The Oscilloscope and Its Use
In this chapter, one of the most important electronic measuring instruments for ac voltages and currents is discussed — the oscilloscope. It is particularly important because it allows one to “take” an electronic picture of ac waveforms. The construction of an oscilloscope is explained. The function and adjustment of an oscilloscope’s controls are described, and you are told how to use an oscilloscope to perform some electrical measurements.
At the end of this chapter you should be able to:
1. Describe the major sections of an oscilloscope.
2. Explain how the vertical and horizontal deflection systems of an oscilloscope function.
3. Adjust the basic controls of a typical oscilloscope.
4. Use an oscilloscope to measure the peak and peak-to-peak voltages of a waveform, and use these to calculate rms values.
5. Use an oscilloscope to determine the period of a sinusoidal waveform and calculate the waveform’s frequency.
INTRODUCTION
In the first two chapters, discussion concerned how ac is generated and how it is specified in terms of amplitude and frequency. You learned how to determine the instantaneous amplitude at various points in the cycle, and how the sine wave derives its name from the sine function.
Before continuing with an investigation of actual ac circuits, however, one of the instruments which is very important for making electrical measurements in the circuits which you will learn about throughout this book is the oscilloscope (or scope for short). A typical oscilloscope is shown in Figure 3.1.
In the last two chapters, it was explained how dc and ac voltages versus time are graphed as shown in Figure 3.2. The oscilloscope is capable of displaying an ac or dc voltage graphed versus time as shown in Figure 3.3. The oscilloscope, then, might be called a “visual voltmeter.” But, in fact, it is more than just a voltmeter; the scope actually displays waveforms so that the intricacies of waveforms can be observed clearly. It is an instrument that converts electrical signals to visual waveforms on a screen.
An oscilloscope performs three basic functions: One of these is waveform observation. The scope allows the size and shape and type of waveform to be observed. A second function is amplitude measurement. The oscilloscope vertical deflection is calibrated on the screen so that actual voltage amplitudes can be measured. The third function is a measurement of time. The oscilloscope sweep across the screen horizontally is calibrated in time increments which allows the measurement of time periods or time duration.
There are many different types of oscilloscopes in use today. Figures 3.1 and 3.4 show typical analog and digital oscilloscopes. Oscilloscopes were initially developed using analog circuits. Analog scopes use amplifiers to increase the signal strength of measured signals and use the amplifiers’ outputs to directly control the deflection of an electron beam whose path is visible on the screen of the oscilloscope. The observed waveform is therefore an instantaneous or “real time” presentation of voltage variations versus time being observed in the circuit under test.
More recently, with the development of digital integrated circuits and digital sampling methods, digital oscilloscopes have quickly filled the role of the analog oscilloscope. Digital oscilloscopes sample the measured signal waveform at regular intervals and convert each sampled voltage level into digital code, which is stored in a digital memory section within the oscilloscope. A microprocessor (miniature digital computer) control system then retrieves the digital code from memory and uses it to create a reproduction of the original waveform. This method also has the ability to store a digitally recorded waveform for viewing and/or printing (via a computer printer) at a later time. The observed waveform seen on the screen of a digital oscilloscope may actually be a “real time” presentation, or instead it may be playback of a digitally stored waveform.
The analog oscilloscope will be discussed first, because historically it was developed first and many are still in use today. The digital oscilloscope will then be introduced using the analog scope as a comparison reference.
A BASIC OSCILLOSCOPE
As shown in Figure 3.5, the scope can be divided into two major segments: 1) the display system, and 2) the controlling circuits.
The Display System
For display systems in analog scopes, CRT (cathode-ray tube) technology is the traditional type of display system employed. As shown in Figure 3.6, the CRT display system consists of three major parts: 1) an electron gun, 2) a deflection system, and 3) a screen.
Electron Gun
The electron gun is located at the rear of the CRT, away from the screen. Its job is to emit a narrow stream of electrons. These electrons are (accelerated) focused into a beam and accelerated toward the screen of the CRT by a high positive potential applied to the electron gun anode placed near the front end of the electron gun. Note: oscilloscopes may employ an acceleration system that accelerates the electrons in the beam either before or after the beam passes through the deflection system. The pre-deflection anode potentials are on the order of 3,000 to 4,000 volts.
Post-deflection anode potentials may be several thousand volts higher. Some may reach over 10,000 volts.
The electron beam travelling toward the screen passes through the deflection system. The deflection system consists of four deflection plates as shown in Figure 3.7. Two plates — one on the top and one on the bottom — are called vertical deflection plates. The two plates on the sides are the horizontal deflection plates. By applying positive or negative potentials to these deflection plates, the electron beam is caused to deflect up or down and right or left as it passes through them.
The more potential applied to the plates, the more the electron beam deflects. Therefore, the amount of this deflection is actually a measure of the voltage, or potentialdifference, applied to the plates. The force bending the beam is electrostatic force and it follows the first law of electrostatics: like charges repel and unlike charges attract.
If a voltage is applied across the vertical deflection plates as shown in Figure 3.8a, the electron beam moves upwards. If the polarity applied to the plates is reversed, as shown in Figure 3.8b, the beam moves downwards.
If a voltage is applied across the horizontal deflection plates as shown in Figure 3.9a, the electron beam will travel from left to right. If the polarity applied to the deflection plates is reversed as shown in Figure 3.9b, the beam moves from right to left.
If voltages are applied to the vertical and horizontal deflection plates simultaneously, the beam moves vertically and horizontally at the same time, diagonally. This is shown in Figure 3.10.
If no potential is applied to the plates, the beam returns to the center of the tube. This was the original position shown in Figure 3.7.
