California State University, Fresno Department of Electrical and
Contents
1. be used by an Electrical or Computer Engineer In the process of learning about the functionality of an oscilloscope you will also learn how to measure the voltage and period frequency of various signals The Tektronix TDS 210 Oscilloscope Oscilloscopes are used by engineers and technicians to display voltage waveforms and signals as a function of time Older displays consist of a glass cathode ray tube CRT an electron gun x and y deflection plates and a phosphor screen that emits light when struck with a focused electron beam The spot of light can be moved by horizontally and vertically deflecting the beam of electrons if moved rapidly enough the moving spot will have the appearance of displaying an image on the screen Horizontal deflection typically occurs from left to right at a constant speed called the sweep rate while vertical deflection occurs in response to the strength of the applied voltage resulting in a plot of the waveform The horizontal sweep is generated by applying a triggered sawtooth voltage waveform created internally across the x deflection plates Although newer LCD screens on digital oscilloscopes do not use a CRT understanding the principles of older oscilloscopes is benefitial toward learning how the current generation of digital scopes operate Figure 1 illustrates the sawtooth voltage waveform applied to the horizontal deflection points At point A of the cycle the beam starts to sweep the screen uni2. the settings discussed so far except perhaps when using XY mode to center an oscilloscope trace on the display and to adjust the INTENSITY if nec essary The position knobs temporarily grounding the inputs and locating the traces for CH1 and CH2 allow the reference level 0 V of the display to be arbitrarily set so become familiar with them Once you are finished set the SEC DIV knob to 500 us and turn OFF CH 2 Sensitivity Controls Vertical deflection of the electron beam is produced by applying an external direct or alternating current DC or AC signal to the input s of the scope The degree of vertical deflection is affected by both the magnitude of the input signal as well as the sensitivity of the scope which can be adjusted by using the VOLTS DIV knob Pressing the CH 1 MENU button you can choose what kind of Coupling you wish to allow into the oscilloscope Ground DC or AC When the Coupling button is pressed so that Ground is chosen the input signal grounded preventing any vertical deflection of the beam This position is typically useful for establishing a reference position somewhere on the display and or finding the beam While the input is grounded wherever the beam is moved to on the screen vertically which can be done arbitrarily using the VERTICAL POSITION knob will determine the vertical location that corresponds to 0 V or ground You should always know where ground is referenced to on the display for both CH1 and C
3. 2 as necessary but this time center the trace toward the top of the screen Once CH 2 has been adjusted press the MATH MENU button and select the Oper ation Select CH1 CH2 and you will see a third trace displayed on the screen which is the algebraic subtraction of the two signals This corresponds physically to the vector subtraction or interference of two parallel signals You can also select CH2 CH1 if you wish or choose to add the two signals together by selecting under Operation By adjusting the VERTICAL POSITION of either beam you will see the vector addition or subtraction move as well Once finished press the MATH MENU soft key button once more to turn off the Math Operation Next press the CH 1 MENU button and be sure that the Coupling is set to Ground Repeat for CH 2 Then adjust both VERTICAL POSITION knobs so that the beams are exactly centered at 0 00 div 0 00 V Then press the DISPLAY button and change the Format option from YT to XY which will turn your line into a dot you will not be able to see it unless you adjust one of the VERTICAL POSITION knobs In this mode the oscilloscope is capable of plotting two dimensional graphs representing a function y f x The plot here is achieved by eliminating the variable time t between the two functions f x and y t where x t is the CH 1 input signal and y r is the CH 2 input signal The physical meaning of this operation is the vector summation or interference of the two p
4. California State University Fresno Department of Electrical and Computer Engineering ECE 90L Principles of Electronic Circuits Laboratory Experiment No 1 Introduction to the Digital Storage Oscilloscope Introduction Welcome to the Principles of Electronic Circuits Laboratory You will find the following equipment on your lab bench Tektronix TDS 210 or 1002 Two Channel Digital Real Time Oscilloscope GW Instek GDM 8245 Digital Multi Meter DMM GW Instek SFG 1013 DDS Function Generator Mastech DC Power Supply HY3005D 3 Function Generator Dual 12 V power supply with breadboard Within the cabinets are BNC cables banana plug adapters variable resistors capacitors and inductors additional power supplies and other equipment that will be made available to you over the course of the semester as you perform your experiments Identify all of the equipment listed above on the lab bench before proceeding Objective The first experiment is intended to be an introduction to some of the hardware available to you in the laboratory specifically the Tektronix TDS 210 or 1002 oscilloscope the GW Instek GDM 8245 Digital Multi Meter DMM and the GW Instek SFG 1013 DDS Function Generator An oscilloscope scope is an electronic instrument that displays a voltage input as a function of time and is typically used to display and measure repetitive waveforms or transient signals It is one of the important diagnostic tools that can
