This paper describes Microwave Workshop for Windows, a suite of three programs which have been designed in the Division of Physics in the University of Portsmouth to perform the dual function of teaching aid and providing a circuit‐design utility in the field of microwave engineering

White, Colin

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Microwave workshop for WindowsColin WhiteDivision of Physics, University of PortsmouthThis paper describes Microwave Workshop for Windows, a suite of three programs which havebeen designed in the Division of Physics in the University of Portsmouth to perform the dualfunction of teaching aid and providing a circuit-design utility in the field of microwaveengineering.IntroductionA suite of three programs has been developed to support the teaching of microwavetheory and design. A secondary function of the package is to support microwaveengineers by providing a library of utilities to assist their design function. All threeprograms were written in Visual Basic and are aimed at supporting both tutor-directedand student-centred learning methodologies. The development team consisted of threefinal-year degree students.Program 1 is Smith Chart. The Smith Chart is one of the most important tools for themicrowave engineer, yet students find the concepts of its application difficult to graspusing traditional teaching methods. Students can choose between running an animatedtutorial or using the chart, in stand-alone mode, to solve problems.Program 2 is Microwave Laboratory. This is a virtual microwave laboratory and includesall the waveguide components one would reasonably expect to see in such a laboratory.The components can be connected in any combination, and a generator and detector canthen be connected to the completed circuit's remaining ports. Important parameters canbe varied by the student, and results in terms of power or Voltage Standing Wave Ratio(VSWR) can be observed.Program 3 is Microwave Device Technology, and is a hypertext interpretation of lecturecourse notes describing a wide range of devices at three levels of complexity. Level 1S7Colin White Microwave workshop for WindowsEU Iocls jftew b«lpRaaxtma R u t e n;Digna IRtfOafl(fi AtMnurfjenVSWR 103.50 IFUlaeL Golf 109.« \t |0«5- Ikj 1-0.5731IMPEDANCE MODEFigure I: Smith Chartreflects the requirements of a Higher Diploma unit, Level 2 corresponds to final-yeardegree level, and the highest level supports postgraduate teaching within the institution.All three programs include extensive help files which describe not only program operationbut also context-sensitive microwave theory and device descriptions.Smith ChartThe currently accepted form of the Smith Chart was developed by Philip H. Smith in1936, and is now extensively used by microwave and radio-frequency engineers to aiddesign work. The chart is a two-dimensional graph with real and imaginary axes (arganddiagram) on which can be mapped complex impedance points such as those which mayexist along a high-frequency cable or transmission line. The chart may be used tocalculate the value of the impedance at the end of the line (the load impedance), oralternatively the length of a transmission line necessary to achieve an impedance match.The chart does not provide answers which could not be derived by someone of a goodmathematical grounding armed with a calculator; rather, it performs the function of aspecial-to-type slide rule to speed up such calculations.The individual steps required to perform the calculations using the chart are actuallyquite trivial. The difficulty students have in grasping the operations is, I believe, due inpart to their inability to equate real and measurable quantities, such as voltage and powerwhich exist along a transmission line, into coordinates and lines on the somewhat abstractconcept of a chart. Indeed, some students successfully complete calculations on the chartby rote, with little knowledge of the underlying philosophy. The students' cognitivedifficulty is further compounded by the fact that the two axes of the chart are circles andsegments of circles respectively, and they are also highly non-linear. Finally, the confusion58ALT-} Volume 3 Number Iis exacerbated by virtue of one of the major attributes of the Smith Chart: eachcoordinate point represents firstly a complex impedance value, secondly the value of thereflection on the transmission line (the VSWR) and finally the distance of the point on theline from the line's termination. It is small wonder that students have trouble when facedwith a real problem to solve, and given only a blank, black-and-white chart which isremote from any real hardware.In such situations a coloured CAL animation becomes an extremely powerful learningtool. The Smith Chart tutorial takes the student in a step-wise manner from constructionof the chart itself through to incremental problem solving. Students are at all times incontrol of the progression of the program, and hence their pace of study. Colour is usedto highlight relevant areas of an otherwise invariable chart, and this is mirrored withhighlighted points along a drawing of an actual transmission line. Movement around thechart corresponds to movement along the transmission line, and student-selectableanimations are used to good effect to illustrate these aspects.The, Smith Chart utility is a fully operational chart on which all functions that wouldnormally be carried out with a paper chart, pencil, ruler and compasses (and probablyrubber!) can be performed by keyboard and mouse-click and displayed as accurately asthe screen resolution will allow. A Magnify button is included for situations where screenresolution is a limiting factor. Results are as accurate as the computer's arithmeticprecision has been set, rather than governed by the thickness of the pencil lead, and it is,consequently, orders of magnitude better than the traditional method. As the cursor ismoved around the chart by the mouse, the corresponding impedance value is updated inreal time. This dynamic nature, again, aids the students' understanding of the chart asthey acquire a better feeling for the impedance values in different areas of the chart thanthey