Module 16—Introduction to Test Equipment
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THE OSCILLOSCOPE AND SPECTRUM ANALYZER
Upon completing this chapter, you should be able to:
1. Describe the purpose of the CRT used in the oscilloscope.
2. Explain the operation of an oscilloscope.
3. Describe the purpose of the controls and indicators found on an oscilloscope.
4. Describe the proper procedure for using a dual-trace oscilloscope.
5. Describe the accessory probes available for use with a dual-trace oscilloscope.
6. Explain the operation of the spectrum analyzer.
7. Describe the purpose of the controls and indicators found on the spectrum analyzer.
One of the most widely used pieces of electronic test equipment is the OSCILLOSCOPE. An oscilloscope is used to show the shape of a video pulse appearing at a selected equipment test point. Although some oscilloscopes are better than others in accurately showing video pulses, all function in fundamentally the same way. If you learn how one oscilloscope operates, you will be able to learn others.
As you will learn in this chapter, there are many different types of oscilloscopes—varying in complexity from the simple to the complex. Before we get into our discussion of the dual-trace oscilloscope, we will first present a general overview of basic single-trace oscilloscope operation. Shortly, we will see how oscilloscopes use a CATHODE-RAY TUBE (CRT) in which controlled electron beams are used to present a visible pattern of graphical data on a fluorescent screen.
Another piece of test equipment used is the SPECTRUM ANALYZER. This test equipment is used to sweep over a band of frequencies to determine what frequencies are being produced by a specific circuit under test, and then the amplitude of each frequency component. An accurate interpretation of the display will allow you to determine the efficiency of the equipment being tested.
A detailed discussion of CATHODE-RAY TUBES (CRTs) is presented in NEETS, Module 6, Electronic Emission, Tubes, and Power Supplies. Before continuing with your study of CRTs in this section, you may want to review chapter 2 of that module.
Cathode-ray tubes used in oscilloscopes consist of an ELECTRON GUN, a DEFLECTION SYSTEM, and a FLUORESCENT SCREEN. All of these elements are enclosed in the evacuated space
inside the glass CRT. The electron gun generates electrons and focuses them into a narrow beam. The deflection system moves the beam horizontally and vertically across the screen. The screen is coated with a phosphorous material that glows when struck by the electrons. Figure 6-1 shows the construction of a CRT.
Figure 6-1.—Construction of a CRT.
The ELECTRON GUN consists of a HEATER and a CATHODE to generate electrons, a CONTROL GRID to control brightness by controlling electron flow, and two ANODES (FIRST and SECOND). The main purpose of the first (FOCUSING) anode is to focus the electrons into a narrow beam on the screen. The second (ACCELERATING) anode accelerates the electrons as they pass. The control grid is cylindrical and has a small opening in a baffle at one end. The anodes consist of two cylinders that contain baffles (or plates) with small holes in their centers.
Q-1. What element controls the number of electrons striking the screen?
Q-2. What element is controlled to focus the beam?
Cathode and Control Grid
As in most conventional electron tubes, the cathode is indirectly heated and emits a cloud of electrons. The control grid is a hollow metal tube placed over the cathode. A small opening is located in the center of a baffle at the end opposite the cathode. The control grid is maintained at a negative potential with respect to the cathode to keep the electrons bunched together.
A high positive potential on the anodes pulls electrons through the hole in the grid. Because the grid is near the cathode, it can control the number of electrons that are emitted. As in an ordinary electron tube, the negative voltage of the grid can be varied either to control electron flow or stop it completely. The brightness (intensity) of the image on the fluorescent screen is determined by the number of electrons striking the screen. This is controlled by the voltage on the control grid.
Electrostatic Lenses and Focusing
The electron beam is focused by two ELECTROSTATIC FIELDS that exist between the control grid and first anode and between the first and second anodes.
