Practical Application Of The Cathoderay Oscillograph

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Practical applications of the cathode-ray oscillograph, there are plenty. To present a list of its practical uses and to enumerate again these units in connection with the actual work, would be just so much duplication. Accordingly, we shall omit the usual list and consider each application in turn.

When talking about certain operating characteristics of cathode-ray oscillographs, we must take for granted that you who read these pages possess the type of equipment which is covered by this text. Of course, it is possible that the unit you own or operate may not be as versatile as that which we discuss. In that event a portion of what is said in these pages will apply to your equipment and you will be familiar with what other effects are experienced or introduced as a result of the manipulation of the controls existing in more elaborate equipment. However, there are certain subjects which can be discussed with total freedom, since they encompass operating features common to all types of cathode-ray oscillographs.

In as much as the general operation of the instruments is generally useful information, we shall start the discussion of practical applications by describing those points which come under the general operation category.

Spot Position

Spot positioning is a very important operation. While it is true that tubes should be manufactured in such manner that the normal position of the spot is in the center of the screen, certain slight discrepancies may arise, which will cause the spot to be off-center. This may be experienced when cathode-ray tubes are changed, or as a result of aging of the spot positioning resistors, or perhaps imperfect adjustment during some test. Unless the tube is so designed, that the normal position of the spot is off center (which is not the case in units now being sold), the first adjustment is the correct positioning of the spot. If the unit on hand does not include spot positioning controls, then some other means, to be mentioned later, must be used. In the meantime, we shall assume that spot positioning controls are available.

The spot can be moved in two directions, vertically and horizontally across the tube. Figure 64 illustrates nine positions of the spot upon the cathode-ray screen. You will note a slight halo around the spot. This indicates an incorrect adjustment, as will be described later. However we deliberately made the spot larger than normal and exposed the film for a long time in order to bring out the full limits of the tube screen. Consider these illustrations from the angle of spot position only and not from the size of focus of the spot. These position adjustments were obtained by varying the vertical and horizontal spot position controls. Of these nine, only one is the correct position, and that is figure A. All the other pictures indicate location of the spots in incorrect positions.

Location of the spot in the correct position is important, because it influences the placement of the image upon the screen. The screen is not a perfectly flat surface and with the spot off-center, examination of a waveform, which may not have equal amplitudes for its positive and negative alternations, may place one portion of the image off the screen. Then again, spot positioning is also important, in order to permit the viewing of just such an image which does not have equal amplitudes both sides of the zero line. Whereas under one condition, the spot must be in the center of the screen, in the other, it may be necessary to move the spot above or below the center along the "Y" axis.

Often, when viewing resonance curves, when a comparison of amplitude is being made, the spot may move off the screen at the peak point. To bring the image within the screen limits the spot would be moved down from the center, along the "Y" axis. For an inverted resonance curve, with the conditions cited, the spot would be moved up from the exact center, along the "Y" axis. However, normal operation requires exact centering. After each change, for certain types of test, the spot should be moved back to its original position.

Whatever the type of cathode-ray instrument, the horizontal and vertical plates must be connected to ground, when adjusting the spot position. This ground can be effected by connecting a 500,000-ohm resistor between the free vertical deflecting plate and ground and a similar resistor between the free horizontal plate and ground. Instruments, which are equipped with gain control in both deflecting

Fig. 64. A shows the correct position of the spot at the screens center. B, C, D and B show the spot above, below, to the right and to the left of the center respectively. The spot in E happened to coincide with the reflection of the light on the external surface of the tube. F, G, H and I show the spot in the different quadrants. It should be noted that the size of the spot was made very large intentionally and it was too intense, as is evidenced by the halo surrounding it.

Fig. 64. A shows the correct position of the spot at the screens center. B, C, D and B show the spot above, below, to the right and to the left of the center respectively. The spot in E happened to coincide with the reflection of the light on the external surface of the tube. F, G, H and I show the spot in the different quadrants. It should be noted that the size of the spot was made very large intentionally and it was too intense, as is evidenced by the halo surrounding it.

plate circuits, automatically are connected to ground. However, units which are arranged in such manner that direct connection is possible to the plates of the cathode-ray tube, require such grounding resistors. If the plates are allowed to float, they will collect electrical charges, which will interfere with correct positioning of the spot.