The Screen
The third and remaining part of a CRT is the screen. After the beam is emitted and travels through the deflection system, it strikes the screen at a point determined by the deflection plates. As shown in Figure 3.11, the inside surface of the screen of a CRT is coated with a phosphor material which has the property of phosphorescence. Phosphorescence, in this case, is defined as the ability of a material to emit light after being struck by electrons. The trace observed when viewing the scope is caused by the electron beam striking the phosphor material of the screen. The very high positive potential (typically from 3,000 volts for small scopes to 25,000 volts for tv picture tubes)accelerates the electrons to the screen to provide the energy for light emission.
Control Circuits
The other portion of an oscilloscope shown in Figure 3.5 and again in Figure 3.11 is its control circuits. These are electronic circuits that perform several functions. They cause the CRT to emit electrons, regulate how many electrons make up the beam current, and control the direction of the beam of electrons. The control circuits are connected to the electron gun and deflection plates in the CRT through a connector at the base of the CRT as shown in Figure 3.11.
Most controls and inputs for the control circuit are located on the front panel of the oscilloscope as shown in Figure 3.12. While the scope in Figure 3.12 is typical and very similar to other scopes, other scopes do have differences. It does, however, provide basic oscilloscope functions, and therefore it will give you a good idea of a scope’s basic controls, how a waveform is displayed, and how to use it to measure voltages. It is a dual-trace scope, and therefore it has two identical sets of vertical input controls so that it can display the traces of two different input signals at the same time.
The controls can be divided into three major subgroups: 1) the mainframe group, 2) the vertical control group, and 3) the horizontal control group. This is shown in Figure 3.13.
MAINFRAME CONTROLS
An enlarged view of the mainframe controls is shown in Figure 3.14. Once the scope is connected to a standard wall receptacle, power can be applied to all circuits in the scope with the power switch. After the power switch is activated, one to two minutes are required before the scope circuits areoperational. After this “warm up” period, a trace may or may not be observed on the screen of the CRT.
Beam Control — Intensity
If a trace is not observed, check the intensity control. The intensity control regulates the number of electrons in the beam striking the screen. As the intensity control knob is turned clockwise and nears mid-position, a spot or a trace should be visible. If the intensity control is set too high as shown in Figure 3.15, the spot or trace (horizontal line across screen) on the screen will not be crisp and sharp; it will instead appear overbright and be fuzzy. This is called “blooming.” If a high intensity like this is allowed to continue, the phosphor can be burned off the inside of the screen where the beam strikes it, and that portion of the screen will be damaged. It is advisable to increase the intensity only to the point where the trace is adequately visible. You should remember to turn the intensity down before turning the scope off to keep the CRT from blooming the next time the scope is turned on. In addition, it is not advisable to leave the intensity control turned up when there is a single spot on the screen rather than a trace. A sufficiently intense beam held at a single spot could burn the screen at that spot.
Beam Control — Focus
The focus control is used to focus the trace. There are two basic ways this can be done. One, if the trace appears as a spot travelling slowly from left to right across the screen, focus the spot to a point. Two, if the trace appears as a horizontal line, use the focus to sharpen the line to a narrow trace. A properly-focused trace is shown in Figure 3.16.
Trace Control — Rotation
To the right of the focus control in the mainframe group on the LS1020 oscilloscope shown in Figure 3.14 is the rotation control. This is a small recessed screwdriver adjustment used to adjust the trace alignment so that its path appears level on the screen display. During movement of the oscilloscope, some jarring of the internal circuitry of the CRT is possible. This could cause the electron beam path that strikes the rear of the CRT face to be knocked out of alignment with the graticule markings (the graph markings on the face of the CRT). The rotation control is provided to correct any misalignment between the actual trace from the electron beam and the scope graticule.
Beam Control — Astigmatism
A third beam control that may be found on some oscilloscopes is the astigmatism control. This control is used to adjust the roundness of the spot. The scope shown in Figure 3.14 does not have an astigmatism control.
Beam Control — Position
Thus far it has been assumed that the trace should be immediately evident after the intensity has been increased. But suppose the intensity is increased and the trace is still not visible. A beam-finder button on some scopes can be pushed to help locate the trace. Often when the scope is turned on, previous settings may be such that the trace is deflected off the screen. The beam-finder button normally reduces all deflection potentials so that the trace appears on the screen, By noting the trace position, you know the direction in which the trace is deflected. This enables you to adjust the position controls to move the trace back near the center of the screen. (The position controls, located in the vertical control section, will be described in a moment.)
Calibration
In the middle of the mainframe control group is the calibrator or “CAL” loop. A signal of a specific frequency and amplitude is applied to this loop. A typical squarewave calibration signal is shown in Figure 3.17. These signals with their known specifications can be used to check the operation and accuracy of the vertical and horizontal deflection systems of the scope. The procedure, involving the use of a known signal, is called calibration. The typical calibration waveform on the LS1020 oscilloscope is a 0.5V (PK-PK) 1 kHz square wave.
Screen Graticule
Above the mainframe controls in Figure 3.13 is the CRT. Figure 3.18 shows an enlarged view of the front of the screen. Notice that the screen has been marked off into eight vertical divisions and ten horizontal divisions. Each division has been further marked off into five equal increments. Each increment represents two-tenths of one division. This scale is called a graticule.
Equipped with a graticule, the oscilloscope provides an electronic graph of voltage against time. It essentially is a calibrated scale with the vertical divisions of the graticule representing voltage values and the horizontal divisions representing time increments.
Illumination Control
The switch marked “ILLUM” on the LS1020 oscilloscope in the mainframe group of controls shown in Figure 3.14 is the illumination switch. It allows the user to turn on small lamps that light the graticule markings so that they are seen more easily either by the user or when oscilloscope traces are being photographed. Notice that the illumination control on the scope shown has three positions: off, dim, and bright.
VERTICAL CONTROL
The vertical controls shown in Figure 3.19 adjust circuits that are connected to the vertical deflection plates and performs two control functions: 1) they control the trace’s vertical position, and 2) they select the amount of voltage each vertical division represents.
The vertical position control, is used to position the electron beam which forms the spot or trace on the graticule vertically. Any initial vertical reference level can be selected.
