REQUIREMENTS

Attendance

A laboratory is a practical experience requiring proper equipment and involving team work. Therefore attendance at all laboratory sessions is required. Students who miss a laboratory session must make up for it at the first opportunity and arrange it with the instructor. It is easiest to arrange a make-up session during regular meetings of other laboratory sections. A student absent at a regular lab session has to make his or her measurements and not use the data obtained by the group partner.

Each set of experiments is preceded by a pre-laboratory assignment, which prepares you for work in the laboratory. Prelabs are completed at home by each student individually and are to be handed to the instructor prior to doing the experimental work. Laboratory Reports are prepared by each group of students, who have worked together on experiments, after all measurements are completed.

Reports should be typed and have the standard properly filled cover page. All report pages must be numbered. All graphs must be on proper graph paper (e.g. log-log graph paper) or, better, they should be generated on a computer. The axes of the graphs must be labeled and the units indicated. Schematics of all circuits should be included and the conditions under which data were obtained (such as input voltage, frequency etc.) must be clearly indicated. The material indicated in bold print on the pages of the manual should be discussed in the report and the questions in the text answered. See next chapter for more details on preparation of reports.

Each group is required to have a Laboratory Notebook which should have a current record of laboratory procedures, schematics and data. The Laboratory Notebook provides documentation of your experimental work and will be reviewed periodically by the instructor and used for evaluating your performance.

At least one test, involving experimental work and data analysis will be taken by each student.

Your instructor may modify these requirements, as he or she sees fit.

THE LABORATORY REPORT

The report you turn in after completion of each series of experiments is the main product of the team’s work which will be used for grading. Therefore you should devote enough attention to this final but critical step in the laboratory experience. Writing good technical reports is a valuable skill, which in the future will help advance you professional career. Start practicing it now.

The purpose of the laboratory report is to provide information on the measurement procedure, obtained results, analysis, and interpretation and discussion of these results. The discussion and conclusions are very important in a report because they show what knowledge you gained by doing the experiments.

There is no one best format for all technical reports but there are a few simple rules concerning technical presentations which should be followed. Adapted to this laboratory they may be summarized in the following recommended report format:

Detailed descriptions of these items are given below.

1.   The Cover Page should have the names of the team members, and a designation such as Group 3, if the groups are numbered, section and course numbers (e.g. EE291, Section 003). It should also contain the number and the title of the experiments, such as "Experiments VII RC circuits; Passive filters". Cover page should also have the date of the report delivery, not the due date.

2.   The Introduction should contain a brief statement in which you state the objectives, or goals of the experiments. It should also help guide the reader through the report by stating, for example, that experiments were done with three different circuits or consisted of two parts etc. or that additional calculations or data sheets can be found in the appendix. It is also a good place to state that the experimental results were as expected or that there were some problems, as explained in the Conclusions. You may state what was learned in this set of experiments

3.   The Procedure describes the experimental setup and how the measurements were made. Include here circuit schematics with the values of components. Mention instruments used, their settings and describe any special measurement procedure that was used.

4.   The Experimental Data section should be presented clearly with a reference to the procedure the schematic used in measurements. Tables are often a good way of presenting results. This section can also include some calculations or data analysis. For example, in describing the measurements of a frequency distribution, make a table with frequency in one column and the peak-to-peak voltage measured with an oscilloscope in another. The third column might contain the voltage rms values, calculated from the first column, which may be compared with the data obtained with a voltmeter, listed in fourth column. Another column may contain values calculated from theory.

In many student’s reports in the past there was not enough information in this section. For example, when reporting measurement of an amplifier gain, input and output voltages were recorded but no information would be given on the frequency at which the measurements were performed or whether the data were taken with a voltmeter or an oscilloscope. Remember: giving more information is not a mistake, less may be.

The best form of presentation of some of the data is graphical. In engineering presentations a picture is often worth more than a thousand words. There are some

simple rules concerning graphs and figures which should always be followed. If there is more than one figure in the report, the figures should be numbered. Each figure must have a caption following the number. For example: Fig. 1 Schematic of the resonance circuit used in experiment 1, or Fig. 3. Dependence of voltage VR on frequency. All components in the schematics should be labeled with symbols (C₁, R₂) or values (10 nF, 100k). If symbols are used, particular values used in experiments should be listed in the text. All graphs, beside captions, should have clearly labeled axes. Axes, beside labels, should have scales and units. For example, in a graph of a diode characteristic, a horizontal axis may be labeled with a symbol VD and have marks (ticks) indicating voltage scale: 0.1, 0.2, 0.3, etc. with the word "volts" or symbol V written under the axis. Similarly the vertical axis may have symbol ID, for current, with numbers and units designated by the symbol mA, for miliamperes. The lack of proper figure identification and labeling is a very often seen shortcoming in student’s reports.

