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Abstract - Conventional instruction on analog circuits has been based essentially on quantitative analytical methods. These methods are specially suited for analysis purposes; however, the process itself often obscures the desired intuitive understanding of the major aspects of the behavior of devices and circuits. Moreover, the graphs commonly utilized to depict the results of the analyses are not always suitable for a clear reading. This is particularly true for first-year B.Sc. undergraduates. The present work suggests a visual instruction method, based on an animated three-dimensional display of circuit operation, called 3D-Schema. While a conventional schematic diagram is two-dimensional in nature, 3D-Schema adds a third dimension, using perspective, to provide an apparent display of circuit node voltages.
The search for new methods and models to depict different aspects of the operation of electronic devices and circuits has been an important goal for research in the electronics instruction field . Schematic diagrams, mathematical models of devices, network theory and certain sorts of graphs are examples of established models and methods. Most of them were developed to be used by hand, with paper and pencil, the simplest tools available .
When powerful personal computers became available at a low cost, the use of computer aided design (CAD) tools came into the field of computer aided instruction (CAI). This is the case for schematic acquisition and SPICE based simulation tools . They are probably the most widely used, either separately or as integrated environments. Such tools can be placed between the traditional theory lectures and practical laboratory work.
The ways utilized to show the results of simulation have evolved from the original SPICE to current commercial environments. Initially, the results from the different analyses were collected and written either in the form of tables (.PRINT command) or as "line printer" plots (.PLOT command). Later, PC graphic capabilities allowed the implementation of graphics post-processors (.PROBE command) able to show oscilloscope-like displays . The combination of the schematic diagram and oscilloscope-like windows is an interesting way to develop presentations and textbooks .
In spite of their unquestionable interest, the techniques described previously have a common restriction: they are all the computer version of traditional methods and do not take advantage of the many capabilities of a personal computer. Two of these capabilities, frequently used in other fields, are three-dimensional representation and animation . Both of them have interesting properties from the instruction standpoint, i.e. they are apparent and engaging for students.
The present work shows a system called 3D-Schema able to display an animated three-dimensional model of analog circuit operation on the PC screen. The main goal is to give life to the traditional schematic diagrams. While these diagrams are a static two-dimensional representation (idle schematic), 3D-Schema adds a third dimension in order to show the voltage level present at every circuit node (active schematic). This way, the diagram raises, becoming a clear image of circuit state, where each node height is proportional to its voltage.
Among the different analyses a simulator can accomplish, the static transfer function or direct current sweep (.DC) and the transient simulation (.TRAN) are the best suited for a 3D-Schema representation. The inspection of the .DC analysis results via the 3D-Schema display provides a fast way for verifying the qualitative response of the whole circuit to the input signal. When a .TRAN analysis is performed, the transient response is displayed by 3D-Schema as an animated model. In both cases, the user controls the speed of animation, either forward or backward, and other display parameters.
Fig. 1 shows a picture extracted from an animation and by clicking the icon under the figure you can gain access to an animated example.
Fig. 1. Idle and active schematic diagrams.
Click here to see the animation
The cabinet perspective was selected for its good visibility and simplicity. In order to enhance the perspective illusion, a projection in the form of a fine-lined shadow of the schematic is provided at ground level.
Fig. 2 depicts the methodology applicable when using the 3D-Schema system. It consists of the following steps: (i) schematic acquisition using a graphic editor; (ii) SPICE-like source code generation; (iii) simulation; (iv) gathering of node voltages picked from simulation results; (v) display of the active schematic diagram .
The previously described sequence is supervised by 3D-Schema in order to ease and speed up the process. If the results obtained are not those desired, the user can easily go back to the schematic editor and modify whatever he or she wants. Selected pictures of active schematic diagrams can be printed, plotted or sent to a file for post-processing.
Fig. 2. 3D-Schema methodology.
The current version of 3D-Schema was written using C language and runs on a personal computer under the MS-DOS operating system. It makes use of VGA graphics mode #0x12 with a resolution of 640x480 pixels and 16 colors. A new version that runs under the Windows platform is being developed.
This section includes several examples of 3D-Schema utilization in order to show its value as a complementary instruction method in the field of analog electronics.
Fig. 3. Idle schematic diagram of a bipolar inverter.
The goal in the first example is to achieve the static transfer function of the bipolar inverter depicted as a conventional or idle schematic diagram in Fig. 3. The user edits the schematic diagram, selects the static transfer function analysis option (.DC), and then instructs the system to simulate the circuit. Once the simulation is completed, the computer screen shows the schematic diagram in perspective. The display accords with the state of the circuit for the first sample of the sweep. The user can now control the perspective format, including the point of view and scaling (height) of node voltages. Finally, the response of the whole circuit can be inspected for the full range of the sweep, either step by step or as a movie. Fig. 4 shows three pictures drawn for three different values of input voltage. The selected pictures depict the cut-off, active and saturation states of the transistor respectively.
