THE OSCILLOSCOPE AND SPECTRUM ANALYZER
Upon completing this chapter, you should be able to:
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.
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.
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.
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 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.