5. H2 while examining signals on the oscilloscope Unlike Ground when the Coupling button is in the AC position only the DC constant component of the signal will be filtered from the input which means that the AC changing component of the signal will still be displayed And finally it is only when the button is in the DC position that the oscilloscope will display the actual waveform input into the device which includes both both the DC and AC components of the signal The advantage of using AC coupling over DC coupling occurs when you are trying to measure small variations of a signal that contain a large DC bias As an example consider the relatively small ripple 5 mV on the output of a typical DC power supply say 5 V With DC coupling increasing the input sensitivity in an attempt to see the small variation will send the trace off the top of the screen However with AC coupling the 5 V level will be suppressed and the sensitivity can be greatly increased while retaining the trace on the center of the display Use the 5 V DC power supply to confirm this behavior turn ON the power to the Mastech DC Power Supply HY3005D 3 Use a BNC banana adapter in conjunction with a BNC cable to hook up the 5 V 3A voltage supply on the Mastech Power Supply to the oscilloscope Make sure the tab marked GND on the banana plug adapter is plugged into the ground plug of the voltage supply However before you attach the output of the power supply to th
6. e scope first verify that you have a 5 V signal by connecting the output to the leads of the GW Instek Digital Multimeter DMM Note that you must plug the leads for the DMM into the appropriate plugs and have selected the DC voltage button to measure the voltage of the power supply Once this has been done using DC coupling on the oscilloscope re verify that the output voltage of the power supply is correct If you then try to examine the ripple more closely by increasing the sensitivity turning the VOLTS DIV knob to the right the trace will soon driven off the edge of the display With AC coupling the DC level is suppressed and the ripple can be observed in detail measure its amplitude and compare it to the DC level What happens if you gently wiggle the cable Why Trigger Controls The triggering circuitry is the means by which the oscilloscope is able to provide repeated snapshots of a repetitive signal By controlling the threshold voltage of triggering and slope polarity the user is provided with great flexibility in the appearance of the display and the utility of the scope for capturing elusive signals Some further details of scope operation are required to understand the concept of triggering As mentioned previously the horizontal sweep of the electron beam is controlled by the timebase circuitry the SEC DIV knob The beam sweep is driven by the application of a ramp voltage to the horizontal deflection plates which serves
7. e from 6 Hz all the way to 6 MHz by powers of 10 and adjust the time base setting as necessary to continue to provide a clean display Comment on your results Returning to a timebase around 500 us div and a frequency of 600 Hz confirm with the oscilloscope that the period of the 5 V AC signal is 600 Hz Adjust the trigger level if necessary to achieve a stable display Measure the horizontal distance between the two closest points where the waveform crosses the baseline 0 V level with the same slope Note that there are two zero crossings per cycle each with different slope Multiply this number of divisions by the timebase setting time div the resulting time T is known as the period How short a time can be read by this oscilloscope This can be estimated by the smallest timebase setting In practice this setting may result in a trace too weak to be seen thus the so called writing speed of the oscilloscope may also limit the shortest time that can be accurately determined as will the bandwidth of the input amplifiers Dual Trace Operation The availability of two independent input channels greatly facilitates comparison of two signals To do this turn on both Channels While displaying the 5 V 600 Hz sinusoidal signal on CH 1 using it as a trigger source observe the signal from a loose BNC cable on CH 2 You will probably observe spikes and other types of electrical noise depending on the sensitivity level You should attemp
8. erpendicular signals When both signals are periodic the resulting graph is known as a Lissajoux figure Press the Format button once again to return to the previous mode of operation YT Figure 2 The front panel of a Tektronix Oscilloscope Model 210 In this figure the input signal is applied to Channel 1 whose sensitivity is set to 10 0 V div The time base is set to 250 us div meaning that the beam will travel each division in 250 us or the entire screen in 2 5 ms The trace on the screen shows a continuous change of the input voltage from 2 0 divx 10 V div 20 0 V to 2 0 divx 10 V div 20 V with a period over 4 div x250 us div 1 0 ms time interval Time Base Controls The time base SEC DIV is an important feature of these oscilloscopes as it allows for the measurement of different frequencies and time scales within an electrical circuit Adjusting the time base adjusts how slowly quickly the signal sweeps across the screen Since the sweep rate is constant the distance which is measured in screen divisions and more specifically centimeters traveled by the spot is a measure of the corresponding travel time Experiment a bit with the SEC DIV knob noting the different rates which the display can be swept You should notice that for very slow sweep rates you can see the voltage versus time signal slowly sweep from left to right across the screen Whenever you turn on the scope you should be able to quickly adjust all of
