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Introduction to the Oscilloscope

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Introduction to the Oscilloscope INTRODUCTION TO THE OSCILLOSCOPE Methods & diagrams : 1 Graph plotting : - Tables & analysis : - Marking scheme : Questions & discussion : 6 Performance : 3 Aim: In this experiment the basic functions of a modern digital oscilloscope will be studied and applied to some electrical measurements. Since the oscilloscope is probably the single most versatile instrument available for such measurements it is important to gain experience in referring to the technical “User Manual” provided. Note: In this laboratory there are two experiments devoted to the TDS210 digital real–time oscilloscope. This experiment is the first of the two and should be done before the second experiment called “APPLICATIONS OF THE OSCILLOSCOPE”. For this experiment you are NOT required to write a formal report, but to spend the whole of your laboratory time gaining familiarity with the use of the oscilloscope. You must keep a detailed record in your laboratory journal and it is this journal report that will assessed. Equipment List: Tektronix TDS digital real–time oscilloscope Signal generator User Manual for oscilloscope 0 V – 30 V Power supply 240 V – 12 V Transformer 6 V Battery Preliminary theory: An oscilloscope measures voltages as they vary with time. In the Tektronix oscilloscope a voltage can be applied to either of two input channels — CH1 or CH2 as shown above. Each channel can be represented by a greatly simplified block diagram. 52 The voltage is sampled, digitized into an 8–bit number and then represented by a point on a liquid crystal display (LCD) screen, where the vertical position of the point on the screen depends on its numerical value. Its horizontal position depends on when it was sampled. The rate of sampling (Samples/second) can be varied from 50 S/s to 1 GS/s by manual adjustment of the “HORIZONTAL” time–base controls. On the screen, the voltage is represented graphically against time. The numerical value of the voltage can also be displayed simultaneously on the screen. To start the oscilloscope sampling and displaying, a “trigger signal” is required. The voltage being measured can be used as the trigger (“internal trigger”) or a separate voltage (“external trigger”) can be used. The external trigger input is illustrated in the first figure. However, in this experiment only the internal triggering will be used. Root Mean Square (RMS), Mean and Peak–to–Peak Values Consider the sinusoidally varying voltage shown. The peak–to–peak value of the voltage (Vpp) is the voltage difference between the maximum and the minimum of the waveform. The mean value of the voltage (Vm) is the average value and is a measure of the DC component in a signal that contains both AC (alternating current) and DC (direct, non–alternating current) components. The root mean square (rms) voltage is a characteristic voltage of the signal which has the same heating effect as a numerically equal pure DC voltage. For example, an AC voltage with an rms 53 value of 5 V has the same heating capability as a 5 V DC voltage. AC voltages are usually quoted in rms such as the 240 V rms mains AC supply in Australia. There is a relation between the peak–to–peak and rms values for a pure AC signal but this depends on the type of waveform being used. A summary is presented in the following table. WAVEFORM RELATION p sine Vrms=Vpp/(2 2) square Vrms=Vpp/2 p triangle Vrms=Vpp/(2 3) A voltage signal can contain both AC and DC components such as that shown below. In this case the following relations hold between Vrms,Vpp and Vm. WAVEFORM RELATION p 2 2 2 sine Vrms= Vm+[Vpp/(2 2)] 2 2 2 square Vrms= Vm+[Vpp=2] p 2 2 2 triangle Vrms= Vm+[Vpp/(2 3)] Experimental Tasks: The Tektronix oscilloscope is a modern, high quality digital oscilloscope and differs significantly from earlier generations of analogue instruments. They are capable of far more sophisticated functions and provide a much greater versatility and accuracy. 54 1. User Manual Read the “Operating Basics” section of the Tektronix oscilloscope User Manual and then answer the following questions in your journal. (Note that it may be necessary to refer to other sections of the manual such as Glossary, Index, Reference or Specifications). • Name and explain the three acquisition modes of the oscilloscope. • What is the function of the “AUTOSET” button? • What does it mean when “Trig’d” is displayed on the screen? • Where are the CH1 and CH2 scale factors displayed on the screen and what do they mean? How are they changed? • Where is the timebase setting displayed on the screen? What does it mean and how is it changed? • Where is the “RUN/STOP” button and what does it do? Reference to the User Manual should be made frequently throughout the remainder of this experiment. 