ECE 480: SENIOR DESIGN LABORATORY

ECE 480: SENIOR DESIGN LABORATORY DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING MICHIGAN STATE UNIVERSITY I. TITLE: Lab I - Introduction to the Oscilloscope, Function Generator, Digital Multimeter and Power Supply II. PURPOSE: The oscilloscope, function generator and digital multimeter are the basic tools in the measurement and testing of circuits. This lab introduces the first time operation of these instruments along with the use of a compensated probe. The concepts covered are: 1. equivalent circuits of the oscilloscope inputs, function generator output and digital multimeter inputs; 2. the use of a balanced bridge to compensate for the stray capacitance of a measuring cable and the equivalent impedance of the oscilloscope; 3. accuracy of components and instruments. The laboratory techniques covered are: 1. voltage amplitude and time measurement with an oscilloscope; 2. a procedure for compensating an oscilloscope probe; 3. procedures for setting up multiple DC supplies. III. BACKGROUND MATERIAL: A) The Oscilloscope The oscilloscope used in this course is a two channel digital storage oscilloscope that allows the observation of low frequency repetitive signals and transients over a wide range of frequencies. In this lab and the following labs we will be covering most of the available features. The oscilloscope, or scope for short, is divided into five blocks: the Display Block, the Vertical Amplifier Block, the Sweep Block, the Trigger Block and the Storage Block. The Display Block consists of the cathoderay tube (CRT) and associated controls. The CRT is a device which provides a visual display of a voltage. This voltage is applied to the scope at the Vertical Amplifier Block. The scope creates a CRT display by capturing and overlaying successive windows of time. These windows 1

begin at the same trigger point on the voltage being examined and end at a specific increment of time. The trigger point is controlled by the Trigger Block and the time increment is controlled by the Sweep Block. The Storage Block allows us to save the wave form for future reference or hard-copy. In measuring any circuit we will need to connect the measuring instrument to our circuit. This changes the original circuit by connecting the equivalent circuit of the instrument into the circuit. We will always need to know what the equivalent circuit of the instrument is, so that we can either neglect loading effects or include loading effects in the calculations of the circuit's response. The equivalent circuit for the A and B input terminals are shown in Figs. 1-3 for the three settings of the AC/DC and GND buttons. This is the load seen by our circuits. In the DC position the measured signal is Directly Coupled to the vertical amplifier and we see displayed the actual wave form. In the AC position, a blocking capacitor is inserted in series with the measured signal. This blocks any dc signal in steady-state and we see displayed only the time varying component of our measured signal. This is useful for measuring transistor amplifier circuits where the time varying signal is very small compared to the dc level. In these cases displaying the actual wave form makes it difficult to see the characteristics of the time varying signal. In the GND position the load seen by our circuit is an open circuit but the vertical amplifier, internal to the scope, sees a short and displays 0 volts. This feature is useful for setting a reference line. Figure 1. DC position Figure 2. AC position 2

Figure 3. GND position B) The Function Generator The function generator is a precision source of sine, triangle, positive and negative ramp, haversine, square, positive pulse and negative pulse wave forms plus dc voltage. The frequency of the wave forms is manually and remotely variable from 100:Hz to 50MHz. 4. The equivalent circuit for the OUTPUT terminal is shown in Fig. C) Probes Figure 4. Generator In some instances we may wish to measure a voltage which exceeds the input rating of the oscilloscope (approximately 400 volts). To do this we can attenuate the signal with a simple resistive voltage divider, measure the voltage and solve for the actual voltage. However, stray capacitance may distort our measurement. It will be shown that it is possible to compensate for stray capacitance and measure the true attenuated voltage. The resistive voltage divider of Fig. 5 would simply multiply the input signal by a factor of y = R 2 /(R 1 +R 2 ) independent of frequency if it were not for the stray capacitance C 2 which shunts R 2. The capacitance 3

