CHAPTER 4 OSCILLOSCOPES 4.1 Introduction The cathode ray oscilloscope generally referred to as the oscilloscope, is probably the most versatile electrical measuring instrument available. Some of electrical parameter can be observed with the oscilloscope are: 1. dc and ac voltage 2. indirect measurement of ac and dc current, 3. time 4. phase relationships 5. frequency 6. a wide range of waveform evaluations such as rise time Many nonelectrical physical quantities such pressure, strain, temperature and acceleration can be measured by using a transducer to convert the physical parameter to an equivalent voltage. The oscilloscope consists of the following major subsystems: 1. Cathode ray tube or CRT 2. Vertical amplifier 3. Horizontal amplifier 4. Sweep generator 5. Trigger circuit 6. Associated power supplies The heart of the instrument is cathode ray tube. 56
4.2 The cathode ray tube The cathode ray tube used in an oscilloscope is very similar to the picture tube in the television set. Figure 4.1 shows a cross section of a CRT. Figure 4.1: Cathode ray tube with the major components identified The major components of a general purpose CRT are: 1. Evacuated glass envelope 2. Electron gun assembly 3. Deflected plate assembly 4. Accelerating anodes 5. Phosphor coated screen Screen size diameters of approximately 1 to 25 in or larger. Most laboratory quality oscilloscopes use a CRT that has a circular screen approximately 5in in diameter. All electrical connections except the high voltage connection are made through the base of the CRT. The glass envelope is evacuated to a fairly high vacuum to permit the electron beam to traverse the tube easily. The electron gun assembly consists of the triode section and the focus section which are shown in Figure 4.1. The purpose of the electron gun assembly is to provide a source of electrons, converge and focus them into a well defined beam, and accelerate them to 57
toward the fluorescent screen. The electrons that make up the beam are given off by thermionic emission from the heated cathode. The cathode is surrounded by a cylindrical cap that is at a negative potential. This cap, which has a small hole located along the longitudinal axis of the CRT, as shown in Figure 4.2, acts as the control grid. Because the control grid is at a negative potential, electrons are repelled away from the cylinder walls and, therefore, stream through the hole where they move into the electric fields of the focusing anodes. Figure 4.2: Triode section of the CRT The focus lens consists of the first anode, focus ring and astigmatism aperture or second anode. The purpose of this section is to converge and collimate the beam to obtain the minimum size and best defined spot on the phosphor screen of the CRT. The section of the CRT just beyond the electron gun assembly is the deflection. Waveforms can be displayed on the CRT s phosphor screen only if there is some means of deflecting the electron beam both horizontally and vertically. This is the function of the deflection plates. The deflection experienced by the electrons beam in the CRT of an oscilloscope is called electrostatic deflection, which means that the electron beam is deflected by the force exerted on each electron by the electric field. Electrostatic deflection offers the advantages of higher frequency operation as well as space saving inasmuch as the deflection plates are mounted inside the CRT. During the period of accelerating in the electrons gun assembly, the electrons have gained kinetic energy as they gain velocity. 58
The next section of CRT is the postdeflection area. After electron pass beyond the deflection plates, they may or not may experience not experienced additional acceleration. This depends primarily on the maximum frequency to be applied to the CRT. In general, if the maximum frequency to be displayed on the CRT is less than 10MH, no deflected accelerating is used. If generally necessary to increase the brightness of the trace which otherwise may be quite dim. However, using postdeflection acceleration generally requires that the length of the CRT be extended to achieve the required beam deflection as a result of the CRT is extended to achieve the required beam deflection as a result of the increased velocity. An alternative to increased CRT length is to add a dome-shaped mesh to CRT just beyond the deflection plates as shown in Figure 4.3. However, the mesh reduces trace brightness and increases the size of spots unless the postdeflection accelerating voltage is increased significantly. Figure 4.3: CRT with dome shaped mesh to increase deflection When the electron beam strikes the phosphor coated face of the CRT, a spot of light is produced because the phosphor absorbs kinetic energy from electrons that strikes it and gives up the energy in the form of light. This property if emitting light when stimulated by electrons bombardment is called fluorescent; therefore, we can say that phosphor is fluorescent material. 59
Phosphor possesses a second desirable characteristic called phosphorescent, which mean that the phosphor continues to emit light for a period of time after the source of excitation is removed. The length of time phosphorescent continues is a measure of the persistence of the fluorescent material. Table 4.1 lists several different phosphors and some of their applications. Phosphor Trance Persistence Application Trace colour P1 Yellow green Medium General purpose CRO P2 Blue Medium Observation of low & medium speed signals P4 White Medium short Television picture tube P7 Blue Long Observation of low & medium speed signals P11 Blue Medium short Photographic applications P31 Green Medium short Observation of low & medium speed signals. Most frequently used in general purpose CRO 4.3 Basic oscilloscope controls that directly affected the beam An intensity control as shown in Figure 4.4 is always connected to the control grid. The potential of the control grid is negative with respect to the cathode. Therefore, as the intensity control is adjusted, the number of electrons that pass through the small hole in the control grid is affected, which in turn affects the brightness of the spot on the screen. 60