ever would by merely reading off the scales. Finally, although unnecessary in thenormal course of events, a calculator button is included for emergencies.Although this is not the only software implementation of the Smith Chart available, it isthe only one designed primarily for teaching purposes. It is also the most intuitive and,perhaps most significantly, the only implementation which runs under Windows.Microwave LaboratoryMicrowave Laboratory is an accurate visual representation of a real teaching microwavelaboratory. All the common components are represented. They are taken individually, asrequired, from the virtual component store, and drag-dropped onto the virtual test benchwhere they are automatically connected together if dropped adjacent to anothercomponent. In this manner, a complete circuit may be built up, and the input/outputports are connected to the microwave generator and receiver respectively. These, again,are accurate representations of commercially available equipment.Devices capable of parameter variation are adjusted using a control pane launched bydouble-clicking the device. To aid the construction of an elegant arrangement of thecircuit on the workbench, any device can be reoriented in 90° steps by pressing the Rotatebutton. Further, any device can be deselected by dragging it to the Mac-style dustbin.59Colin White Microwave workshop for WindowsOnce a circuit has been constructed, the generator power and frequency can be selectedand then switched on. First, the circuit is checked for validity by confirming that theworkbench contains one launcher, one or two detectors, no open ports and no loops.Each device in the circuit defines a square array-type variable of order equal to thenumber of ports. This device array fully defines the device characteristics. A matrixmultiplication routine calculates the power at the detector and displays it on the virtualAmp in digital and analogue format. If a standing wave detector has been selected, poweris also displayed graphically in a pane below the workbench, as a function of length alongthe transmission line.The usual Windows menus or short-cut keys allow any useful circuits to be saved andthen opened at a future date or, indeed, to be printed.Certain common devices are not available in the device store. There are, however, soundpedagogic reasons for this. By way of example, consider the wavemeter. This is a deviceused to measure very accurately, but with much time and effort, the output frequency ofthe generator. It is obvious that the frequency accuracy of the virtual generator is definedby the program's mathematical precision and is certainly orders of magnitude better thanthe real wavemeter. There is, therefore, no reason to specify a wavemeter and, perhapsmore importantly, the measurement of accurate frequency can detract from the main aimsof the experiment.This laboratory simulation will model all the usual microwave experiments quickly,efficiently and accurately, but it important that students do not perceive it as the realworld. The program is intended to support traditional experimental procedure but not toreplace it. Typically, the student is given some time on the package, constructing thecircuit and measuring relevant parameters before doing it for real. By this means he or sheacquires an overview of what each device does, what happens when they are connected ina particular configuration, and the particular measurement technique to be learned by theexperiment. An important part of the learning strategy is in the interpretation ofmeasurement discrepancy between the virtual and the real laboratories.Microwave Device TechnologyThe third program is simply a hypertext implementation of microwave course notes. Thenotes reside in three levels representing HND, final level degree and MSc teaching. The levelcan be switched at any point in the device descriptions to enable a lower-level student toread about a particular device at a higher level, if required (or even desired!). The programhas been used in a traditional lecture environment using a laptop and overhead tablet as acleaner alternative to chalk. Students have access to the notes on the network and they canprint out any or all of the levels, as required, for study remote from a terminal.Future workFuture work will involve both improvements to the existing programs and also additionsof new programs to the workshop suite.60ALT-} Volume 3 Number IThe Smith Chart presently supports only calculations on one impedance point.Occasionally it is necessary to work with two points and, for instance, calculate theimpedance difference. A future version will incorporate this facility.The Microwave Laboratory can support only devices specified in the device store. It is atrivial addition to include a user-defined black box which may be connected on theworkbench. In this case, by double-clicking on the black box, the user may write directlyto the defining array the parameters of some device not included in the store.However, a non-trivial addition to the microwave laboratory could be the inclusion of aswept-frequency generator and amplitude analyser. This would open up a range of moresophisticated experiments involving frequency response measurement techniques.The Microwave Device program will soon have device diagrams reinforced by colourscanned images of the devices.As for additional programs, a microstrip version of the laboratory simulator is plannedwhich is thought to be of interest to commercial companies, and an antenna design CALpackage and design utility.61

Microwave workshop for windows

A suite of three programs has been developed to support the teaching of microwave theory and design. A secondary function of the package is to support microwave engineers by providing a library of utilities to assist their design function. All three programs were written in Visual Basic and are aimed at supporting both tutor-directed and student-centred learning methodologies. The development team consisted of three final-year degree students

Colin White

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Microwave workshop for windows

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