It is usually positioned without any signal applied to the scope input. The most common level is on the center horizontal line of the graticule as shown in Figure 3.20. The spot can be moved vertically by applying avoltage to the vertical input jack. The input is applied through the input jack to electronic circuits which amplify, or attenuate, the input and apply the potentials to the vertical deflection plates.
The Scope as a Voltmeter
Rememeber, the scope basically acts as a voltmeter; therefore, as shown in Figure 3.21, it must be connected across circuit components to measure voltages across those components. However, instead of one connection being a negative lead, it is a common lead on most scopes, and it is at ground potential. If measurements are to be made in a circuit with a ground, the scope common (ground) lead should be connected only to the circuit ground as shown in Figure 3.22. Failure to practice this procedure will often cause damage to the circuit or the scope, or it could expose the operator to a possible electrical shock.
Vertical Deflection — DC Voltage
What does happen when a dc voltage is applied to the vertical input of a typical scope? First, assume that the scope is turned on and that its controls have been set to produce a spot at the center of the screen without any signal applied to its vertical input. The screen is as shown in Figure 3.23. Now a dc voltage from a power supply is connected to the scope’s vertical input, connected as shown in Figure 3.24. When the power supply is connected to the scope the spot will move up or down depending on the polarity of the connections between power supply and scope input. The spot moves in the direction of the most positive vertical deflection plate. One deflection plate is more positive than the other because one side of the power supply applied is more positive than the other.
Therefore, the spot — which is actually a beam of negatively-charged electrons — isattracted to the more positive of the deflection plates. If the connections between the power supply and scope input are reversed as shown in Figure 3.25, the spot will move in the opposite direction it moved before the connections were reversed. This is because the opposite vertical deflection plate is now more positive than the other. The important fact is that the spot can be made to move up or down from a reference position by applying a voltage of the proper polarity to the vertical input. The examples here were for constant dc voltage inputs.
Vertical Deflection — AC Voltage
If an ac voltage is applied to a scope’s vertical input, the spot will deflect up and down periodically from the center reference point of Figure 3.23. It does this because, as you know, ac is sinusoidal, constantly changing polarity from positive to negative and back again with time in a sine wave fashion. Therefore, a straight vertical line will appear on the scope face as shown in Figure 3.26a. The line is being created by the spot moving up and down very rapidly between the rapidly-changing positive and negative potentials on the vertical deflection plates. Even though a sinusoidal ac voltage is being input to the scope as shown in Figure 3.26b, it doesn’t look like a sine wave because the voltage is being applied only to the vertical deflection plates, and therefore, there is no horizontal deflection. The line is simply an indication of the voltage amplitude changes of the applied ac voltage, as shown in Figure 3.26b. The length of the line will become longer or shorter as the ac voltage amplitude is increased or decreased. It obviously is not the standard graph of a sinusoidal waveform.
Voltage Measurements
Some of the amplitude units of measurement applicable to sinusoidal waveforms can beobserved and measured directly on the scope face using the graticule. As shown in Figure 3.27, the maximum positive deflection from the reference position is the positive peak voltage of the waveform. The maximum negative deflection from the reference position is the negative peak voltage. Therefore, the peak-to-peak voltage is the sum of the positive and negative peak voltage values. Or, the peak-to-peak voltage is the value of voltage measured from the positive peak deflection to the negative peak deflection.
Vertical Deflection — Volts per Division
The amplitudes of ac waveforms can be any voltage value from zero to many hundreds of volts. Therefore, some method is needed to determine the peak or peak-to-peak voltage values of a wide range of possible input voltages. This is done by using the scope’s graticule markings and a control called the volts-per-division selector. It is shown on the scope’s vertical control panel in Figure 3.19. Two vertical volts-per-division selectors are shown on the panel because it is a dual-trace scope designed to provide two separate waveform traces from two separate vertical input sources. Only one vertical volts-per-division selector and input are needed to display a single trace on a scope.
The magnitude of voltage represented by a vertical division on the graticule is determined by the setting of the volts-per-division selector. For example, if the dial is set to 1 volt per division, the vertical deflection circuits have been adjusted so that each major vertical division on the graticule equals 1 volt.
Now, a dc voltage is applied to the scope’s vertical input. If the spot deflects up from a reference position exactly one division, as shown in Figure 3.28, a potential of +1 volt is being measured. If the spot deflects down exactly one division as shown in Figure 3.29, a potential of −1 volt is being measured.
Each major vertical division is further divided into five subdivisions. The voltage interval represented by each subdivision is two-tenths of the voltage of one major division.
If the spot deflects up one major division from a reference position plus four-tenths of the next major division (two subdivisions) asshown in Figure 3.30, then the number of divisions is 1.4. The potential being measured is calculated:
(1.4 divisions)(1 volt-per-division) = 1.4 volts
The voltage being measured is 1.4 volts. Other applied voltages are calculated the same way. The value of the voltage is determined by multiplying the number of graticule divisions by the setting on the volts-per-division selector.
An Example
For example, assume that an ac voltage whose voltage level is unknown is applied to a scope’s vertical input. The waveform on the scope’s CRT is as shown in Figure 3.31. The scope’s vertical selector is set to 2 volts-per-division. What are the peak and peak-to-peak voltages of the input voltage?
To evaluate the scope measurement, count the number of graticule divisions from the zero (center) horizontal graticule reference to the top of the waveform. In this example that is two and one-half divisions. This is the peak voltage of the ac voltage. It can be calculated using equation 3–1.
The positive peak deflection is 5 volts. Since the negative peak voltage is the same as the positive peak voltage as shown in Figure 3.32, the peak negative deflection is also 5 volts. Recall that the peak-to-peak voltage is calculated:
The peak-to-peak voltage is 10 volts.