2. Measuring a DC voltage Turn the oscilloscope on using the switch on the top of the instrument. To begin with something easy, use the oscilloscope to measure the DC terminal voltage of a battery by connecting the CH1 probe to the terminals of the battery. Press the “RUN/STOP” button so that “STOP” is NOT displayed at the middle top of the screen. Now press the “AUTOSET” button. You should see a straight horizontal line on the display representing the voltage of the battery. If not, ask your demonstrator. Note that the “AUTOSET” button instructs the oscilloscope to read the voltage present at its input and set itself up automatically to give a readable display of that voltage. If you ever have trouble displaying a voltage, try the “AUTOSET” button. When a control button is pressed, a menu usually appears on the right–hand side of the screen. Particular menu items are then accessed by the vertical column of buttons next to the menu display. Press “ACQUIRE” and ensure that the oscilloscope is in “Sample” mode. Press the “TRIGGER MENU” button and check that the “Edge”, “Slope Rising”, “Source CH1”, “Mode AUTO” and “Coupling DC” settings are selected. Press the “CH1 MENU” button and select the “Coupling DC”, “BW Limit OFF”, “Volts/Div Coarse” and “Probe 10×” settings. Notice that by hitting the “CH1 MENU” button, the CH1 display toggles on and off. This feature is also present on CH2. 55 Press the “MEASURE” control button. Using the top menu button “Source Type”, set the next box down to “CH1 Mean”. Record the numerical value in this box. • What does it mean? What is the CH1 vertical scale factor and how does it relate to the number in the “CH1 Mean” box? • Summarise your findings about the battery voltage. Disconnect CH1 of the oscilloscope from the battery and connect it instead to the output of the variable DC power supply. Display the voltage in the same way as with the battery. Vary the output voltage using the knob on the front of the power supply. • How does the voltage on the oscilloscope compare with that indicated on the meters on the front of the power supply? • Report your observations. 3. Measuring an AC voltage In this section the pure AC signal from a transformer will be measured. Disconnect the CH1 probe from the DC supply and connect it to the output of the transformer. Press “AUTOSET” to get a reasonable display. A 12V sinusoidal signal should appear on the display screen. • Adjust the “POSITION” and the “VOLTS/DIV” knobs. What happens to the signal when you adjust the “VOLT/DIV” knob? • Adjust the “HORIZONTAL POSITION” and “SEC/DIV” (also known as timebase) knobs. Record the consequences of making these adjustments. • Is the input signal changed when these changes to the “VOLTS/DIV” and the “SEC/DIV” are made? What is actually been done to the signal? Change the vertical scale to 5 Volts/div and the horizontal scale to 500 µs/div. Adjust the horizontal and vertical position knobs until the peak–to–peak voltage and the frequency can be read easily from the screen. • Count the number of squares (or division) from peak to peak of the signal along the vertical axis. The peak–to–peak voltage can be found by relating the number of squares counted with the vertical scale factor. Do the same to measure frequency along the horizontal axis. • Using this Vpp value and the relationship between the peak–to–peak and rms values from page 54, check that the measured values are consistent with a sinusoidal type of waveform. Press “MEASURE” and then use the top menu button and the other menu buttons to set the menu boxes in the following order: “CH1 Pk–Pk”, “CH1 CycRMS”, “CH1 Mean” and “CH1 Freq” 56 • Record all the numbers displayed in the menu boxes. Do these numbers correspond to those measured earlier by counting of the squares or division? • What part of the signal does the 12V represent, ie. the Vrms,Vpp or Vmean? • What does the mean value tell you? • The frequency of this signal is the frequency of the mains. What is this frequency? What is the period of the signal? • Sketch the waveform including vertical and horizontal scales as well as the position of zero volts. • Summarise your findings about the transformer voltage. 4. Mixed AC and DC voltage Disconnect your CH1 probe from the transformer and connect it to the upper “PROBE COMP” terminal on the front panel of the oscilloscope. The earth doesn’t need to be connected since there is already an internal connection. Press “AUTOSET”. Select “CH1 Menu” and set the submenu button “Coupling” to “Ground”. Move the signal vertically to the x-axis. Go to the “AC” mode and adjust the “Trigger Level” slightly till a stable signal can be observed.
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