Experiment 2: Basic Circuits and Electronics - Fall 2004 | ENGR 4300, Lab Reports of Engineering

Download Experiment 2: Basic Circuits and Electronics - Fall 2004 | ENGR 4300 and more Lab Reports Engineering in PDF only on Docsity! Electronic Instrumentation ENGR-4300 Fall 2004 Section ____________ K. A. Connor Revised: 9/3/2004 Rensselaer Polytechnic Institute Troy, New York, USA 1 Experiment 2 Basic Circuits and Electronics Purpose: In the following exercises, we continue gaining experience with some of the equipment in the Instrumentation Studio and work with some basic circuits and electronics concepts. Equipment Required: • Oscilloscope (HP 54603B 2 Channel 60 MHz Oscilloscope) • Function Generator (HP 33120A 15 MHz Function/Arbitrary Waveform Generator) • Function Generator (Brand X Function Generator) Background The following material is taken from Basic Electronics, a lab exercise written by James A. Harrington of Rutgers University. Some of the material on the working of the oscilloscope is taken from J.P. Holman, Experimental Methods for Engineers, McGraw-Hill, New York, 1989. The following paragraphs describe the analog oscilloscope. Most new oscilloscopes today are digital, not analog, and offer substantial benefits over the analog scope. Hewlett-Packard (now Agilent) stopped introducing analog scopes over 20 years ago, but most conventional TV sets have worked like analog ‘scopes. The Cathode-ray Oscilloscope This is one of the most versatile pieces of laboratory test equipment that you will use. It is really a type of analog voltmeter with an arbitrary zero. It can read DC voltages as an offset voltage and as well as AC voltages by displaying the true wave form. Most modern oscilloscopes are capable of measuring AC signals over a wide range of frequencies. Check your oscilloscope for its frequency response. Find the minimum and maximum frequency. Note – You can get this information from the oscilloscope link in the Facilities section of the course webpage. You should find that there are two ranges listed, one for DC coupling and one for AC coupling. Either answer is fine. The heart of any oscilloscope is the cathode-ray tube (CRT), which is shown schematically in Fig. 1. Electrons are released from the hot cathode and accelerated toward the screen by the use of a positively charged anode. An appropriate grid arrangement then governs the focus of the electron beam on the screen. The exact position of the spot on the screen is controlled by the use of the horizontal and vertical deflection plates. A voltage applied on one set of plates produces the x deflection, while a voltage on the other set produces the y deflection. Thus, with appropriate voltages on the two sets of plates, the electron beam may be made to fall on any particular spot on the screen of the tube. The screen is coated with a phosphorescent material, which emits light when struck by the electron beam. If the deflection of the beam against a known voltage input is calibrated, the oscilloscope may serve as a voltmeter. Since voltages of the order of several hundred volts are usually required to produce beam deflections across the entire diameter of the screen, the cathode-ray tube is not directly applicable for many low-level voltage measurements, and amplification must be provided to bring the input signal up to the operating conditions for the CRT. Figure 1 - Schematic diagram of cathode-ray tube (CRT) Electronic Instrumentation ENGR-4300 Fall 2004 Section ____________ K. A. Connor Revised: 9/3/2004 Rensselaer Polytechnic Institute Troy, New York, USA 2 A block schematic of the oscilloscope is shown in Fig. 2. The main features are the CRT, as described above, the horizontal and vertical amplifiers, and the sweep and synchronization circuits. The gain of the vertical amplifier is controlled by the Volts/Div knob found above the two BNC input connectors on the HP 54603B 2 Channel ‘scope. (Here you should either look at the ‘scope or the picture on page 4 of this write up.) The sweep generator produces a sawtooth wave which may be used to provide a periodic horizontal deflection of the electron beam, in accordance with some desired frequency. This sweep then provides a time base for transient voltage measurements by means of the vertical deflection. Oscilloscopes provide internal circuits to vary the horizontal sweep frequency over a rather wide range as well as external connections for introducing other sweep frequencies. The frequency of the sweep generator is controlled by the Time/Div knob found above the Power button. If you slow the sweep down (by increasing the time per sweep) you will be able to see the electron beam spot move across the CRT screen. Internal switching is also provided, which enables the operator of the scope to lock the sweep frequency onto the frequency impressed on the vertical input. (This is more generally called sync.) This is what the AUTO SCALE button does for you when you push it. If we use the function generator to provide the input to our device under test, we can connect the SYNC output it provides to the External Trigger input on the ‘scope and then select External for the trigger source. This is generally the best way to connect things, if we have enough cables. It also avoids the problem of accidentally confusing the SYNC with the OUPUT BNC on the function generator. Figure 2 – Block schematic diagram of an oscilloscope A dual-beam oscilloscope provides for amplification and display of two signals at the same time, thereby permitting direct comparison of the signals on the CRT screen. It