The peak-to-peak voltage of the waveform can also be determined another way. The total number of graticule divisions from the top of the positive peak to the bottom of the negative peak of the waveform can be counted. In this example that is five graticule divisions. Using equation 3–1,
This is the same amount of peak-to-peak voltage calculated using equation 3–2. Thus either method can be used to determine the peak-to-peak voltage of a waveform if it is a sine wave or a symmetrical ac waveform. With its volts-per-division controls and graticule markings a scope can be used to determine the peak and peak-to-peak values of any unknown voltage within its measurement range.
Vertical Deflection — Variable Volts-per-Division
The scope shown in Figure 3.19 has a variable volts-per-division control located in the center of the volts-per-division selector. This control must be turned fully clockwise to the calibrated position as indicated by the arrow to the lower right side of the VOLTS/DIV control to read the correct amplitudes from the scope graticule as indicated by the main volts-per-division selector. (This is a detent position called CAL.) If the variable control is taken out of the calibrated position, the number of volts for each graticule division would become more than the setting on the main volts-per-division selector. This control should remain in the CAL position except when making special-purpose measurements, which are usually outlined in the scope manual.
Vertical Coupling Control
Below the vertical input connector on the scope in Figure 3.19 there is a switch that is labeled AC-GND-DC. In general, the vertical coupling control should be set to AC when measuring ac voltages such as in Figure 3.26b. The control should be set to DC to measure dc voltages. If a particular waveform has both ac and dc voltage components as shown in Figure 3.33, set the control on the scope to the DC position to examine bothcomponents and it will appear as shown. If you wish to measure only the ac component of a waveform that has both dc and ac components, change the vertical coupling control to the AC position. The dc component will not be displayed, as shown in Figure 3.34.
Figure 3.34 Waveform of Figure 3.33 Using AC Coupling
The other vertical input control position in Figure 3.19 labeled GND is used to set the trace to zero or reference position before taking measurements. In this position, the input signal is removed, and zero volts, or ground potential, is applied to the scope input circuit. The trace can then be moved to a point on the graticule representing the zero point. This zero voltage reference point is normally set at the center horizontal line of the graticule, but it can be moved to any vertical position on the screen using the vertical position control.
HORIZONTAL CONTROLS
The horizontal control section, identified in Figure 3.13 and shown magnified in Figure 3.35, determines the potentials on the right and left horizontal deflection plates causing the spot to move from side to side on the scope face. This side-to-side movement of the spot is called sweep. The spot is always swept from left to right across the screen of the scope as shown in Figure 3.36.
Horizontal Sweep
Recall that the vertical deflection of the spot is proportional to the voltage amplitude of the waveform. The horizontal sweep of the spot on the other hand, is proportional to the amount of time it takes time for the spot to move from one side of the scope’s CRT face to the other. As stated previously, the vertical deflection of the scope plots voltage and the horizontal deflection plots time.
If the spot is swept with the vertical input grounded, there will be no vertical deflection and only the effect of the horizontal controls on the spot will be viewed.
Horizontal Positioning and Triggering
The position control in the horizontal control section shown in Figure 3.35 determines the place at which the spot begins its sweep. The scope position control shown in Figure 3.35 has a “X10 MAG” feature. If the position control is pulled outward, the sweep display is magnified by a factor of 10. This means that a portion of the sweep that normally occupies one division will be spread out over 10 divisions. This is particularly useful when you wish to view a small portion of a displayed waveform in more detail.
The act of starting the sweep is called triggering. Basically, the triggering circuits and their controls shown in the lower part of the horizontal section in Figure 3.35 allow you to select when a sweep will begin relative to some reference signal. Generally, the most useful triggering method is to set the scope trigger controls to internal auto trigger. No trigger source selection is labeled internal on the triggering controls in the horizontal section of the scope shown in Figure 3.35. On this scope, there are actually three selections for internal triggering. They are labeled ALT, CH 1, and CH 2. With the source switch set to any of these three positions, the scope is set to internal triggering using the signal from the designated channel, or from either channel when in the ALT position. The ALT position is the recommended default position so that a signal appearing at either the Channel 1 or Channel 2 input will trigger itself when displayed. The selection of auto triggering is done using the center knob of what is labeled in the triggering section as the LEVEL-HOLDOFF control. You may note that the scope is set to automatic (auto) triggering mode when this knob is pushed inward. When pulled outward, the scope is set to normal (norm) triggering mode. So with the controls on this scope set to ALT–AUTO triggering, the signal applied to either vertical input, and observed on the screen, actually acts to trigger its own sweep.
In the internal auto trigger mode, when the incoming applied signal reaches a specific point on its waveform, determined by the scope’s slope and level controls, the sweep begins. By adjusting these controls the sweep can be caused to begin at any desired point of the waveform displayed. If the input signal is removed, the horizontal sweep would not be triggered. By changing the trigger controls to the auto mode the scope will begin triggering itself (free run) so that there is a reference trace on the CRT face.
Horizontal Sweep Time
The spot is swept across the screen by the horizontal deflection circuits changing the electrical potentials on the horizontal deflection plates. The time interval required for the trace to travel across the screen is controlled by the horizontal deflection time selector or time/division control. The selector is calibrated in units of seconds, milliseconds, or microseconds per division. This is comparable to the volts-per-division units of the vertical deflection selector. The sweep is measured in units of time, usually in seconds or some fraction of a second.
When a time in seconds or decimal fractions of a second is selected by a horizontal time selector setting, the spot then sweeps one division on the graticule horizontal scale in the selected time. For example, if the time division selector is set to one second per division, the spot moves from left to right on the scope face at a rate of one major graticule division per second.
In Figure 3.36 for instance, it would take one second for the spot to move across the scope face from one major graticule mark to another. There are ten major divisions across the graticule. Therefore, with a horizontal setting of one second per division, it would take ten seconds for the spot to move from the left-hand edge of the scope face to its right-hand edge.