C 2 represents the parallel combination of the input capacitance of the oscilloscope and the stray capacitance of the probe's cable. Figure 5. Divider with stray cap. Using Thevenin's theorem, the circuit of Fig. 5 is replaced by its equivalent in Fig. 6 where R TH = R 1 5R 2. For a step input of V P volts for V TH we have that, Solving for t, we find that V O = V P [1 - e -t/r THC 2 ] (1) t = R TH C 2 ln [V P /(V P - V O )] (2) The rise time of a signal (t r ) is the time required for the signal to go from 10% (at time t 1 ) to 90% (at time t 2 ) of its final value, that is, t r = t 2 - t 1. Therefore, Solving for the rise time, we find that t 2 = R TH C 2 ln [V P /(V P - 0.9V P )] (3) t 1 = R TH C 2 ln [V P /(V P - 0.1V P )] (4) t r = 2.197 R TH C 2 (5) Figure 6. Thevenin equivalent circuit If R 1 = 9MS, R 2 = 1MS and C 2 = 30pF then the rise time for a step input is 59:seconds. If the input is a square wave then half the period 4

should be much greater than 59:sec, say 590:sec, so as not to be noticeable on the scope. Then the frequency of the input voltage would be less than 1/1180: = 847Hz. This is a severe limitation. Figure 7. Compensated attenuator It is possible to compensate this attenuator such that the attenuation is independent of frequency by adding a capacitor in parallel with R 1 as shown in Fig. 7. To explain the operation of this circuit, consider the capacitive voltage divider of Fig. 8. If a voltage V S is applied, I 1 flows through C 1 and C 2 such that q 1 = * I 1 dt (6) then V S = (q 1 /C 1 ) + (q 1 /C 2 ) (7) = q 1 (C 1 +C 2 )/C 1 C 2 (8) and V a = q 1 /C 2 = V S C 1 /(C 1 +C 2 ). (9) Likewise, consider the resistive voltage divider of Fig. 8 where V b = V S R 2 /(R 1 +R 2 ) (10) Figure 8. Cap. and res. attenuator If V a and V b are the same voltage then the difference between these two node voltages is zero. Any element can be connected between these two 5

points without affecting the voltage because no current will flow. This is the concept of a balanced bridge. In Fig. 7, the element between points a and b is a short circuit. Setting Eqns. 9 and 10 equal, we find the condition needed for compensation is R 1 C 1 = R 2 C 2. (11) If R 1 = 9MS, R 2 = 1MS and C 2 = 30pF then C 1 = 3.33pF and V O = V S /10. What happens if Eqn. 11 is not satisfied? For a step input, the voltage across the capacitors must change rapidly. This will require a very large current of short duration. So we can neglect the currents in R 1 and R 2 at t = 0 + and V O is basically determined by the capacitive voltage divider. In steady-state the capacitors are open circuits and V O is determined by the resistive voltage divider. If x = C 1 /(C 1 +C 2 ) > R 2 /(R 1 +R 2 ) = y (12) then there will be an overshoot on the response at 0 +. Solving Eqn. 12 we find that R 1 C 1 > R 2 C 2. This is called over-compensation and is illustrated in Fig. 9. Likewise for R 1 C 1 < R 2 C 2 we find an undershoot on V O at t = 0 + and this is called under-compensation. This is shown in Fig. 10. Figure 9. Over-compensated Figure 10. Under-compensated The probe used in lab is shown in Fig. 11. R 1 = 9MS, C 1 = 14pF, and C 3 is effectively the parallel combination of the capacitance of the coax cable (.10pF/foot of cable) and a variable capacitor. The probe also 6

has a switch for connecting a resistance of 470S to ground. This makes a voltage divider that is very small and effectively applies zero volts to the scope input. This allows the user to display the zero volt reference while holding the probe. Although we started out with trying to measure large voltages by using a voltage divider, it is desirable to use this compensated divider in any application where the loading effects of the cable and the oscilloscope are significant. The only limit would be the smallest voltage scale of the oscilloscope. For the PM3365 this is 2mV/div. For at least a 1 division display with an attenuation of 10, the measured voltage would need to be greater than 10 x 2mV = 20mV. It is important to note that the PM3365 senses whether the X10 probe is connected and correctly displays the proper volts per division. Most other scopes do not do this and the user must make this correction. Figure 11. Probe schematic BECAUSE THERE IS A 9MS RESISTOR INSIDE THE SCOPE PROBE YOU SHOULD NEVER USE THE SCOPE PROBE ON THE FUNCTION GENERATOR. THIS WILL FORM A VERY LARGE VOLTAGE DIVIDER. D) Accuracy The scope has a horizontal and vertical accuracy of ±3 % for both the analog and digital readings. The digital multimeter s accuracy is different on each scale. But if the maximum number of digits are displayed, it is better than ±0.02%. IV. EQUIPMENT REQUIRED: 1 Philips PM3365 Digital Storage Oscilloscope 1 Philips PM5193 Programmable Synthesizer/Function Generator 1 HP 34401 A or Fluke 8840A Digital Multimeter 1 HP E3630 A Triple Output Power Supply 1 Philips PM8926/59 10:1 Passive Probes 7