5.   The Discussion is a critical part of the report which testify to the student’s understanding of the experiments and its purpose. In this part of the report you should compare the expected outcome of the experiment, such as derived from theory or computer simulation, with the measured value. Before you can make such comparison  you may have to do some data analysis or manipulation. The simplest example would be conversion of peak to peak voltage obtained from an oscilloscope to rms values or conversion of a waveform period to frequency. When comparing experimental data with numbers obtained from theory, make very clear which is which. The best way of analyzing strings of data, such as a frequency distribution, is to make an appropriate graph on which the theory is represented by a continuous curve and experimental data by points. In such case you do not need to join the points with a continuous line; their distance from the curve will be the measure of agreement of the experiment and theory. If there is no theoretical curve on the graph, the data points may be joined by a continuous line which is to represent the measured function. A caution is advised if there are just a few data points on the graph. Computers may draw a meaningless zigzag stick figure through such scattered points. You may do a much better job by drawing a smooth line between the points by hand, unless you use software with capabilities of fitting a spline or other function, based on statistical analysis of the data.

A critical part of discussion is error analysis. In comparison of theory and experiment you may not get and usually do not get a perfect agreement. Sometimes the agreement is poor. It does not necessarily mean that your experiment was a failure. The results will be accepted, provided that you can account for the discrepancy. Precision and accuracy of the instruments or your ability to read the scales may be one limitation. The value of some circuit components may not be well known and a nominal value given by the manufacturer does not always correspond to reality. Very often, however, the reason for the difference between the expected and measured values lies in the experimental procedure or in not taking into account all factors that enter into analysis. A good example comes from a student’s report. Low-pass filter characteristic obtained by measuring voltage across a capacitor did not agree with the theoretical curve. The measurements appeared to be precise and the scatter of experimental points was negligible. The instructor pointed out a small value of capacitance, which students choose for the filter. After some discussion they realized that the capacitance of the cable connecting the circuit to the meter changed its characteristics. When this capacitance was taken into account the agreement was quite good and the report was accepted. The above example shows that data analysis requires an open mind and a critical approach to your own work and that routine methods may not be sufficient.

6. The Conclusions , should contain several short statement closing a report. They should inform the reader if the experiments agreed with the theory. If there were differences between measured and expected results, try to explain possible reasons for these differences. You may also say what could have been done differently, how experiments may be improved, or make other comments on the laboratory. Constructive and original statements are highly valued.

CIRCUIT SIMULATION

In this laboratory course you should become familiar with an important circuit simulation program SPICE. The program, developed long time ago (before there were PCs) at the University of California in Berkeley, become the world standard for simulation of electronic circuits. PSPICE® was the first version of SPICE available for IBM PC, introduced in 1984. Initially, simulation of a circuit required writing a line code (netlist) describing the configuration of circuit parts and nodes. The code is easy and efficient but, in the spirit of the times, a visual graphic interface with the image of the circuit schematic on the screen was introduced. Parts, wires, and sources are placed from windows menus with a mouse. The latest review version of the software can be downloaded from the web site of OrCAD Corporation ( http://www.orcad.com ). Students can also request from the company a free of charge CD-ROM.

The powerful program solves very quickly complex circuit equations for various signals and conditions and also displays results graphically. The best way to learn it is by trying to simulate some of the circuits explored in this laboratory. Comparison of simulations with your measurements will, hopefully, give you a better insight in the operation of these circuits. Remember however that while simulations are very useful, they are never a substitute for real data taken from real physical systems, which are the true realm of engineering activity.

There is no such thing as a perfect or ideal measurement which provides the "true value" of the measured quantity. There are a number of reasons for this, from limitations of the instrumentation used and those of the observer, to the variations in the devices in the circuit on which the measurement is made. This does not mean that a good, useful measurement is not possible. Obtaining it, however, requires not only adequate instruments but also some attention and vigilance against gross mistakes which seem to lurk in any laboratory setup. Gross mistakes are such errors as connecting a voltmeter lead to a wrong point in a circuit or entering data incorrectly into a notebook or a computer. These can be avoided by following proper procedures, careful data recording etc. Here we are concerned with two other important concepts: accuracy and precision.