Also note in Figure 3.36 that each major horizontal division is divided into five subdivisions. This is further clarified in Figure 3.37. The time interval represented by each subdivision represents two-tenths of the time of one major division. For example, if the time division or horizontal time (sweep) selector is set to one second per division, each subdivision on the horizontal axis represents two-tenths of one second. If, however, the selector is set to two microseconds per division then the spot moves across the screen at the rate of one major graticule division every two microseconds, and each subdivision represents 0.4 microseconds as shown in Figure 3.38.
Figure 3.37 Time Required to Move Spot Across Graticule When Horizontal Control Is Set to 1 Second per Division
Figure 3.38 Time Required to Move Spot Across Graticule When Horizontal Control Is Set to 2 Microseconds per Division
As the sweep time interval per division is decreased by changing the setting of the horizontal time control, the sweep moves more rapidly across the scope face. Eventually, the sweep (the spot) is moving so rapidly that it appears to be a continuous line. Actually, the sweep ends and starts just like on the larger time per division sweep settings, but the motion is too fast for the eye to see.
The range of selection of horizontal time units may be different on different scopes. With the scope shown in Figure 3.35, the trace can be made to move across the screen from a rate of 0.2 seconds per division to a rate up to 0.1 microseconds per division. The common name for the horizontal time control is horizontal sweep selector, therefore, that name will be used from now on.
Variable Sweep Time
To the upper right of the horizontal time/division selector knob is a time variable-control knob. This knob is similar to the variable control of the vertical deflection control. For the seconds-per-division control setting to indicate the correct sweep time it must be in the calibrated position, turned fully clockwise as indicated by the arrow. This is the CAL position. This control should be kept in the CAL position except when making special-purpose measurements which are outlined in the scope manual.
Triggering Control Functions
Recall that the sweep is the action of the electron beam moving from left to right across the scope face. Each sweep must begin at the same time-base point on a repetitive waveform as shown in Figure 3.39 so that each succeeding sweep overlays precisely on the preceding sweep. The trigger circuits determine when each sweep begins, thus providing a stable CRT display. If the sweep is not stabilized, the waveform appears to run across the screen, creating a random display pattern, or it produces a waveform envelope as shown in Figure 3.40.
Figure 3.39 Relationship of Trigger Point and Displayed Waveform (Reproduced from Basic Oscilloscope Operation,Tektronix, Inc., 1978)
As stated previously, the sweep trace seen on the face of the oscilloscope is the result of an electron beam moving from left to right (as we view it) across the face of the CRT. Depending on the setting of the time/division control the beam takes a certain amount of time to traverse the ten horizontal divisions on the face of the CRT. When the trace reaches the right side of the CRT, the beam is turned off (turning off the beam by the scope circuits is called blanking) and the voltage on the horizontal deflection plates is reset (resetting the voltages is called retrace) so that the beam, when turned back on, will reappear at the left side of the CRT. If the oscilloscope is in “free run” (non-triggered) mode, or if the oscilloscope display is set so that the trace is not triggered, the start of the sweep begins again as soon as the beam reaches the right side of the screen and enough time elapses for completion of blanking and retrace. This free run operation will typically result in a display of multiple waveforms that seem to randomly superimpose themselves one on top of the other.
The repeating waveform pattern of Figure 3.41 when displayed in free run mode wouldoverlay the series of successive waveform time-base durations (shown in Figure 3.42) so that the oscilloscope display would appear similar to that shown in Figure 3.43. This effect is a random display pattern typically seen in free run mode.
On the other hand, a triggered oscilloscope display utilizes the scope’s triggering circuits to determine the exact point at which a given waveform will begin to be displayed on the face of the CRT. For example, if the waveform of Figure 3.41 is displayed using a triggered oscilloscope where each successive display of the waveform begins at exactly the same point on the waveform as determined by the triggering circuits, then the display will appear as a stable waveform as shown in Figure 3.44.
Triggering Sources
The control labeled source determines where the sweep obtains its triggering signal. There are three possible sources: internal, line, and external.
The triggering source must be of the same or related frequency as the vertical input signal frequency. If not, the waveform displayed will not remain stabilized on the screen. The waveform must be stable if it is necessary to examine it closely or measure it accurately. When the waveform remains stationary on the CRT display, it is said to be “synchronized.”
Internal Trigger Source
On the LS1020 oscilloscope, internal triggering is set by selecting either ALT,CH 1, or CH 2 as the source. When ALT is selected, either CH 1 or CH 2 vertical input will initiate the sweep — whichever receives an input first. CH 1 position is used to indicate that only signals arriving at the Channel 1 vertical input may be used to trigger the sweep. Similarly, CH 2 position is used to indicate that only signals arriving at the Channel 2 vertical input may be used to trigger the sweep.
Line Trigger Source
When line is selected, the power line frequency is chosen as the triggering frequency. This source is most often used when line voltages are being measured, or if the waveform frequency being measured is related to the power line frequency. Because the power line frequency in the United States is 60 hertz, this triggering mode is most effective when measuring signals withfrequencies that are exact multiples of the line frequency, such as 60, 120, or 180 hertz.
As an example, Figure 3.45 shows a 1-kilohertz waveform input on Channel 1 with the scope set at CH 1 internal triggering mode. As long as internal triggering mode is used, the waveform remains stable because a 1-kilohertz waveform is being observed. However, if the scope is switched to the line triggering source, then a 60-hertz line frequency is being used to trigger a 1-kilohertz waveform. Because 1 kilohertz is neither the same as, nor an exact mulitple of, the 60-hertz line frequency, the 1-kilohertz waveform will not be stabilized. It will run freely across the screen as shown in Figure 3.46. An unstabilized (unsynchronized) waveform is difficult to examine or measure accurately.