V. PARTS REQUIRED: 1 BNC-BNC cable 1 BNC-to-Banana cable 1 Small screwdriver VI. LABORATORY PROCEDURE: A) First Time Operation of the PM3365 Oscilloscope 1. Before actually turning on the oscilloscope, there are a few controls you can preset to facilitate the startup procedure. On the right side there are two columns of 4 round knobs. The outermost column are labeled Y POS, Y POS, X POS and TRIG LEVEL. These knobs should be pointing straight up. The other column of knobs are labeled VAR, VAR, VAR and HOLD OFF. These knobs should be turned fully clockwise and pointing to CAL, CAL, CAL and MIN, respectively. 2. Press in the POWER switch found in the upper left corner. If the trace that appears is extremely bright, turn the INTENS control counterclockwise. CAUTION: If the cathode ray tube (CRT) is left with an extremely bright dot or trace for a very long time, the fluorescent screen may be permanently damaged. If a measurement requires high brightness, be certain to turn down the INTEN control immediately afterward. 3. Turn the INTENS control to adjust the brightness to the desired level. 4. Turn the FOCUS control for a sharp trace. B) The PM5193 Programmable Synthesizer/Function Generator 1. The laboratory function generator is a precision source of sine, triangle, positive and negative ramp, haversine, square, positive pulse and negative pulse wave forms plus dc voltage. 2. Press the POWER switch found in the lower left corner to the on position. Sometimes an ERR3 display comes up during power up. This indicates a low back up battery. We won t be saving our settings so this isn t critical to our work. Just press any button and it should clear itself. C) Wave form Measurement 1. Coaxial cable is the most common method of connecting an oscilloscope 8

to signal sources and equipment having output connectors. The outer conductor of the cable shields the central signal conductor from hum and noise pickup. These cables are usually fitted with a BNC on each end. Connect a BNC-to-BNC cable from the OUTPUT of the function generator found on the lower right side to the A input terminal of the scope found in the lower center. Our first task will be to generate a voltage equal to 4 + 5 sin (2B 1000 t). 2. Press the button with the sinewave. This is the upper left button in the set of nine buttons under WAVE FORM. A red light should be on. This indicates that we have selected this particular wave form. 3. We next need to set the frequency of the wave form. This is done with the START button. This is the upper left button in the set of six buttons under FREQUENCY. Again a red light will indicate your selection. The value is entered with the number key pad on the right side. Press 1 0 0 0. This value will appear on the left most display. Press ENTER which is found on the lower right side. The frequency is now selected to be 1000 Hz. 4. The parameters of our wave form are set with the LEVEL buttons. Push the button labeled Vpp. This is the peak-to-peak value of the wave form selected. Enter a value of 10. (This is done by selecting the number with the key pad followed by enter.) You now have generated a voltage with the expression 5 sin (2B 1000 t). 5. To add a dc offset onto our wave form, we need to select LEVEL button Vdc. Enter a value of 4. We now have a voltage of 4 + 5 sin (2B 1000 t). 6. If you make a mistake or want to change any of the above settings, just repeat the specific step. 7. A wave form should appear on your scope screen. If not ask your lab instructor for help. 8. The wave form on the screen may appear small or crowded. This is because of the setting selected by the scope during power-up. We will learn shortly how to change this. However we can quickly re-set the scope so that it will try to make our wave form easier to view by pressing the green AUTO SET button on the scope. Do this. 9. The scope has two different modes of operation. These are called analog and digital. In the analog mode the wave form is very smooth and continuous. In the digital mode the wave form is being sampled and displayed and may appear jagged on the screen. 9

