CAD tools since the complexity and the number of variables that might be involved in a design can be significant, especially when compared with those encountered in school. This paper introduces the Computer-Aided Microwave Design System (CAMDS). CAMDS is a system of CAD tools developed for solving problems in various areas of microwave engineering. CAMDS was designed and developed in MATLAB 7 to deal with design problems in microwave engineering encountered in textbooks such as Microwave Engineering by D. M. Pozar (2005).

Proceedings of the 2008 ASEE North Midwest Sectional Conference

2 ADVANTAGES AND DISADVANTAGES OF CAD TOOLS Azemi and Stook (1996) clearly identify the advantages and disadvantages of the use of CAD tools in teaching microwave engineering courses. One of the advantages is that graphs of functions become very helpful in understanding the phenomena. These packages provide the capability to solve fairly complicated problems that would be rather unrealistic for a paper-andpencil solution. The greatest advantage is an increased interest among students in the subject material (Azemi and Stook, 1996; Gupta, 1990). Industrial use of soft designs and simulation capabilities, provided by advanced CAD tools, help lower the cost of making prototypes during the development phase of new or modified designs (Gupta, 1990). As for disadvantages, many CAD tools require a special computer configuration to run. Not all tools are freely available. Some require the user to buy a license which could be expensive for students. The learning curve for students and instructor can be steep. There is also a disadvantage in defining a baseline in the curriculum for teaching these software packages (Azemi and Stook, 1996). COMPUTER-AIDED MICROWAVE DESIGN SYSTEM (CAMDS) The Computer-Aided Microwave Design System (CAMDS) overcomes these disadvantages and still has the advantages mentioned above. CAMDS was designed and developed in MATLAB 7. It does not interface with any other program or subroutine written in a different language. MATLAB is widely used in various undergraduate courses in electrical engineering. MATLAB is commonly installed in university computer clusters, and a moderately priced student version is available. CAMDS can be used in upper division or graduate courses. At these levels there is virtually no need for an extra effort to learn MATLAB operations. The package is selfexplanatory and is easy to use. PROGRAM STRUCTURE OF CAMDS CAMDS is an interactive program. Consisting of a set of “.m” files, it covers a wide range of topics in microwaves. Most of the files can be run independently. Files created for filter design have associated subroutines. A file called “takename.m” is a separate .m file which cannot run on its own. Other files, which fetch inputs from the user, call this file. CAMDS as a package includes just one graphical user interface named “StartHere.m.” This file was created to help the user navigate through CAMDS. For smooth operation, all files must be stored inside the same folder. CAMDS is described in detail in a thesis written by D. Bais (2008a), and the code is available in a separate technical report (Bais, 2008b). CAPABILITIES CAMDS currently is capable of solving a total of 19 different types of problems. Some of these are microstrip, stripline, parallel-plate waveguides (TEM, TE, and TM modes), rectangular waveguides (TE and TM modes), circular waveguides (TE and TM modes), network parameters

Proceedings of the 2008 ASEE North Midwest Sectional Conference

3 (ABCD, Z, Y, and S), single-stub tuners (series and shunt), double-stub tuners, L-match tuners, filter design by Kuroda’s method, and stepped-impedance design. For waveguide problems the user can calculate a maximum of ten propagating modes for a given signal frequency in a waveguide. This feature is embedded inside the design programs for waveguides. For stub-matching problems the user can choose to plot the absolute value of the reflection coefficient verses frequency for a defined band. Filter design by Kuroda’s method and stepped-impedance design make CAMDS unique. The current version is limited to six-element filters. CAMDS is capable of generating all possible parameters for a particular design as output, although GIGO (“garbage in, garbage out”) applies. Interested readers can see Bais (2008a) for details on the program including examples. A user’s guide is also available (Bais, 2008b). CAMDS EXAMPLE CAMDS uses only one graphical user interface (GUI) file in the suite. The file “StartHere.m” was developed to help the user navigate through the program. The corresponding “listbox” as shown in Figure 1 lists all programs that can run in CAMDS. The user can select one program at a time and press the “GO” button.

Figure 1. User Interface for CAMDS Program.