Figure 6-2 shows you how electrons move through the electron gun. The electrostatic field areas are often referred to as LENSES because the fields bend electron streams in the same manner that optical
lenses bend light rays. The first electrostatic lens cause the electrons to cross at the first focal point within the field. The second lens bend the spreading streams and return them to a new, second focal point at the CRT.
Q-3. Why are the electrostatic fields between the electron gun elements called lenses?
Figure 6-2.—Formation of an electron beam.
Figure 6-2 also shows the relative voltage relationships on the electron-gun elements. The cathode (K) is at a fixed positive voltage with respect to ground. The grid is at a variable negative voltage with respect to the cathode. A fixed positive voltage of several thousand volts is connected to the second (accelerating) anode. The potential of the first (focusing) anode is less positive than the potential of the second anode. The first anode can be varied to place the focal point of the electron beam on the screen of the tube. Control-grid potential is established at the proper level to allow the correct number of electrons through the gun for the desired image intensity.
Q-4. What is the function of the second anode?
ELECTRON BEAM-DEFLECTION SYSTEM
The electron beam is developed, focused, and accelerated by the electron gun. The beam appears on the screen of the CRT as a small, bright dot. If the beam is left in one position, the electrons will soon burn away the illuminating coating in that one area. To be of any use, the beam must be able to move. As you have studied, an electrostatic field can bend the path of a moving electron.
As you have seen in the previous illustrations, the beam of electrons passes through an electrostatic field between two plates. You should remember that electrons are negatively charged and that they will be deflected in the direction of the electric force (from negative to positive). This deflection causes the electrons to follow a curved path while in the electrostatic field.
When the electrons leave the electrostatic field, they will take a straight path to the screen at the angle at which they left the field. Because they were all deflected equally, the electrons will be traveling toward the same spot. Of course, the proper voltages must exist on the anodes to produce the electrostatic field. Changing these voltages changes the focal point of the beam and causes the electron beam to strike the CRT at a different point.
Factors Influencing Deflection
The ANGLE OF DEFLECTION (the angle the outgoing electron beam makes with the CRT center line axis between the plates) depends on the following factors:
· Length of the deflection field;
· Spacing between the deflection plates;
· The difference of potential between the plates; and
· The accelerating voltage on the second anode.
LENGTH OF DEFLECTION FIELD.—As shown in figure 6-3, a long field (long deflection plates) has more time to exert its deflecting forces on an electron beam than does a shorter field (short deflection plates). Therefore, the longer deflection plates can bend the beam to a greater deflection angle.
Figure 6-3.—Factors influencing length of field.
Q-5. What effect do longer deflection plates have on the electron beam?
SPACING BETWEEN PLATES.—As shown in figure 6-4, the closer together the plates, the more effect the electric force has on the deflection angle of the electron beam.
Figure 6-4.—Spacing between plates.
Q-6. What effect does closer spacing of plates have on the electron beam?
DIFFERENCE OF POTENTIAL.— The potential on the plates (figure 6-5) can be varied to cause a wider or narrower deflection angle. The greater the potential, the wider the deflection angle.
Figure 6-5.—Differences of potential.
Q-7. Is the deflection angle greater with higher or lower potential on the plates?
BEAM ACCELERATION.—The faster the electrons are moving, the smaller their deflection angle will be, as shown in figure 6-6.
Figure 6-6.—Beam acceleration.
Q-8. Is the deflection angle greater when the beam is moving faster or slower?
Vertical and Horizontal Plates
If two sets of deflection plates are placed at right angles to each other inside a CRT (figure 6-7), the electron beam can be controlled in any direction. By varying the potential of the vertical-deflection plates, you can make the spot (beam) on the face of the tube move vertically. The distance the beam moves will be proportional to the change in potential difference between the plates. Changing the potential difference between the horizontal-deflection plates will cause the beam to move a given distance from one side to the other. Directions other than up-down and left-right are achieved by a combination of horizontal and vertical movement.
Figure 6-7.—Deflection plate arrangement.
