CarQuad: caracterizador de quadripolo

Abstract

This paper presents the development of CarQuad, a teaching kit able to obtain the frequency response, input and output impedance and other common parameters in electrical engineering. These parameters are used to evaluate the behavior of circuits for a given frequency range. Usually the data are collected manually, a procedure that is time consuming and inaccurate. At first, tp develop the kit, a few questions were raised related to the generation of signals to be applied to the circuit under analysis at different frequencies and how to transform the measurements made in this circuit into values of gain, phase and impedance. Thus, it was designed in terms of hardware a motherboard addressing all these issues. The results of the measurements from the designed hardware are sent to a processing unit, the board eAT55 from eSysTech, in which they are converted into equivalent values of amplitude and delay. These values are then sent to a PC via serial communication and are saved in a file. Subsequently, a software was developed in Visual Basic for Excel, which loads the file and automatically generates the spreadsheet with the information obtained from the circuit and further calculations of gain, phase and impedance. The software also builds the curves of frequency response of the circuit. As proposed in CarQuad`s project, an ISHM was developed through a touch screen graphical display. To integrate this device to the kit was necessary to design and assemble a board with the touch screen controller and other display controls and a board to interface the display with eAT55. Finally, a need to include in the kit circuits that serve to demonstrate its operation appeared. Eight tests circuits have been defined, designed and mounted on printed circuit boards as part of the kit. It was verified at a practical assessment of these circuits, as it is done in the laboratory, that it takes the average time of 3 hours and a half to measure each circuit, considering an average of 40 measurement points. On the other hand, with the measuring system created with the CarQuad, it was obtained an analysis average time of 25 minutes to each circuit with the assessment of 100 points. In a comparison of the expected gain response and the one obtained in practice it was calculated a gain error for the test circuits with active components, and it reached a maximum of approximately 8%. For the passive circuits the gain error reached the maximum value of 33% compared to the expected value. In frequency analysis there are several factors that can influence the behavior of circuits, such as errors in the used components, in the assembly process of the circuits, environmental influences among others. Thus, it was possible to see, in the development of CarQuad, the complexity involved in the evaluation and measurement of parameters in the frequency circuits, the limitations imposed by the circuits themselves and how much it is not trivial to solve.