An L-section matching network for a load impedance of ZL = 200 – 100j is shown as an example. The line impedance is 100 and the design frequency is 500 MHz. The process is illustrated starting with Figure 2 where the complex value of ZL is indicated by the user. Additional inputs and computational requirements are specified in the boxes that pop up next in the MATLAB environment (Figures 3 through 6). Proceedings of the 2008 ASEE North Midwest Sectional Conference

4 Depending upon the input variables and selections made by the user while defining the inputs for the selected design, the appropriate parameters are calculated and displayed. The first result displayed in this case (Figure 7) is a plot of the reflection coefficient versus frequency for each of the two L-section solutions that are possible (Bais, 2008a).

Figure 2. Input box for the load impedance for L Match.

Figure 3. Input box for the characteristic impedance for L Match.

Figure 4. Input box for the matching frequency for L Match.

Figure 5. Question box for plotting reflection coefficient vs. frequency for L Match.

Proceedings of the 2008 ASEE North Midwest Sectional Conference

5

Figure 6. Input box for the frequency range for L Match.

Figure 7. Plot of the reflection coefficient vs. frequency for L Match. The final results are displayed in the MATLAB command window (Figure 8). All results displayed in the command window end with two dotted lines at the bottom to indicate one complete run. These lines are also helpful to distinguish one set of results from another in case multiple programs are selected by the user. As can be seen in Figure 8, all outputs are displayed with the commonly used symbols and units for the respective outputs.

Proceedings of the 2008 ASEE North Midwest Sectional Conference

6

Figure 8. L Match design output as seen in the command window.

AVAILABILTY CAMDS is available for educational use without charge from the NDSU Department of Electrical and Computer Engineering (Bais, 2008a and 2008b). Users can modify the code for their personal use. Selling the code for profit is considered as misuse. CONCLUSION CAMDS deals with problem solving in various areas of microwave engineering. Designed solely in MATLAB, this package reduces the time normally required to become familiar with new software by its users. This CAD system requires no Internet connection. No special skill set is required for running this program. It can be stored in a local drive and occupies very little space. This program is not recommended for use in elementary courses, but it is useful in upper division and graduate courses dealing with radio frequency and microwave engineering. It can be very helpful in designing homework problems or in checking homework solutions.

Proceedings of the 2008 ASEE North Midwest Sectional Conference

7 CAMDS is unique in its design since it generates a detailed list of parameters based on the userspecified input and still leaves to the final judgment of the user the actual implementation of results. CAMDS can be extended to include more topics in microwave engineering, such as couplers, amplifiers, power dividers, transmission lines, etc. It can also be extended to include the areas of electromagnetic compatibility and antenna design. The same code could be transformed to use more appealing graphical user interfaces.

REFERENCES Azemi, A. and Stook, C. (1996). Using MATLAB in Graduate Electrical Engineering Courses, Proceedings of the 1996 ASEE Annual Conference (Session 1532, Paper No. 01637). Washington, DC. Bais, D. (2008a). Computer-Aided Microwave Design System. Unpublished master’s thesis, North Dakota State University, Fargo. Bais, D. (2008b). User Manual for CAMDS: A MATLAB-Based Computer-Aided Microwave Design System (Tech. Rep.). Fargo, ND: North Dakota State University, Department of Electrical and Computer Engineering. Gupta, K. C. (1990). Teaching CAD to Microwave Students. IEEE Transactions on Education, 33, 140-144. Pozar, D. M. (2005). Microwave Engineering (3rd ed.). Singapore: John Wiley and Sons, Inc.

BIOGRAPHICAL INFORMATION
DIVYA BAIS is a graduate student of Dr. David A. Rogers. She earned her M.S. in Electrical and Computer Engineering from North Dakota State University, USA in 2008 and the B.E. in Electronics and Telecommunication from Pt. Ravi Shankar Shukla University, Raipur, India in 2003. DAVID A. ROGERS is a Professor of Electrical and Computer Engineering at NDSU. He earned the B.S.E.E. and Ph.D. (E.E.) degrees from the University of Washington, the M.S.E.E. from Illinois Institute of Technology, and the M. Div. (Ministry) degree from Trinity Evangelical Divinity School.

Proceedings of the 2008 ASEE North Midwest Sectional Conference

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