9. formly from left to right At point B the beam passes through the center of the screen and at C moves to the end of its trace on the right The beam then quickly jumps back to the starting point at A within a few nanoseconds where a new cycle begins Both the sweep rate and the vertical sensitivity can be adjusted by using controls on the front panel of the oscilloscope an example of which is shown in Figure 2 Figure 1 The sawtooth shape of the sweeping voltage V t At point A of the cycle the beam starts to sweep the screen uniformly from left to right At point B the beam passes through the center of the screen and at C is at the right end of the trace The beam then quickly jumps back to the starting point at A within a few nanoseconds where a new cycle begins Display Controls To begin turn on the oscilloscope by pressing the POWER button on the top of the device Because the Tektronix 210 1002 is a dual input oscilloscope you can choose to display a voltage waveform from either 1 of 2 inputs or both simultaneously which are marked CH1 and CH2 respectively on the front panel Repeatedly pressing the MENU button for CH 1 will turn the scope trace for Channel 1 ON and OFF Likewise pressing the MENU button for CH 2 will turn the scope trace for Channel 2 ON and OFF You have the option of displaying one of the scope traces at a time or both simultaneously if multiple signals need to be measured Initiall
10. function of the second channel based upon what you see on the screen Next switch the inputs so that the sinusoid is input into CH 2 and the DC voltage into CH 1 What changed and why Additional Display Options Next press the MEASURE button and note that there are various measurements about the waveforms that can be displayed for both Channels 1 and 2 Experiment with the various measurement options and explain as many as you can in your laboratory notebook Next press the CURSOR button and choose Time under the Type option You can then move the two VERTICAL POSITION knobs to give you a measurement of the time difference between the two vertical lines This can be used to help you make measurements in time Similarly if you choose Voltage under the Type option you can make similar measurements with respect to voltage Once finished turn the CURSOR Type to Off Finally press the ACQUIRE button and note that you can continually sample the voltage versus time plot by selecting Sample choose to display the peak voltages by selecting Peak detect or display an average of a number of voltages by selecting Average and then adjusting the Averages button Explain the difference between the waveforms for each case and describe what the oscilloscope is doing Conclusion Provide a brief conclusion based upon what you have learned today and both partners will turn in the lab book at the beginning of the next laboratory session No group report is nece
11. moving the trigger toward the right the starting point of the trace changes in this case toward the left Likewise adjusting the threshold for the trigger toward a negative voltage moving the trigger increasingly toward the left the starting point of the trace moves in the opposite direction Refer to Figure 3 for a pictorial representation of the various triggering combinations Switch the trigger SLOPE and observe the changes to the displayed waveform The SLOPE selector chooses whether the trigger event will occur when moving through the threshold from a high voltage to a low voltage falling edge negative slope or from a low to high voltage rising edge positive slope Refer again to Figure 3 Finally setting the trigger level too low or too high outside the bounds of the signal amplitude results in loss of triggering and an unstable running waveform You can observe this if you decrease the input amplitude of the signal TRIGGER TRIGGER TRIGGER TRIGGER LEVEL SLOPE LEVEL SLOPE LEVEL SLOPE LEVEL SLOPE x vy CF 1G z ey KI 2H APN ASS threshold threshold threshold threshold slope slope slope slope Figure 3 Effects of slope and the threshold on triggering by using the AMPLITUDE knob on the function generator Decreasing the TRIGGER LEVEL down toward the voltage level of waveform will restore a stable trace Notice also what happens if you switch the trigge
12. r Source to CH2 Explain your results for both cases Next examine of the effects of the external trigger selection EXT It is with this setting that an additional better conditioned signal can be used to trigger the scope while observing a relatively weak signal one that is too weak when using internal triggering as discussed above For example you may again observe the ripple on the DC power supply or the stray signal picked up by a loose lead using the 600 Hz AC signal to trigger the scope externally Ex ternal triggering is accomplished by connecting the signal to be used for triggering to the external trigger input EXT TRIG of the scope and selecting Ext on the trigger Source button When you are finished display a 600 Hz 5 V peak to peak sinewave using CH 1 on the scope Make whatever ad justments are necessary to display a stable waveform Then start from 500 us div and increase the timebase to larger settings slower sweeps which is simply a longer snapshot of the signal being displayed but with less detail Re member that the 600 Hz signal has not changed the beam sweeps more slowly so only more transitions are displayed Moving to much smaller timebase settings faster sweeps more detail can be seen at the expense of the big picture However notice the decrease in intensity of the trace as the electron beam moves more swiftly over the screen at faster time base settings Finally adjust the frequency of the sinewav