Many digitizing scopes have phantom results displayed due to the capture process. Although digitizing scopes are pretty good, there is no substitute for seeing the actual waveform to truly verify the measurement wave shape is correct. However, digitizing scopes are great for wave form calculations and storage. The Philips PM3365 is one of the rare (and very expensive) scopes that allows you to see both with the same instrument. Many employers expect engineers to be able to use analog scopes for advanced measurement. This is why we are using an analog /digital scope in ECE 480. Press the white DIGITAL MEMORY button on the scope. You will see these two modes. Place the display back in the analog mode. 10. The top number found in the menu window is the value for each of the vertical divisions. These are the large squares of the screen grid. Counting the number of these divisions from the highest to lowest point of your sine wave and multiplying this times the setting displayed in the menu window is the peak-to-peak value of your sine wave. This may be difficult depending on where your wave form is on the screen. 11. We can set and/or place the zero volt reference of the input. This is done by first pressing the GND button. There are two such buttons. The top one is for the A input and the second one two rows down is for the B input. Press the A input GND. This is zero voltage reference for this channel. Rotate the Y POS for this channel and place this line in the center of the screen. Now press the A input GND button again. Calculate the measured peak-to-peak value of your sine wave using the setting of the scope, that is, the voltage per division times the number of divisions. Record this, and all data that follows, as indicated in the Lab Report. If your calculated peak-to-peak voltage is not what was set in part 4, ask your instructor for help. 12. Press the A input AC/DC button. Now you should be in the DC mode. Your wave form still has a zero reference at the center line of the screen. Calculate the dc level of your wave form. If this is not what was set in part 5, ask your instructor for help. What is the purpose of the AC/DC button? (Answer this and all questions in a complete sentence in the Lab Report.) 13. The other number in the menu window is value of the horizontal divisions. Count the number of divisions per cycle and calculate the period of your sine wave. The frequency of your sine wave is the 10

reciprocal of the period. Calculate the frequency. If this is not the same as set in part 3, ask your instructor for help. 14. What function does the AC OFF perform on the function generator? What advantage do you see in this feature? Restore the wave form to that in part 13. 15. The PM 3365 can also perform wave form measurement. Place the scope in the digital mode. Press the right most white button on the screen bezel so that the words: CURSORS SETTINGS SHIFT and TEXT-OFF appear. Press the CURSORS button, then the CONTROL button. CURSOR CONTROL should appear. The measurement cursors are the small cursors. They float as necessary but are bounded on the left and right by the large cursors. By moving the large cursors you can force an automatic measurement on a wave form. To have the greatest degree of freedom we need to move the cursors to the extreme right and left position. Play with this feature and leave the large cursors at the extreme right and left position. Press RETURN twice. Press the CURSORS button. A new menu appears on the bottom of the screen. Press the CALC button. A new menu appears. We can select the measurement of amplitude or time. Select TIME. Again a new menu appears. Select FREQ. The frequency of your sine wave should now appear in the upper portion of the screen. Record the value displayed in the Lab Report. If the measurement is constantly changing, this may be due to noise in the lab. There is a LOCK located in the center column of buttons and one row from the bottom. If you press it don t forget to press it again to take new measurements. Now press RETURN and this time select AMPL. Record the peak-topeak in the Lab Report. Also measure the MEAN (average) and RMS values. 16. At the time of manufacturing, the scope supported two printers. These are no longer available. We will use a digital camera in the future to record wave forms. 17. Do not turn off or change the settings of the function generator. D) Digital Multimeter 1. If your lab bench has Fluke 8840A Digital Multimeter proceed to Section E). 11