External Trigger Source
Sometimes it may be necessary to observe or measure a waveform that is related to another signal, or to synchronize it to another waveform. To do this, external triggering is used. An input voltage is applied to the external input jack. This is the BNC input labeled EXT TRIG IN at the bottom right corner of the horizontal trigger section of the LS1020 oscilloscope shown in Figure 3.35. The voltage applied to this input is used to trigger the sweep. In this mode, one waveform is used to trigger the sweep in order to observe another waveform displayed with respect to the external input triggering waveform.
Coupling
The coupling control determines how the source signal being used to trigger the sweep is connected to the triggering circuits. If AC coupling is used, the signal is connected so that dc voltages do not affect the triggering point. This is the most common general purpose setting for this control. This is the position you should set this control to if you are not sure how to set the coupling control.
The HF-REJ position is the same type of coupling as AC coupling, except that a filter circuit is used to block any high frequency variations in the triggering signal. This position is useful if you suspect that high frequency voltages are present in the triggering signal and you wish them to be blocked because they are causing the waveform you are observing to jump or remain unsynchronized.
The DC coupling position allows both dc and ac components of the triggering signal to determine the point at which the sweep is initiated.
TV-V and TV-H (television-vertical and television-horizontal) are triggering coupling positions typically used in conjunction with the external triggering source input to synchronize complex television video signals so that they may be stabilized and measured on the oscilloscope display.
LEVEL–HOLDOFF
Both slope and level specifications must be used to describe the exact triggering point on a given waveform.
Slope
The slope control determines whether the rising or falling portion of the triggering waveform initiates the sweep. Note that portions of the ac waveform shown in Figure 3.47 are considered to be positive-going (rising), and other portions are considered to be negative-going (falling). If positive slope is selected, the positive-going portion of the waveform initiates the sweep and therefore appears at the beginning of the sweep as shown in Figure 3.48 (assuming internal triggering). If negative slope is selected, the negative-going portion of the waveform initiates the sweep and appears at the beginning of the sweep as shown in Figure 3.49 (assuming internal triggering). Remember, because internal triggering is being used, the waveform observed is the triggering waveform itself.
Level
The level control sets the instantaneous voltage level on the specified slope of the triggering waveform at which the sweep will be initiated. Adjusting the control determines the exact point at which the sweep begins on the positive- or negative-going portion of the triggering signal. As the control is rotated clockwise (the direction marked + in Figure 3.35), the voltage level at which the sweep starts is adjusted higher, or more positive, as shown in Figure 3.50. As the control is rotated counterclockwise (the direction marked – in Figure 3.35), the voltage level at which the sweep starts is adjusted lower or more negative, as shown in Figure 3.51.
Holdoff
The holdoff control is used to delay for a finite amount of time before the scope trigger circuits are allowed to initiate the sweep. As the holdoff is increased, the amount of time delay is increased. In Figure 3.35, if no holdoff is desired, then the control is left in the norm (normal) position. As the holdoff control is rotated clockwise, the amount of holdoff (time delay) is increased. Trigger holdoff is particularly useful when viewing complex repetitive waveforms. In waveforms where multiple points exist that satisfy both the slope and the level criteria for triggering the sweep, the scope might try to initiate the sweep at an inappropriate time and cause the displayed waveform to run across the screen and appear unsynchronized (or seem to free run). Trigger holdoff can delay the oscilloscope trigger circuits from looking for a particular slope and level combination for a set amount of time, thus enabling the user to cause each successive scope sweep to retrace the previous sweep’s path so that the displayed waveform is stabilized, as shown in Figure 3.52.
Figure 3.52 Trigger Holdoff (Reproduced from XYZ’s of Oscilloscopes, Tektronix, Inc., http://www.tek.com/Measurement/App_Notes/XYZs/controls.html)
THE DIGITAL OSCILLOSCOPE
In the 1990s digital oscilloscopes were developed. These oscilloscopes appear to operate similarly to analog oscilloscopes. However, the method used to acquire and display the waveform being observed is different.
Until now, this chapter has concerned itself with a discussion of basic oscilloscope operation as it pertains to an analog oscilloscope such as the LS1020. For many applications the basic analog oscilloscope is adequate. The highest frequency an analog oscilloscope is able to display is determined primarily by its phosphor display system. Analog electronic circuitry is well capable of amplifying and processing very high frequencies, but analog oscilloscope CRT-type displays are limited to a maximum frequency of about 1 GHz. A distinguishing feature of analog oscilloscopes is that they present measured voltage and time information in “real time”; that is, the display is created instantaneously as actual measured voltage versus time events occur.
In contrast to the analog oscilloscope, the data displayed on the screen of a digital oscilloscope is not necessarily “real time” data. The digital oscilloscope samples the measured analog voltage at regular intervals, then each sample is converted into digital data by an analog-to-digital converter. The digital data from the converter is stored temporarily in a digital memory circuit until enough samples are acquired to describe a waveform. Then the samples are read from memory sequentially and used to electronically reassemble the waveform on the output display. The output display is typically a liquid crystal display in most digital oscilloscopes. It uses what is known as a “raster” type display technology. (Raster display is the traditional type of display technology originally used in television and computer monitor displays.) The digital oscilloscope, known as a DSO (digital storage oscilloscope), is able to sample, store, and display higher frequency signals than many analog oscilloscopes due to its method of acquiring and displaying data.
A typical digital oscilloscope is shown in Figure 3.4. The DSO shown in the figure is a 60-MHz dual vertical channel oscilloscope somewhat equivalent to the LS1020 analog oscilloscope. The TDS series oscilloscope has many additional features not found on an analog oscilloscope primarily because of its digital nature. Because of the way a digital oscilloscope samples and stores waveform information digitally, it can store and retrieve waveforms as well as perform and display mathematical calculations regarding waveforms, such as determining the peak-to-peak voltage, period, frequency, and average and true RMS value of a displayed waveform.