is a very good idea to always use both inputs, so that you can display both the signal into and out of the device you are studying. On the HP 54603B ‘scope, we can use one of the two inputs as the horizontal input shown on the diagram above. Figure 3 – Use of oscilloscope for phase measurements The ‘scope may be used to measure phase shift in an electronic circuit, as shown in Fig. 3. An oscillator is connected to the input of the circuit under test. The output of the circuit is connected to the ‘scope vertical input, whereas the oscillator signal is connected directly to the horizontal input. The phase-shift angle ϕ may be determined from the relation, where B and A are measured as shown in the figure. For zero phase shift the ellipse will become a straight line with a slope of 45° to the right; for 90° phase shift it will become a circle; and for the 180° phase shift it will become a straight line with a slope of 45° to the left. Electronic Instrumentation ENGR-4300 Fall 2004 Section ____________ K. A. Connor Revised: 9/3/2004 Rensselaer Polytechnic Institute Troy, New York, USA 5 very accurately compare the two signals by displaying them separately. We can now generate a Lissajous pattern by clicking on one of the ‘scope buttons. On the ‘scope face you will notice that there are several blocked off sections that address different functions. We have used the VERTICAL section to adjust the display of the signal amplitudes and the HORIZONTAL section to adjust how much time we are displaying. By changing the Time/Div, for example, we can change the two cycles shown to many more cycles of the sine wave. Click on the MAIN/DELAYED button to produce a new set of functions for the soft keys at the bottom of the CRT. Then click on the soft key labeled XY to combine the two signals that were displayed separately into a single Lissajous pattern. Describe what you see. (You can use the examples shown in Figure 4). Use the benchmark software to take a picture of the image. Your TA or instructor can help you with this. Include this picture with your report. [Hint: Use the STOP button on the ‘scope to freeze your figure at the point you wish to take a picture of.] You should now play with the equipment and produce several other representative patterns. Create three patterns that you find particularly interesting. Which ones have you done? Next, we will do a different kind of comparison of the two sine waves, one that will prove to be very important in the development of measurement techniques. In this measurement, we will compare two signals to see how close they are to one another by subtracting one from the other. Click on the MAIN/DELAYED button again in the HORIZONTAL section. Change back to the MAIN display by clicking on the left most soft key. This should bring back the two individual sine waves. Adjust them again so that they are as identical as possible. (Try to get them displayed on top of one another.) Once you have done so, click on the channel math button (this is labeled with both a plus and a minus and is half way between the buttons for the two channels in the VERTICAL section). Click on the soft key labeled 1-2, which will produce a third trace that is the difference between the two channels. If you have adjusted the two sine waves to be identical, their difference should be zero. How well did you do? Note that the amplitude, the frequency, and the phase must be identical to make the difference zero. Now make the two signals as identical as possible by adjusting the difference signal away. What did you have to do? From this exercise, you should see the value of comparing one signal against some kind of reference. It is possible to tune a guitar, for example, by comparing the tone a string makes with an electronic tone. This permits a perfectly tuned instrument, even when the player has less than perfect pitch. You have also seen how we make what are called differential measurements. There are many advantages to making differential over absolute measurements. You have seen one of the key reasons since differential measurements allow you to focus on smaller quantities since you are working with the difference between two signals. We will see another good reason for differential measurements when we address bridge circuits. Part B Voltage Divider and Bridge Circuits In Experiment 1, we looked at one of the simplest useful circuits – the voltage divider. In many simple applications of electronics, we have only a small number of standard voltages in whatever circuits we are building. When we use a 9 volt battery as our source we have only one voltage level available, unless we use a voltage divider to get smaller voltages. We can also use a divider to measure resistance, if we have some device with an unknown resistance. For example, if we connect an unknown resistor in series with a known resistor, then the voltage across the unknown resistor can tell us the value of the resistance. An even better measurement can be done by combining two voltage dividers in a configuration like the one shown in the following figure. Note that if R1 = R2 = R3 = R4, the voltages at the two points marked Vleft and Vright will be equal to half of the source voltage. Thus, their difference should be zero. Set up the circuit shown using PSpice, following the procedure given in Experiment 1. Use a 100mV, 1kHz source and setup the transient analysis so that three cycles of the sine wave are shown. Once you finish the analysis and display the voltages at the two points, click on Trace on the probe menu and add the trace