13. ssary for this laboratory Acknowledgments In addition to my own work several people have allowed me to adapt portions of their lab manuals into this laboratory procedure I would like to gratefully acknowledge the following people for giving me permission to use their materials Dr Larry Owens Professor Electrical and Computer Engineering Department California State University Fresno Dr George Watson Associate Dean College of Arts amp Sciences University of Delaware Dr Jeffrey Urbach Associate Professor and Chair Department of Physics Georgetown University
14. t to explain why this noise is locked to the signal on CH 1 For a more dramatic effect touch your finger to the CH 2 input conductor from the BNC cable The noise probably has much higher amplitude and is still locked to the calibration signal Finally while holding the bare lead in one hand touch the metal frame of the table and observe the noise signal on CH 2 Attempt to explain your observations Next apply the 0 20 V input from the Mastech power supply to CH 2 on the oscilloscope using the red and black banana plugs making sure that the plug with the GND tab is attached to the common black output from the power supply While displaying both Channels on on the oscilloscope adjust the Coupling control to DC for both channels and observe your results when using the MATH MENU options As you do so adjust the amplitude of the DC signal using the VOLTS knob on the dual power supply What happens and why Finally try various combinations of AC GND and DC and observe your results When the knobs are in the AC position and you vary the amplitude of the DC voltage from the dual power supply what happens Why X Y Mode Once you are comfortable with the dual trace operation of the oscilloscope set the amplitude of the DC signal into CH 2 to 5 V and select XY operation of the scope using the Format option after hitting the DISPLAY button Now the voltages into CH 1 and CH 2 are functions of each other Can you justify which channel is a
15. to move the beam across the face a distance proportional to the elapsed time from a trigger event The occurrence of the trigger event marks the start of the linear ramp By slowing the timebase to about 1 s div the actual deflection of the beam can be slowed to the point where the resulting luminous waveform can be observed crossing the display from left to right To examine the effects of the trigger settings on the triggering of the display use the output available on the SFG 1013 function generator to generate a 600 Hz triangle wave use the WAVE button as the input signal for CH 1 Hook up the input to CH 1 on the scope and increase the sensitivity to 5 V div Adjust the amplitude on the function generator so that the amplitude of your triangle wave is exactly 5 V peak to peak and select a 500 us div sweep speed on the oscilloscope With the trigger Source set for CH1 note there are 5 options CH1 for Channel 1 CH2 for Channel 2 and Ext for an external trigger Ext 5 which divides the external trigger by 5 and AC Line which expects an AC signal the TRIGGER LEVEL knob rotated to be just above 0 V and the Slope option set to detect on the rising edge of a pulse a stable trace showing the triangle wave should appear Adjust the horizontal position of the trace until its starting position is seen on the display Next adjust the TRIGGER LEVEL by adjusting the knob to the left and right Notice that as the threshold for the trigger is increased
16. y we are only interested in looking at the voltage on Channel 1 so press the appropriate MENU buttons to turn CH 1 ON and CH 2 OFF Under the TRIGGER MENU make sure that the Auto option is selected you can toggle between options under Mode using the 2nd soft key button from the bottom just to the right of the LCD Also make sure that the Edge option is selected you are using a Rising Slope your Source input is CH1 and you are using DC Coupling Next turn the SEC DIV knob to 250 us and make sure the VOLTS DIV knob for CH 1 is turned all the way to the left After the scope warms up you will see a horizontal line on the screen If you cannot find the beam use the VER TICAL POSITION knob to center the line at 0 00 divs 0 00 V Because we are only looking at CH1 only the left hand knob for the VERTICAL POSITION will be useful Rotating the knobs applies a constant negative or positive voltage across the horizontal vertical deflection plates causing the beam to move horizontally vertically on the screen depending on the applied voltage how far you turn the knob Experiment with the knobs for a bit to move the trace to arbitrary locations on the display Can the display be completely moved off the screen What happened and why Explain this in your lab notebook Once the trace has been centered use the DISPLAY option to adjust the brightness of the trace using the Contrast Increase and Contrast Decrease buttons Repeat the process for CH
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