Figure 4.4: Basic circuit of controls that determine position, intensity, and focus of the electron beam on a CRT The focus control is connected to the focusing anode. The focusing and accelerating anodes form an electrostatic lens to collimate the electrons into a well defined beam. Generally, a beam that is in sharp focus at the centre of the screen will be out of focus near the edge of the screen because the lengths of the electron paths are different when the beam is deflected. Adjusting the astigmatism control gives a sharp over the entire screen. The beam can be positioned anywhere on the screen by adjusting the controls marked horizontal position and vertical position. When the horizontal and vertical position controls are set to their midpoint position, the deflection voltages divide equally across both halves of the potentiometers. There is therefore no deflection of the beam; it simply travels along the axis of the CRT and strikes the centre of the screen. Adjusting the horizontal and vertical position control deflect the beam to any desired position on the screen. 61
4.4 Vertical Amplifier The vertical amplifier is the principal factor in determining the sensitivity and bandwidth of an oscilloscope. The gain of the vertical amplifier determines the smallest signal that the oscilloscope can be satisfactory reproduce on the CRT screen. The sensitivity of an oscilloscope is directly proportional to gain of the vertical amplifier; that is, as gain increases sensitivity increases, which allows us to observe smaller-amplitude signals. The vertical sensitivity is a measure of how much the electron beam will be deflected for a specified input signal. Bandwidth of an oscilloscope determines the range of frequencies that can be accurately reproduced on the CRT screen. The greater the bandwidth, the wider the range of frequencies that can be observed with the instrument 4.5 Horizontal amplifier The horizontal amplifier basically serves two purposes: 1. When the oscilloscope is being used in the ordinary mode of operation to display a signal applied to the vertical input, the horizontal amplifier will amplify the sweep generator output. 2. When the oscilloscope is being used in the X-Y mode, the signal applied to the horizontal input terminal will be amplified by the horizontal amplifier. 4.6 Sweep generator Oscilloscopes are used to display a waveform that varies as a function of time. If the waveform is to be accurately reproduced, the beam must have a constant velocity. Since the beam velocity is a function of the deflecting voltage must increase linearity with time. A voltage with this characteristic is called a ramp voltage. 62
Figure 4.5: Typical sawtooth waveform applied to the horizontal deflection plates. If the voltage decreases rapidly to zero with waveform repeatedly reproduced, as shown in Figure 4.5, the pattern is generally called a sawtooth waveform. During the sweep time, T s, the beam moves from left to right across the CRT screen. The beam is deflected to the right by the increasing amplitude of the ramp voltage and the fact that the positive voltage attracts the negative electrons. During retrace time, T r, the beam returns quickly to the left side of the screen. The control grid is generally gated off, which blanks out the beam during retrace and prevents an undesirable retrace pattern from appearing on the screen. 4.7 Attenuators The voltage at input terminal of the vertical amplifier, which causes the beam to be deflected off CRT screen, is quite low in amplitude. So that, high amplitude signals may be displayed, an attenuator network is placed between the vertical input terminal and the input of the vertical amplifier. The term attenuate means to reduce in size. The purpose of the attenuator is to reduce the amplitude of the vertical input signal before applying it to the vertical amplifier. The most basic attenuator is a simple resistive voltage divider such as the one shown in Figure 4.6. With this circuit, the input voltage V i, will be attenuated by a factor of 10000 with the switch S set to position A as shown, in position B, C, and D, the attenuation factors will be 1000, 100, and 10, respectively in switch position E there is no attenuation. 63
Figure 4.6: Resistive attenuator network The attenuation in any switch position can be determined from the ratio of the total resistance from the position of interest to ground to the total series resistance, written as V o = V i R R t Where R = total resistance from the desired attenuator terminal to ground R t = total series resistance The attenuation factor is the reciprocal of the voltage divider ratio. 64
4.8 Determining frequency with Lissajous Patterns The oscilloscope can be used in X-Y mode to determine the frequency of a signal. The frequency is determined by applying the signal of unknown frequency to either the X or the Y input terminal and a signal of known frequency to other input terminal. The pattern observed on the screen is called Lissajous Figure. The particular Lissajous pattern observed depends on the ratio of the two frequencies. f f v h = number of number of horizontal tan gencies vertical tan gencies Figure 4.7: Lissajous Patterns for integral frequencies 65
Figure 4.8: Lissajous Pattern for Non-Integral frequencies 4.9 Phase angle computation Oscilloscopes can also be used in X-Y mode to determine the phase angle between two signals of the same frequency. The pattern displayed on the CRT screen may vary from straight line with a positive slope, if the signals are in phase, to a straight line with a negative slope for signals 180 0 out of phase, as shown in Figure 4.8. If the phase angle is any angle between 0 0 and 360 0 besides 180 0, a circle or ellipse, as shown in Figure 4.9, will be displayed. The phase angle is easily determined from the ellipse. The ratio of the Y axis intercept represented as Figure 4.9, and the maximum vertical deflection, Y2 is equal to the sine of the phase angle, that is, Where θ = phase angle in degrees Y 1 = Y axis intercept Y 2 = maximum vertical deflection Sin θ = Y Y 1 2 66
Figure 4.9: Lissajous patterns for selected phase angles 67
Figure 4.10: Evaluation of phase relationship 68