Digital Oscilloscope Controls
The control method characteristic of digital oscilloscopes is the use of menu systems. In Figure 3.53 you may notice that the DSO has six buttons grouped together at the top of the control section labeled MENUS. You will also notice the presence of five buttons on the right side of the screen. As different menus are selected, the function of the five buttons on the right of the screen changes. There is also a menu button over each of the Channel 1 and 2 volts/div knobs, a horizontal menu button over the sec/div knob, as well as a trigger menu button under the level knob. These buttons (when pushed) bring up four additional menus, each related to the menu button pushed. Each menu appears at the right side of the LCD screen with its title (the menu has the same title as the menu button pushed).
For example, the Channel 1 menu button allows one of the five selection buttons on the right side of the screen to be used to select the vertical input coupling method (ac-ground-dc), whether a bandwidth limit is imposed on the display of vertical input data or not, whether the volts/div knob adjusts the vertical scale to set values in a “coarse” 1-2-5 (normal) manner or in a “fine” 0.1 V/div manner, and what the multiplier factor of the probe being used is (×1, ×10, ×100, or ×1,000).
Other than the menu system, the controls on a digital oscilloscope are similar to those found on an analog oscilloscope. Notice that the digital oscilloscope has the same three groups of controls as the analog oscilloscope. The vertical section, horizontal section, and trigger section labels can be easily seen on the Tektronix TDS220 shown in Figure 3.4. Depending on the manufacturer, the layout of the controls may or may not be as obvious. Some manufacturers use one control knob to perform several functions — the function of the knob is dependent on the immediately prior pressing of a menu or function button.
Digital Oscilloscope Displays
Because of the use of CRT display technology, the physical size and weight of the analog oscilloscope is larger and heavier than the digital oscilloscope. The digital oscilloscope utilizes LCD technology. Because the LCD can be made in the from of a relatively thin, flat panel and is lightweight, the digital oscilloscope tends to be smaller and lighter than the analog oscilloscope. Compare the long CRT of the analog oscilloscope in Figure 3.54 to the depth of the case (approximately 5 inches) of the digital oscilloscope shown in Figure 3.55.
Digital Oscilloscope Outputs
Many digital oscilloscopes provide output ports either as a standard feature or as an option. The Tektronix TDS220 has an optional “Communications Extension Module” (Figure 3.56)that includes an RS232 Serial port, a Centronix Parallel Printer port, and an IEEE 488 GPIB port. Because waveform data is stored digitally, this allows the waveform to be dumped to an external PC, printed on a printer, or output through a GPIB.
MEASUREMENT OF AN AC WAVEFORM
An important measurement that can be done easily using a scope is the measurement of the period of an ac waveform. The period measurement is significant because the period information can be applied to the time-frequency equation to calculate the frequency of the waveform.
The first step is to apply an ac voltage to the scope’s vertical input with the scope’s coupling control set to measure an ac voltage. Next the horizontal sweep selector should be adjusted to provide a display of one or two cycles of the waveform. For this example, assume that the horizontal sweep selector is set at two-tenths milliseconds (0.2 ms) per division. Make sure the inner variable control is set at CAL.
Recall that the period of a waveform is the time duration, T, of one cycle as shown in Figure 3.34. One cycle exists between any two points that have the same value and where the waveform is varying in the same direction. The points most commonly used to measure the time of one cycle are the points where the waveform crosses the zero axis line on the positive-going part of the waveform.
For this example, the waveform displayed on the face of the scope is as shown in Figure 3.57. Two hash marks have been drawn to indicate one cycle of the waveform. To determine the amount of time between the two hashmarks, the number of graticule divisions between them is counted. There are exactly five divisions between them.
Recall that the scope’s horizontal sweep selector is set to two-tenths milliseconds (0.2 ms) per division. The period of the waveform is then calculated by multiplying the time per division by the number of divisions:
The period of the waveform is one millisecond.
The period of any sinusoidal waveform can be determined in the same way.
Now that the period of the waveform is known, the frequency of the waveform can be calculated easily using this equation:
The frequency of the waveform is 1 kilohertz.
You have now learned how to use a scope to determine the frequency of an ac waveform. Earlier in this chapter, the method of determining the amplitude of an ac waveform was discussed using equations 3–1 and 3–2. Another example will help summarize the technique. We will start with an unknown waveform and use the scope to determine its amplitude and frequency.
Figure 3.58 shows the unknown waveform as it would appear displayed on the face of a scope when the trigger control is set for internal. The scope’s vertical output control is set to 2 volts per division. The scope’s horizontal sweep selector is set to 2 milliseconds per division.
The waveform’s peak-to-peak amplitude can be determined, as stated previously, one of two ways. In this example, the number of graticule divisions between the top of the positive peak and the bottom of the negative peak will be counted. Then that number and equation 3–1 will be used to calculate the peak-to-peak voltage of the waveform. Thus, since the waveform amplitude covers six divisions, the peak-to-peak voltage is:
The unknown waveform is 12 volts peak-to-peak.
The frequency is calculated by determining the period of the waveform by counting horizontal graticule divisions. In this case, the number of graticule divisions for one period of the waveform is six. The time per division is 2 milliseconds. Using equations 3–3 and 3–4, the period and the frequency of the waveform can be calculated:
and
The waveform’s frequency is 83.3 hertz.
Therefore, the unknown waveform is a 12-volt peak-to-peak ac voltage with frequency of 83.3 hertz.
SUMMARY
Discussion in this chapter has tried to familiarize you with one of the most important pieces of electronic measuring equipment — the oscilloscope. The construction of a scope was described. It was explained why the scope is capable of measuring the voltage and time of electrical signals very accurately. The function of a scope’s controls and how they must be adjusted, were described, and you were shown how to use a scope to measure and determine amplitude, period, and frequency of ac waveforms. In following chapters these qualities will be required of many circuits that will be studied. The scope is a very useful instrument for these measurements.
One other point: there are a great number of different types of scopes in use. A scope is supplied, generally, with an operational instruction manual. If you need any additional information regarding any control on a particular scope you are using, you should refer to the manual for that scope. Determine the values of voltage, period, and frequency for the following waveforms shown as they would appear on an oscilloscope.