Referring to figure 64, A is the correct position. In B, the spot has been moved up along the "Y" axis. In C, the spot has been moved down along the "Y" axis. In D, the spot has been moved to the right along the "X" axis. In E, the spot has been moved to the left along the "X" axis. In F, the spot has been moved into the first quadrant, to the right of the vertical reference line and above the horizontal reference line. In G the spot is in the second quadrant, to the left of the vertical reference line and above the horizontal reference line. In H, the spot is in the third quadrant, below the horizontal reference line and to the left of the vertical reference line. In I, the spot has been moved to the fourth quadrant, to the right of the vertical reference line and below the horizontal reference line.

There are several ways of positioning the spot when the normal voltage divider adjustments are not available. The old suggestion to position the spot by means of an external magnet, placed near the deflecting plates on the outside of the tube is not satisfactory, because it distorts the image formed when the spot is made to move. For that matter, such a condition must be guarded against. Time and again, an external field may cause the spot to be out of position. To insure against such action, the metal case housing the cathode-ray unit, must be grounded. If the tube is used without a metal housing or shield, it should be kept away from all steel or from units which develop strong, magnetic fields, unless, of course, a test is being made, which requires that a strong magnetic field act upon the spot.

A fairly satisfactory method of centering the spot, when the proper controls are not available, is to sweep a horseshoe magnet back and forth across the tube, near the plates. By properly orienting the magnet with respect to the plate and noting the movement of the spot, correct placement will be achieved. The magnet need not be closer than perhaps six inches from the tube. The average horseshoe magnet about 3 inches long is big and powerful enough. It may be necessary to place the magnet closer to the tube. Experience will indicate the correct distance. As a general rule, the amount of correction required is not very great, for a fairly high degree of uniformity exists between tubes. The magnet should be moved past the tube, parallel with the neck of the tube and then the change in spot position noted.

Take care that such a remedy is applied, only when it is needed. Make certain that the off-center position of the spot is not due to circuit adjustment, as, for example, in the National Union instrument, the normal position of the spot, when the sweep is in operation and adjusted to minimum amplitude, is at the extreme right hand side of the viewing screen.

The use of a biasing battery in the vertical or horizontal deflection plate circuit is quite in order. The battery is connected into the free deflection plate lead. However, this battery can be utilized only if direct connection to the plates is possible. If a blocking condenser is used in the circuit, the battery biasing arrangement cannot be employed. The amount of voltage required for the battery is dependent upon the amount of spot position shift desired. The battery voltage may be controlled with a potentiometer. The complete biasing voltage source in series with a 3.0 or 5.0 megohm resistor may be connected between the free deflecting plate and the ground plate. The signal voltage to be observed or applied across the same set of plates, then is applied to the cathode-ray tube in normal fashion. The high resistance in series with the battery serves to maintain the input impedance of the biased set of plates at a reasonably high value. The circuit which supplies the signal voltage to be observed is connected across the free plate and the grounded plate. This signal feed circuit may incorporate the blocking condenser if desired. Since most of the commercial units incorporate blocking condensers, it will be necessary to make proper contact inside the unit, in order to apply the biasing voltage.

Spot Focusing

We have made reference to the fact that the spot intensity or brilliancy and focus influenced the detail of the image appearing upon the screen. Once the spot has been properly centered, which operation need be carried out but once for each tube, the next item is the focusing and brilliancy adjustment. As a general rule, for any one focus adjustment arrived at in conjunction with the brilliancy adjustment, it is necessary to operate but one control, to maintain the same detail of image. This is true for certain periods of time. Naturally, as the tube ages, the emission characteristic varies and the rectifier tube emission varies, so that both brilliancy or intensity and focus controls must be readjusted.

Figure 65 shows three photographs of spot images. A is the cor rect spot size, attained by proper adjustment of the intensity and focus controls. Note that the spot is uniformly round, without any jagged edges. B shows a spot which is out of focus. Note that the spot is somewhat elliptical and has a slight fringe. C shows a spot which

Fig. 65-A. Left. The proper size of the spot. Note that it is round and has smooth edges. Fig. 66-A. Right. The line resulting when a voltage is impressed on the horizontal deflecting plates. Note that it is uniform in thickness.