If your lab bench has an HP 34401A Digital Multimeter proceed with the following steps. Turn On the multimeter by pressing the button on the lower left corner. The display should show mvdc. If not, press the DC V button. Disconnect the function generator from the scope without changing the setting on the function generator. Obtain a BNC-to-Banana cable and connect the function generator to the HP 34401A Digital Multimeter with the red banana connector inserted into the right most HI input and the black (ground) banana connector into the right most LO input. 2. If the DC value is way off from the expected value, toggle the AC OFF button on the function generator. Sometimes powering up the multimeter charges up the blocking capacitor and this will help clear it. Turning the multimeter on and off should have the same effect. This is common in digitizing equipment and that is why having a true reading with the analog scope can help resolve these phantom results. Record the value of the DC voltage displayed on the multimeter in the Lab Report. This may be off by several hundred millivolts from what was set on the function generator. This is due to the fact that the DC offset tolerance is several hundred millivolts. If this is critical to an application we an null this by adjusting the dc level on the function generator. 3. Press the AC V button. Record the RMS value of the mutimeter in the Lab Report. The values measured in steps 2 and 3 are much more accurate than readings we can get off of the scope and this is why we have the meter. 4. Calculate the RMS value by dividing the value set peak-to-peak on the function generator by 2 times the square root of 2 which equals 2.828. How does this compare to the measured value in part 3? 5. Proceed to Section F). E) Fluke 8840A Multimeter 1. If your lab bench has a Fluke 8840A Digital Multimeter proceed with the following steps. Turn On the multimeter by pressing the button on the lower right corner. The display should show mvdc. If not, press the V DC button. 12

Disconnect the function generator from the scope without changing the setting on the function generator. Obtain a BNC-to-Banana cable and connect the function generator to the Fluke Digital Multimeter with the red banana connector inserted into the left most HI input and the black (ground) banana connector into the left most LO input. 2. Record the value of the DC voltage displayed on the multimeter in the Lab Report. This may be off by several hundred millivolts from what was set on the function generator. This is due to the fact that the DC offset tolerance is several hundred millivolts. If this is critical to an application we an null this by adjusting the dc level on the function generator. 3. Press the V AC button. Record the RMS value of the multimeter in the Lab Report. The values measured in steps 2 and 3 are much more accurate than readings we can get off of the scope and this is why we have the meter. 4. Calculate the RMS value by dividing the value set peak-to-peak on the function generator by 2 times the square root of 2 which equals 2.828. How does this compare to the measured value in part 3? Proceed to Section F). F) Probe Compensation and Use 1. Locate a scope probe. Hold the base such that the set screw of the adjustment capacitance trimmer is face up. Connect the probe to the A input connector such that the set screw is still face up. Pull back on the probe flange to expose the hook on the tip of the probe and attach to the CAL 1.2V connector. 2. Press the AUTO SET button. Measuring the peak-to-peak value of this wave form would be easier if we could enlarge the picture on the screen. The setting of the vertical voltages per division can be changed by the user with the rocker switch marked A. Push on one end and then the other and watch what happens on the screen. Set the value to 0.2 Volts (per division). Now measure the peak-to-peak value of this by counting divisions on the screen and record this value. How does this compare with the value stamped on the scope? 13

3. CAUTION: Excessive turning of the adjustment trimmer screw will permanently damage the probe. The replacement cost of one probe is approximately $100. With a small screwdriver, adjust the set screw of the capacitance correction trimmer of the probe no more than a 1/8 turn in either direction to display an under-compensated wave form (rounded edges). Make a rough sketch of this wave form in the Lab Report indicating maximums, minimums and levels. 4. Adjust the probe to display an over-compensated wave form (edges with peaks). Make a rough sketch of this wave form in the Lab Report indicating maximums, minimums and levels. 5. Adjust the probe so that the wave form is a correctly compensated square wave. 6. Determine the DC (average) value and period by counting divisions. Record and determine the frequency of this calibration signal. 7. Measure the average (mean) value and frequency using the digital mode of the scope. 8. Because there is a 9MS resistor inside the scope probe you should never use the scope probe on the function generator. This will form a very large voltage divider. 9. Disconnect the probe from the scope. G) HP-E3630A Triple Output Lab Bench DC Power Supply 1. Got to a lab bench with an HP-E3630A power supply. In using integrated circuits, it is often necessary to supply positive and negative voltages to operate the chip. Suppose we need to supply +15 VDC and!15 VDC. We will use the Lab Bench Power Supply to do this. This power supply has 3 adjustable DC voltages as indicated in Fig. 12. With no external connections to the HP-E3630A Triple Output Lab Bench DC Power Supply, turn it On by pressing the button in the lower left corner and press the +20V button in the set labeled METER. The display should be lit and indicate the voltage and current of the 0 to +20 V supply. Check that the Tracking Ratio knob in the upper right 14