of the difference between the two voltage. You can use the differential voltage marker, type it in directly as a mathematical expression or click on the symbols in the node list (the latter two choices are found using the Add Traces window). You should find that the difference is indeed equal to zero. Electronic Instrumentation ENGR-4300 Fall 2004 Section ____________ K. A. Connor Revised: 9/3/2004 Rensselaer Polytechnic Institute Troy, New York, USA 6 0 R1 1k R2 1k R3 1k R4 1k V1 Vleft Vright Now, change R4 to be equal to 1.1k ohms. Do the analysis again and add the trace of the difference between the two voltages. What is the amplitude of the difference voltage as a percentage of the source voltage? Print out this plot and include it with your report. Now analyze this circuit by hand, and find the voltages at the two points and their difference. Make sure that your answer agrees with the PSpice simulation. Finally, assume that R1 = R2 = R3 are known resistors equal to R, and that R4 is unknown. Derive a formula for R4 in terms of R , the source voltage V1, and the voltage difference between the two divider voltages (Vleft- Vright). We have been doing only one kind of simulation with PSpice. You may have noticed that there seem to be quite a few other options. Two that we will use quite a bit are DC and AC analysis. For the former, we will need to have a source with a DC (constant with respect to time) voltage. For the latter, we will not learn much unless our circuit also contains some capacitors or inductors. We have not reviewed the properties of these other circuit devices, but we will analyze some simple cases to see how they work. Let us consider the simple voltage divider again to see how a finite DC source value will be reflected in the circuit analysis. Use PSpice to configure the circuit shown below. This is the same source – VSIN – that we have used a couple of times already. Set up VSIN in the same manner as we did in Experiment 1, except now make VOFF = 1V, which will add an offset of 1 volt to the 100mV, 1kHz sine wave. Place a voltage marker on either side of resistor R1. You should find that the sine waves displayed by PROBE will no longer oscillate around zero, but will be positive at all times. Change the value of R2 to 100 ohms. How do the DC values (offset) differ from the case where both resistors are 50 ohms? 0 R2 50 R1 50 V1 Electronic Instrumentation ENGR-4300 Fall 2004 Section ____________ K. A. Connor Revised: 9/3/2004 Rensselaer Polytechnic Institute Troy, New York, USA 7 Now change R2 back to 50 ohms and add a 1uF capacitor in parallel with R2. Run the analysis again. You should see little or no difference between this and the previous results. Capacitors can cause a circuit to behave differently at some frequencies and have no effect at other frequencies. 0 R2 50 R1 50 V1 C1 1uF In order to see this, we will have to do another kind of analysis. Click on the Edit Simulation Settings button. Select the AC Sweep/Noise analysis and set it up as shown. The AC Sweep Type has been chosen as Logarithmic - Decade since frequency effects usually only become obvious when we change orders of magnitude. This generates a log scale for frequency. The start frequencies and end frequencies are chosen to cover an interesting range. Usually this range is selected from some knowledge of the expected performance of the circuit. However, since we are assuming that we know very little about this circuit, we can set the range to be roughly that covered by the HP function generator. Note that 15MHz had to be written as 15MEG, since PSpice uses both lower case m and upper case M to mean milli. We have to make one more IMPORTANT change before we can do AC analysis. Double click on the voltage source V1 again to bring up its attribute spreadsheet. You will now have to give a value to AC. This can really be anything, but make it equal to the sine wave amplitude 100mV. In general, it is not a bad idea to do this from the beginning in any analysis involving an AC (or VSIN) source to avoid problems when the analysis type is changed. Electronic Instrumentation ENGR-4300 Fall 2004 Section ____________ K. A. Connor Revised: 9/3/2004 Rensselaer Polytechnic Institute Troy, New York, USA 10 resistance of the bottom half of the pot and thus this voltage divider is loaded down significantly. The voltage across the 200 ohm resistor is determined more by the 200 ohms than the 500 ohms of the bottom half of the pot. To get additional information about the exact value of your voltages, you can look in the output file. In the OrCAD/Pspice demo window (which displays the simulated ‘scope traces), choose Output File from the View menu. The display will switch to the text file containing the output. Scroll down until you see a heading Initial Transient Solution. Just below that, you should see a series of node names and voltages. Each of the uniquely defined non-zero nodes should have a voltage value. In this case, there are three such nodes, one for the voltage at the top of the voltage source and one for each of the wiper voltages. Once you have found these voltages, write them down. Then, you should look through the rest of the file and see how PSpice describes this circuit. To return to the ‘scope display, use the tab in the lower left corner of the screen. Draw the equivalent circuit (as just regular resistors, not with pots) and then calculate the voltages you expect to see at the three points with voltage markers. Check to be sure that your answer agrees with the PSpice Output File. Now we want to use another function of PSpice to see what happens as we vary the position of the tap. To do this, we have to set the tap position as a variable, rather than having it fixed. Use the following procedure: 1) Double click on the left pot and change the value of the SET attribute to setvar. You can make the name anything you want, but this reminds us what we are doing. This makes the value of the left set point a variable we set when we set up the analysis. 