1. The waveform shown is a dc voltage. Since it is below the OV-reference level, it is a negative voltage:
But remember, this is a negative voltage; therefore,
A constant dc voltage has no frequency.
2. The waveform is an ac sine wave.
Therefore its amplitude can be specified in terms of peak, peak-to-peak, or rms voltage. From its positive peak value to its negative peak value it spans 6 vertical divisions. Thus its peak-to-peak voltage is
Its peak voltage is one-half its peak-to-peak value, so
Its rms voltage is 0.707 times its peak voltage value. Therefore,
The period of the waveform is contained within 8 horizontal divisions. Thus
Knowing the period of the waveform, the frequency can be calculated:
3. The waveform is called a square wave. It is like the waveform that is usually used as a calibration waveform for most oscilloscopes. Note that it alternates between a dc voltage level two divisions above the OV-reference level and two divisions below the OV-reference level. Its amplitude is the difference between these two levels:
This square wave also has a frequency. Its frequency is the rate at which it alternates between its +4-volt and −4-volt levels. The cycle of the waveform exists between the points x and y shown on the graticule. The number of divisions which the cycle spans is four. Therefore, the period of the waveform is
The frequency of the square wave can now be determined as
1. Use the oscilloscope diagram on the following page to identify the following controls and specify the section of the oscilloscope controls in which they are located.
______f. line trigger source control
______g. ac/gnd/dc coupling control(for signal measured/displayed)
______j. time/division control
______l. external trigger source control
______n. input BNC jack (for signal measured/displayed)
______o. up/down position control for trace
______p. up/down position control for trace
2. Using direct coupling, how many vetical divisions are required to display an 8-volt (peak-to-peak) waveform with the vertical volts-per-division control set to 2 V/division?
3. Using direct coupling, how many vertical divisions are required to display a 2.5-volt (peak-to-peak) waveform with the vertical volts-per-division control set to 0.2 V/division?
4. How many horizontal divisions are required to diaplay a sine wave with a period of 1.6 milliseconds with the horizontal seconds-per-division control set to 0.5 ms/division?
5. How many horizontal divisions are required to display a sine wave with a frequency of 5 kilohertz with the horizontal seconds-per-division control set to 0.1 ms/division?
For the following waveforms shown as they would appear on the graticule of an oscilloscope, determine the values of voltage, period, and frequency as specified:
1. Using the oscilloscope diagram on the following page, identify the location of each of the controls listed below by placing the letter designator of the control in the space beside the name of the control:
______c. vertical position control
______d. horizontal position control
______e. vertical volts/division control
______f. horizontal time/division control
______h. vertical input ground jack
______i. vertical input ac/gnd/dc coupling control
______j. variable volts/division control
______l. variable seconds/division control
______m. triggering source control
______n. triggering coupling control
______o. graticule illumination
______q. automatic normal mode control
The following 20 questions are multiple choice; circle the letter of the most-correct answer.
2. The three basic functions that an oscilloscope performs are
a. waveform observation, current measurement, frquency measurement
b. frequency measurement, voltage measurement, period measurement
c. waveform observation, voltage measurement, time measurement
d. period measurement, power measurement, current measurement
3. The two major segments of the oscilloscope are the
4. The CRT is composed of three major parts:
a. screen, deflection system, and screen coating
b. electron gun, deflection system, and screen
5. The controlling circuits are divided into three major groups:
a. mainframe, underframe, and overframe control groups
b. mainframe, vertical, and CRT
6. The type(s) of deflection employed by most oscilloscopes is/are
7. The ability of a material to emit light after being struck by electrons thus enabling you to see the location of the electron beam on the screen of the oscilloscope is called
8. The purpose of the power switch is to control power
9. The purpose of the intensity control is to adjust the
10. The purpose of the focus control is to focus the
11. The purpose of the beam finder control is to
a. automaically reposition the trace
b. turn off the scope power until the beam is found
12. The purpose of the calibrator is to
a. supply a square wave signal of known amplitude and frequency for checking the scope’s accuracy (i.e., calibration)
b. supply an appropriate signal used to self-calibrate the scope
c. automatically calibrate the scope each time it is turned on
13. Each small mark on the center vertical and horizontal graticule lines is
14. The purpose of the vertical position control is to control the
a. vertical position of the trace
b. vertical attitude of the scope
15. The purpose of the vertical attenuator (volts/division control) is to
a. control the level of the signal from the oscilloscope vertical circuits to the circuit under test
b. attenuate the signal to the triggering circuits
c. select the number of volts each vertical graticule major division represents
16. The purpose of the ac/dc vertical input coupling control is to
a. turn on an ac or dc voltage when switched to the corresponding position
b. eliminate the dc component of a waveform when in the ac position
c. eliminate the ac component of a waveform when in the dc position
17. The purpose of the ground vertical input control position is to
a. ground the chassis of the scope to earth ground
b. remove the input signal from the vertical deflection circuits
c. apply ground potential (zero volts) to the vertical deflection circuits in order to set the trace to zero or reference position before taking measurements
18. The purpose of the horizontal position control is to control the horizontal position of the
19. The purpose of the horizontal time/division control is to
a. set the number of seconds/division each major horizontal graticule mark represents
b. set the speed at which the trace (electron beam) moves from right to left across the face (graticule) of the scope
c. assist in determining the period and hence frequency of a measured waveform
20. The purpose of the triggering controls is to select when
21. For most general-purpose measurements, the triggering method to use is to set the scope controls on
a. nonautomatic, line triggering
b. automatic, external triggering
22. Using direct coupling, how many divisions (vertically and horizontally) are required to display one complete cycle of a 3-volt (peak-to-peak), 40 kilohertz waveform with the vertical volts/division control set to 0.5 V/division and the horizontal seconds/division control set to 5 μs division?
number of vertical divisions = ______
number of horizontal divisions = ______