corner is pointing to Fixed. Adjust the ±20 V knob in the upper right side such that the displayed voltage is about 15. The current should be reading 0.00 since we have nothing connected to the +20V terminals. Figure 12. Lab Bench Power Supply equivalent circuit 2. The power supply has a built in current limiter to protect its internal electronics and our circuits from damage. This is the maximum current we can get from this voltage source. For this power supply it is NOT adjustable. So great care must be taken when building circuits. Check you wiring before you turn on the power supply. Obtain a red banana wire from the racks on the wall. Connect the wire from the +20V terminal to the COM terminal. The voltage displayed is the voltage at the terminals which should be around zero. The current coming out of our power supply and through our wire is displayed under AMPS. When we try to exceed the maximum allowed current an OVERLOAD light comes on for that supply. (If this happens when we build and test any circuit, something is seriously wrong.) Record the value of AMPS displayed in the Lab Report. 3. REMOVE THE RED BANANA WIRE from the terminal of the power supply. Press the -20V button in the set labeled METER. The display indicates the voltage and current of the -20 V to 0 supply. The meter should read about -15 because we have the Tracking Ratio invoked. By this we mean that the ratio of these two voltages is one in magnitude. To illustrate this, adjust the ±20 V knob in the upper right side such that the displayed voltage is about -12. Press the +20V button in the set 15

labeled METER. The displayed voltage is now about 12. Measure the short circuit current and record in the Lab Report. Remove the short circuit from the -20V supply. 4. Set the +6 V supply to 5 V. Measure the short circuit current for the + 6 volt supply and record in the Lab Report. Remove the short circuit. 5. Adjust the ±20 V knob in the upper right side such that the displayed voltage is again about 15. We now have a +5 V, +15 V and!15 V battery available. Record in the Lab Report the actual values displayed for the +5 V, +15 V and!15 V settings. 6. Using the digital voltmeter, measure the +5 V, +15 V and!15 V terminals. Record in the Lab Report. Again note that the digital volt meter is more accurate than the settings on the supply. It is good practice to build your circuit without power applied. If the value of the supply voltage is critical you may want to use the lab digital voltmeter. H) Clean up Please return all wires to the racks from which they were taken. Turn off all equipment. Assemble your lab report, staple it and hand it in to your instructor. Please read and sign the Code of Ethics Declaration on the cover. 16

Lab Report Lab I - Introduction to the Oscilloscope, Function Generator, Digital Multimeter and Power Supply Name:. Partner:... Date:. Lab Section Number... Lab Station Number... Code of Ethics Declaration All of the attached work was performed by our lab group as listed above. We did not obtain any information or data from any other group in this lab. Signature... 17

VI-C11 Voltage per division = Number of divisions = Measured Voltage Peak-to-Peak = VI-C-12 Number of divisions = Measured DC (average) Voltage = VI-C-13 Seconds per division = Number of divisions = Measured Period = Measured Frequency = VI-C-14 18

VI-C-15 FREQ = VPP = VMEAN = VRMS = VI-D/E-2 VDC = VMEAN = VI-D/E-3 VRMS = VI-D/E-4 VRMS (calculated) = VI-F-2 Voltage per division = Number of divisions = Measured Voltage Peak-to-Peak = Value stamped on Scope = 19

VI-F-3 Sketch of undercompensated probe below VI-F-4 Sketch of overcompensated probe below VI-F-6 Voltage per division = Number of divisions = Measured dc Voltage = Seconds per division = Number of divisions for one period = Measured Period = Calculated Frequency = 20

VI-F-7 Measured Mean (DC) Voltage = Measured Frequency = VI-G-2 Maximum current of the + 20 V supply = AMPS VI-G-3 Maximum current of the - 20 V supply = AMPS VI-G-4 Maximum current of the + 6 V supply = AMPS VI-G-5 Voltage displayed on the + 6 V supply = Voltage displayed on the + 20 V supply = Voltage displayed on the - 20 V supply = VI-G-6 Measured Voltage of the + 6 V supply = Measured Voltage of the + 20 V supply = Measured Voltage of the - 20 V supply = 21