2) Next we have to tell PSpice that we are using some parameters. To do this, we go to the parts list and select PARAM, which we will find in a library called Special. Place this item in an uncluttered spot Electronic Instrumentation ENGR-4300 Fall 2004 Section ____________ K. A. Connor Revised: 9/3/2004 Rensselaer Polytechnic Institute Troy, New York, USA 11 on your schematic. Double click on this word and you will be able to set its attributes. When you have finished, you will see the parameter and its default value listed under Parameters. To declare a global parameter, use the following procedure: • Place a PARAM part in your design (which you have already done) • Double click on the PARAM part to display the Parts spreadsheet and click on New Column. • In the Property name textbox, enter your chosen name (here use setvar). • Enter 0.5 for the default value, since that will leave the wiper half way up in its default position and click ok. • While this cell is still selected, click Display. • In the Display Format window select Name and Value, then click OK. • Click Apply to update all the changes to the PARAM part. • Close the Parts spreadsheet. When you are finished, your circuit look something like this: 0 V1 5V R4 200 R3 1MEG R2R1 PARAMETERS: setvar = 0.5 3) Next we will set up the analysis. Click on the Edit Simulation Profile button or the New Simulation Profile button, depending on whether or not you have already run a simulation. Select DC sweep. Then, set up the global parameter which we have called setvar to vary linearly from 0 to 1 in steps of 0.01. Electronic Instrumentation ENGR-4300 Fall 2004 Section ____________ K. A. Connor Revised: 9/3/2004 Rensselaer Polytechnic Institute Troy, New York, USA 12 Now perform the simulation and you will see the voltages at the taps of the two pots. You should print this particular plot and then discuss why it looks the way it does. Include this in your report. We will now combine our study of potentiometers and bridge circuits in a practical hardware application. You will need a 1 k ohm resistor, an instrumented beam, and a pot. We used a 1 k ohm pot for this simulation, so try to get one about that value. However, the operation of the bridge is not changed fundamentally as long as a pot with a value from 1 to 10 k ohm is used. The beam comes with two sets of output wires. One, usually covered with black insulation, is connected to a coil located on the support plate near the moveable end of the beam. The other, usually with twisted wires, is connected to a small strain gauge cemented near the fixed end of the beam. We will only be using the strain gauge in this experiment. The strain gauge is relatively fragile, so its wires are usually fixed to the support plate before going to the outside world. It is still a good idea, however, to be very careful with such devices. The ones we are using are quite good (aka not cheap). Before you build the circuit, use the multimeter to determine the resistance of your strain gauge in its rest position, with the beam deflected down until it touches the support plate, and up an equal amount. Do not make this measurement when the beam is connected to the circuit. Do not over extend the beam upwards. Write down the maximum and minimum resistance of your strain gauge. You should see why we indicate this resistance with a variable resistor in the figure below. 0 V1 5V R1 R2 1k R3 1k R4 1MEG R5 1MEG Scope Channel 2 Scope Channel 1 Strain Gauge 1k to 10k Pot The diagram above should have the information you need to build the circuit correctly. Once you get the circuit built, adjust the pot so that there is no voltage when the beam is not deflected. The best way to monitor the difference between the two output points is to connect each point to one of the ‘scope channels and then use the channel math button to display the difference between the two voltages. When you have everything hooked up, set the beam into free oscillation. You should observe a decaying sinusoidal voltage. Estimate the frequency of the oscillation. You will note that the AUTO SCALE feature will probably not work at this low frequency. This is one of the problems with mechanical systems. The natural frequencies of mechanical devices are usually low. Thus, you will have to learn to set your scales manually if you hope to see anything useful. Once you have obtained a clear signal of a decaying sinusoid, stop the ‘scope scan so that you can look at the signal for some time. Then use the Benchlink software to obtain a plot of the signal. Include this plot with your report. Indicate on it how you found the beam frequency. Note that a more accurate estimation of the frequency can be found by averaging over many cycles. Note that there is a very good reference on strain gauges available in the Helpful Info section of the course webpage. Look for the Interactive Guide to Strain Gauge Technology from Vishay Measurements Group, Inc. To apply any of the materials available there to the experiment we are doing, you need to know that the gauge factor of the strain gauge we are using is 125. (Typical for semiconductor gauges.)