Fig. 4. Active schematic diagram of a bipolar-based inverter showing the different states of the transistor: (a) cut-off, (b) active, and (c) saturation.
The static transfer function of an op-amp inverter with voltage gain -1 is obtained in the second example. The goal here is to show the effect of output saturation on the inverting input. Fig. 5 consists of two pictures. In (a) the operational output is not saturated so the inverting input voltage is approximately zero and both resistors share the same slope (voltage drop). In (b), the output is saturated and the inverting input moves away from zero, but naturally the resistors still share the slope (Ohm's law, null input current and equal resistors). The animated display on the computer screen provides a clear perception of the saturation phenomenon.
Fig. 5. a) Idle schematic diagram of a basic operational-based inverter. b) Active schematic diagram of the operational-based inverter with non-saturated output, c) with saturated output.
A third example examines the common relaxation oscillator with an operational amplifier. A simulation of the transient analysis (.TRAN) for approximately two cycles is requested. The animation displayed on the screen is clear enough to make the student understand the basics of circuit operation in a few seconds. Therefore, the student more easily follows the quantitative study that must come later. Fig. 6 shows a picture that results from the animation of this transient simulation. In that picture the operational amplifier is saturated to the positive supply rail and, since the capacitor is getting charged, the voltage level of the inverting input is rising. The animation shows how the inverting input rises until it reaches the level of the non-inverting input. Then, the output falls down to the negative supply rail and the capacitor is charged with the opposite polarity. The system allows the user to select a cycle of the steady state and repeat it in a continuous animation at the desired speed.
Fig. 6. a) Idle schematic diagram of a relaxation oscillator. b) Picture extracted from a 3D-Schema animation showing the evolution of the relaxation oscillator.
3D-Schema can be used in some of the following environments: (i) As a complement to the teacher's dissertation in the traditional classroom; (ii) under student control in a simulation laboratory; (iii) as a tool for developing educational videotapes. In any case, it is not intended to replace but to enhance conventional instruction techniques .
Currently, the system is being utilized at the University of Vigo (Spain) for teaching the operation of some analog circuits especially difficult to explain using chalk and a blackboard. The subject of analog electronics in the second-year of telecommunication engineering studies was chosen. A poll among the students at the end of the current academic course is planned. This poll and the comparison between the results of current knowledge tests and those of the preceding years will allow the evaluation of the benefits and drawbacks of the proposed system.
So far, one advantage and one disadvantage have been found: (i) the system has the property of being apparent and engaging for students; (ii) it needs an (usually expensive) LCD computer display suitable for use coupled to an overhead transparency projector.
The utilization of 3D-Schema under student control is to be performed throughout the next academic year. There is no experience in applying 3D-Schema for videotape recording since this was not the first application in mind. Actually this utilization might decrease its immediacy, an important attribute of 3D-Schema.
This work was sponsored by the University of Vigo with grant reference number I4UB60902 (September 16, 1994).
Click here to download a demonstration version of the program.
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 Autodesk Inc., 3D-Studio release 3, 1993.
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 A. del Río and A. Rodríguez, "Aplicación del modelo 3D-Schema en asignaturas de electrónica analógica", Proceedings of "II Congreso de Tecnologías Aplicadas a la Enseñanza de la Electrónica", Sevilla , Spain, September 1996.
 A. del Río and A. Rodríguez, "3D-Schema: An Intuitive Model for Analog Circuits Instruction", Proceedings of "Computer Aided Learning and Instruction in Science and Engineering", ed. Springer, San Sebastián, Spain, July 1996.
María Dolores Valdés
Departamento de Tecnología Electrónica
Universidad de Vigo
Apartado de Correos Oficial
Alfredo del Río holds a M.S. degree in Telecommunication Engineering (1980) from the Universidad Politécnica de Madrid (Spain), and a Ph.D. in Telecommunication Engineering (1992) from the Universidad de Vigo, Spain. Since 1994 he has been a member of the IEEE and the Education Society. From 1983-1987 he was professor of electronics at the college level. Since 1988 he has been working as a professor in the Electronics Technology Department at the Universidad de Vigo, Spain. Currently he teaches the subjects of microcontrollers and analog electronics and his research interest is in the area of computer aided instruction (CAI).
María Dolores Valdés received the M.S. degree in Electrical Engineering (1990) from the University of Santa Clara. Currently she is doing a Ph.D. at the Electronics Technology Department of the University of Vigo (Spain) where she is a member of the research team. Her current research interest is in the area of field programmable gate arrays and programmable logic devices, comprising the development of new architectures oriented to coprocessing functions.