Accuracy can be defined as the difference between the value obtained from measurement and a real "true" value of a quantity. It can be expressed in absolute numbers, such as 10 mV, or in relative numbers, such as 0.5%. In the first case the measured voltage may be different from the actual voltage by no more than 10 mV, in the second by the given percentage. Accuracy is difficult to determine, because we never know what the real value of the measured quantity is, but it can be roughly estimated if we know the precision of instruments and the reliability of their calibration.

Precision of a measurement is related to the smallest difference between the measured values that can be distinguished. For example, if a voltmeter precision is 0.1 V we could measure the difference between 10.2 V and 10.3 V but no better. A reading of 10.25 may be assigned to either of these values, we could not tell. Precision is often confused with the resolution of the instrument scale. Just because an instrument has a finely divided scale on which we can read numbers "precisely" (true for many digital instruments), it does not necessarily follow that the measurement is precise. It may happen that when you disconnect the meter and connect it again to the same source you get a different reading on the same "precise" scale. It is generally true, however, that more precise instruments are designed with finer scales or more digits in their numerical display.

To understand better the difference between accuracy and precision consider a voltmeter that measures voltage consistently and reliably with the precision of 1 mV. A measurement of the voltage of an accurate standard source used for calibration of instruments gives a voltage 5 mV too high. This last error is the measure of the voltmeter accuracy. Its measurements were quite precise but the instrument was not well calibrated and showed consistently higher values. Such an instrument is still quite useful since we are often interested in comparing different voltages and this meter is able to measure the ratio of two voltages much better than it measures their absolute values.  

In considering the effect of precision of instruments on measurement errors we are usually concerned with relative rather than absolute numbers. An error of 0.1 V for measurement of power line voltage of 117 V is very acceptable, since it gives the relative error of 0.1/117 < 0.1 % The same absolute error in a measurement of an amplifier output of 1 V gives a large relative error of 10%.

The manual itself does not contain all information needed in the laboratory. You are expected to consult the literature or your own lecture notes, as needed. Any good book on basic electronics will do. Highly recommended: Horowitz and Hill The Art of Electronics , Cambridge University Press 1989, and The Student Manual for the above by Hayes and Horowitz.

You will be using a computer to write the Laboratory Reports with a word processing program of your choice. Use a computer also to plot data obtained from your measurements. Represent measured values by symbols such as dots, circles, squares etc. When appropriate, plot theoretical curves on the same graphs. Theoretical curves can be obtained by plotting results of calculations for many points on the graphs, so that continuous lines are formed. Spreadsheet programs (such as MS Excel™) may be used for this purpose or math programs (like Mathcad® -Trial CD request- or Matlab®), as well as specialized graphic software. For circuit simulation a programs based on SPICE code is introduced in this course (see section 4). An evaluation version of the commercial code PSPICE® can be downloaded from the web (http://www.orcad.com) or ordered free of charge from OrCAD company. Assignments for simulation of some circuits studied in this laboratory are described in this manual. You may not be able to plot your experimental data points on graphs generated by simulation programs. In such a case prepare separate graphs of experimental data with a spreadsheet program, using appropriate scales for easy comparison with a model, or plot your data points by hand on the graphs generated by the program.

You will find that time is a precious commodity in the laboratory. It will be wasted if you are poorly prepared. Besides completing the Prelab assignment, go over the text of the Laboratory Manual carefully to be sure that you know what to do. Resolve any questions or uncertainties with your instructor at the beginning of the lab session.

Each group should invest in a notebook for taking notes on all activities in the laboratory. Write as many details as you can; draw circuits that worked and did not work. Do not erase errors; they are real stepping stones to knowledge! If you do not make a note of them you will repeat them later. Good notes save your time and make easier writing reports. Do not believe that you will remember the details later. Information which at first seems obvious or unimportant may save you repeating measurements.

The author acknowledges the use of material in Experiments II, V, and VI from prior Laboratory Manuals for EE291 which were authored by Professors: W. Clemens, K. Sohn, J. Strano and W. Troop. Special thanks go to Professor Joseph Frank for revising this text and many helpful comments and suggestions regarding the experiments.