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 √ sine Vrms=Vpp/(2 2)

square Vrms=Vpp/2 √ 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 √ 2 2 2 sine Vrms= Vm+[Vpp/(2 2)]

2 2 2 square Vrms= Vm+[Vpp/2] √ 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.

• 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. Do the same in the “DC” mode. Repeat this until the signal is stable simultaneously in both “DC” and “AC” modes. Once this is achieved, do not press the “AUTOSET” button again. Press the “Coupling” submenu button several times.

• What happens when the “Coupling” mode is set to “DC”? To “AC”? What do the choices mean?

While in “DC” mode, change the vertical scale to 5 Volts/div and the horizontal scale to 5 ms/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. Find Vm using the same method. • Repeat the above measurements along the horizontal axis to measure the frequency. • Using these measured values and the relationship between the peak–to–peak, mean and rms values from page 54, check that the values are consistent with a square type of waveform.

57 While still in the “DC” mode, press “MEASURE”:

• Read and tabulate Vpp,Vrms,Vm and the frequency of the signal. • Sketch the waveform. Show vertical and horizontal scales and the position of zero volts. Mark where the trace was triggered. • Repeat for “AC” mode. Compare and comment on the measured values obtained from “AC” mode with those from “DC” mode. • Summarise your ﬁndings about the signal.

5. Stray Electromagnetic Signals

Turn the “VOLTS/DIV” to its most sensitive setting and connect a wire to the +ve end of the co-axial cable which is connected to CH1. • Sketch the waveform. Suggest likely origins for such stray signals. • To obtain a strong signal, bring the wire near any mains power source. What is the frequency measurement? Comment on it.

6. Lissajous ﬁgures

The usual function of oscilloscopes is to display voltages against time in YT mode. However they can also be used to display how one voltage varies with respect to another voltage. When used in this manner the oscilloscope is said to be in XY mode.

Connect both CH1 and CH2 probes to the same output of the transformer, making sure that the earths of both probes are connected to the same point. Both channels on the oscilloscope should have their probe settings at 10×. Refer to the “User Manual” in order to set the oscilloscope to its XY format. It may be helpful to check the “Display” button menu.

When conﬁgured correctly, a circle, ellipse or straight line should be observed on the oscilloscope screen. If not, ask your demonstrator. This pattern is referred to as a 1:1 Lissajous ﬁgure, since the ratio of the frequencies of the voltages applied to the X (CH1) and Y (CH2) axes of the screen is 1:1 (the same voltage was applied to both).

A Lissajous ﬁgure is created whenever a point moves under the inﬂuence of two simultaneous sinusoidal signals operating at right angles to each other. Note that a stable Lissajous pattern is only formed when the ratio of the frequencies of the two signals can be expressed as n/m where both n and m are integers. If fx and fy are the frequencies of the signals operating in the x and y directions on the oscilloscope screen, then a stable pattern results when f 1 1 2 2 3 x = , , , , , ··· . fy 1 2 1 3 4

58 In each case a characteristic Lissajous ﬁgure is formed. Since these patterns are sensitive to changes of frequency, they provide a precise method for comparing frequencies.

Connect CH1 and CH2 probes of the oscilloscope as shown in the above diagram. Set the CH1 probe to 1× rather than 10×. Do NOT switch on the signal generator or the transformer until after your demonstrator has checked the wiring.

• Calculate Vpp for the 12 Vrms AC (50 Hz) transformer.

Set the amplitude of the signal generator voltage entering CH2 to 3.4 Vpp so that it matches the amplitude entering CH1. Set the frequency of the generator in turn to 25 Hz, 50 Hz, 100 Hz, 150 Hz and possibly 200 Hz as well. At each frequency, carefully adjust the generator frequency to obtain a stable Lissajous pattern on the screen.

• Sketch and label each Lissajous ﬁgure. • What generator frequency would produce the Lissajous ﬁgure drawn below?

7. Triggering

Connect CH1 probe to the output of the transformer. Press “AUTOSET” to get a display. Press the “MEASURE” control button and then adjust the “SEC/DIV” knob 10ms. Slowly tune the “Trigger Level” knob.

• What happens to the signal when the trigger is set at a trough or a peak of the signal? Comment on the Vpp,Vrms,Vm and the frequency values.

59 Clearly this type of signal can not be used to obtain any meaningful measurements.

• What happens when the trigger is set anywhere between the troughs and the peaks? What is the criteria for the trigger setting to obtain a stable signal?

Adjust the “Trigger Level” knob so that the signal will be triggered somewhere between a peak and the middle of the signal. Press the “Trigger Menu” knob and change the “Trigger Slope” from “Rising” to “Falling”.

• What happened to the signal? Draw to explain, rather than describe it in words.

... If there is any time remaining, employ the “User Manual” to examine other functions of the oscilloscope that have not been investigated so far in this practical. Keep a detailed record of your ﬁndings.