As shown in figure 6-8, position X of the beam is in the center. It can be moved to position Y by going up 2 units and then right 2 units. Movement of the beam is the result of the simultaneous action of both sets of deflection plates. The electrostatic field between the vertical plates moves the electrons up an amount proportional to 2 units on the screen. As the beam passes between the horizontal plates, it moves to the right an amount proportional to 2 units on the screen.
Figure 6-8.—Beam movement on the CRT.
If the amount of deflection from the left and down occurred so that each set of plates acted at the same time, the picture would be like the one in view A of figure 6-9. For example, if the vertical plates moved the beam downward (starting from point X) at the rate of 3 units per second and the horizontal plates moved it to the left at the rate of 1 unit per second, both movements would have been completed in
1 second at point Y. The result would be a straight line.
Figure 6-9.—Deflection of the beam.
In view B, the potentials on the vertical and horizontal plates change at the same rate. In the same time period, say 1 second, both plates move the beam 1 unit. The horizontal plates have completed their task at the end of 1 second, but the vertical plates have moved the beam only one-third of the required distance. In this case, the picture in view B would appear on the screen.
Beam-deflection Plate Action
Recall from your study of chapter 2 of this module that waveforms are described in terms of amplitude versus time. You have just seen how the movement of the CRT beam depends on both potential (amplitude) and time.
Q-9. Waveforms are described in terms of what two functions?
VERTICAL-DEFLECTION PLATES.—We will use figure 6-10 to explain the action of the vertical-deflection plates in signal amplitude measurements. As this discussion begins, remember that vertical-deflection plates are used to show amplitude of a signal, and horizontal-deflection plates are used to show time and/or frequency relationships.
Figure 6-10.—Amplitude versus time.
1. From T0 to T1, the vertical plates maintain their static difference in potential and the beam stays at 0 units; the T0 to T1 change causes an increasing potential difference in the horizontal plates, and the beam moves 1 unit to the right.
2. At T1, a positive potential difference change in the vertical plates occurs, which causes the beam to move up (instantaneously) 2 units. This vertical (amplitude) beam location is maintained from T1 to T4; horizontal beam movement continues moving to the right as 3 units of time pass.
3. At T4, an instantaneous negative change in potential of 4 units in amplitude occurs, and the beam moves from +2 to -2 units.
4. From T4 to T7, the beam remains at -2 units. During this time period, the beam continues moving horizontally to the right, indicating the passage of time.
5. At T7, a positive increase of amplitude occurs, and the beam moves vertically from -2 to 0 units. From T7 to T8, no change occurs in vertical beam movement; however, horizontal movement continues with time.
The vertical-plate potential difference follows the voltage of the waveform. The horizontal-plate potential follows the passage of time. Together, they produce the image (trace) produced on the screen by the moving beam.
Q-10. The vertical-deflection plates are used to reproduce what function? Q-11. The horizontal-deflection plates are used to produce what function?
HORIZONTAL-DEFLECTION PLATES.—Now let's look at horizontal-deflection action. Assume that the resistance of the potentiometer shown in figure 6-11 is spread evenly along its length. When the arm of the potentiometer is at the middle position, the same potential exists on each plate. Since there is zero potential difference between the plates, an electrostatic field is not moved downward at a uniform rate; the right plate will become more positive than the left (you are looking down through the top of the CRT). The electron beam will move to the right from screen point 0 through points 1, 2, 3, and 4 in equal time intervals.
Figure 6-11.—Horizontal plates (top view).
If the potentiometer arm is moved at the same rate in the opposite direction, the right plate will decrease in positive potential until the beam returns to the 0 position. At that point, the potential difference between the plates is again zero. Moving the arm toward the other end of the resistance causes the left plate to become more positive than the right, and the beam moves from screen points 0 through 4. If the movement of the potentiometer arm is at a uniform (linear) rate, the beam moves at a uniform rate.
Notice that the ends of the deflection plates are bent outward to permit wide-angle deflection of the beam. The vertical plates are bent up and down in the same manner.
Q-12. Why are the ends of the deflection plates bent outward?
For ease of explanation, the manual movement of the potentiometer arm is satisfactory to introduce you to horizontal beam movement. However, in the oscilloscope this is not how horizontal deflection is accomplished. Beam movement voltages are produced much faster by sawtooth circuitry. You may want to review the sawtooth generation section in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuitry before continuing. Nearly all oscilloscopes with electrostatic deflection apply a sawtooth voltage to the horizontal plates to produce horizontal deflection of the beam, as shown in figure 6-12.
Figure 6-12.—Sawtooth generator.
In the figure, the sawtooth generator replaces the potentiometer and is connected to both horizontal plates of the CRT. At the reference line, the potential on both plates is equal. Below the line, the left plate is more positive and the right plate is less positive. This causes the beam to move left. Above the line, the right plate is made more positive than the left and the beam moves to the right. The waveform amplitude causes a uniform movement of the beam across the screen (called TRACE). RETRACE time, shown at the trailing edge of the waveform, quickly deflects the beam back to the starting point.
A GRATICULE was used in our previous discussion in figure 6-10. It is simply a calibrated scale (made of clear plastic) of amplitude versus time that is placed on the face of the CRT.
The graticule can be used to determine the voltage of waveforms because the DEFLECTION SENSITIVITY of a CRT is uniform throughout the vertical plane of the screen. Deflection sensitivity states the number of inches, centimeters, or millimeters a beam will be deflected for each volt of potential difference applied to the deflection plates. It is directly proportional to the physical length of the deflection plates and their distance from the screen and inversely proportional to the distance between the plates and to the second-anode voltage. Deflection sensitivity is a constant that is dependent on the construction of the tube.
Deflection sensitivity for a given CRT might typically be 0.2 millimeters per volt. This means the spot on the screen will be deflected 0.2 millimeters (about 0.008 inch) when a difference of 1 volt exists between the plates. Sometimes the reciprocal of deflection sensitivity (called DEFLECTION FACTOR) is given. The deflection factor for the example given would be 125 volts per inch (1/0.008).
Q-13. What term is used to describe the reciprocal of deflection sensitivity of a scope?
In the above example, 125 volts applied between one set of plates would deflect the beam 1 inch on the screen. This means that the deflection caused by small signals would likely not be observed. For this reason, the deflection plates are connected to amplifiers that magnify the signals applied to the vertical input of the scope.
Assume, for example, that a peak-to-peak value of a known voltage applied to the oscilloscope indicates that each inch marking on the graticule is equal to 60 volts. Each of the 10 subdivisions will, therefore, equal a value of 6 volts. Most oscilloscopes have ATTENUATOR controls to decrease or GAIN controls to increase the strength of a signal before it is placed on the deflection plates. Attenuator and gain settings must not be disturbed after the calibration has been made. For maximum accuracy, you should recalibrate the graticule each time a voltage is to be measured.
Cathode-ray tubes are identified by a tube number, such as 2AP1, 2BP4, or 5AP1A. The first number identifies the diameter of the tube face. Typical diameters are 2 inches, 5 inches, and 7 inches. The first letter designates the order in which a tube of a given diameter was registered. The letter-digit combination indicates the type of phosphor (glowing material) used on the inside of the screen. Phosphor P1, which is used in most oscilloscopes, produces a green light at medium PERSISTENCE. Persistence refers to the length of time the phosphor glows after the electron beam is removed. P4 provides a white light and has a short persistence. If a letter appears at the end, it signifies the number of the modification after the
OSCILLOSCOPE CONTROL COMPONENTS
Although the CRT is a highly versatile device, it cannot operate without control circuits. The type of control circuits required depends on the purpose of the equipment in which the CRT is used.
There are many different types of oscilloscopes. They vary from relatively simple test instruments to highly accurate laboratory models. Although oscilloscopes have different types of circuits, most can be
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Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
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