Fig. 65-B. The spot is here out of focus, evidenced by its elliptical shape. Fig. 66-B shows the resulting line when this spot is spread across the screen. Note its non-uniformity of thickness.

is too intense. You can see the halo around the spot. The effect that the intensity and focusing adjustment has upon the image is shown in figure 66, A voltage was applied to the horizontal deflection plates, for each of the conditions shown in figure 65. A is the line developed when the spot is correctly focused and of proper intensity. As a matter of fact, it is necessary to readjust slightly the intensity control when the spot is spread into a line or image, to maintain the intensity of

Fig. 6S-C. Here the intensity control was too far advanced, this being evidenced by the halo around the spot. Fig. 66-C. The spot of Fig. 65-C shows a trace that is broad at one end and narrow at the other.

the spot. This is so because the spot is in motion. Note that the intensity of the line is uniform, its width is even throughout its length and that the width is equal to the diameter of the spot. Actually,, correct intensity and focusing produces a spot which is about 1/64 or perhaps 1 /32 inch in diameter.

The out of focus adjustment, shown in figure 65-B, develops a line which is wider at one end than at the other. Improper intensity and focusing adjustments may switch the wide portion from the right side to the left side, but one side will always be wider than the other. Excessive intensity is indicated in figure 66-C. Note the ragged edges and the excessive width of the line. Such a spot adjustment would kill all detail in a pattern.

As far as intensity adjustment is concerned for any one correct focus adjustment, let it be known that the minimum brilliancy consistent with ease of examination of the image, is the best adjustment. Of course, if the image is to be photographed, as we have done, then far more brilliancy is required. Nothing is gained, when viewing the image with the eye, by making the image very brilliant. As a matter of fact, a stationary image of excessive brightness is apt to damage the screen of the tube, if the image is allowed to stay on the screen for too long a time. A single brilliant spot should not be allowed to stay on the screen for too long a time. It will burn the screen and that part of the screen will thereafter not fluoresce as brilliantly as the balance of the screen.

Sweep Circuit Control

The oscillograph is valuable because it enables observation of various alternating phenomena. One of the reasons why such observation is possible is that the device provides a time base. In the majority of cathode-ray tube instruments, the greater majority at that, this time base is of a variable frequency. In a few isolated cases, the time base is the 60-cycle supply. Since this type of time base, and we are speaking about what is known as the 60-cycle sweep, is of very little, if any value, we do not deem it worthwhile to devote space to it as a sweep circuit. Instead it shall be considered as just a sinusoidal wave applied to the horizontal plates in connection with the development of Lis-sajous' figures. . . . Do not confuse this reference to a 60-cycle sine wave sweep with a 60-cycle synchronizing pulse, which will be discussed in a subsequent paragraph. The two are very definitely different, although the original signal is secured from the same place. In the immediate paragraphs to follow, we shall dwell upon what has been presented as being the linear sweep or timing axis.

There are numerous pertinent facts relating to the operation of a sweep circuit which you should know and understand. The frequency relation between the sweep circuit voltage and the voltage being observed or to be observed, has very much to do with the ease of making such observations. If it is necessary to make photographic records of the pattern upon the screen, even greater stress must be placed upon proper application of the sweep system. As we stated in a preceding paragraph, the amplitude of the sweep voltage has a bearing upon the shape of the image and it is necessary for the person, who is using the equipment, to understand the nature of the circuit employed in the instrument, so that he can employ the device to greatest advantage and with greatest accuracy.

Subsequent paragraphs will be devoted to what can be generally called "the ways and means of developing waveform images upon the screen." However, we hasten to say that this discussion will not include what is really the interpretation of waveforms. That will follow later. Before walking, we crawl. So it is with the operation of the oscillograph, for if the proper means are not employed when establishing the wave upon the screen, the imperfect shape of the wave, due perhaps to improper handling of the instrument, may result in false conclusions. You may find it necessary, as we progress through this discussion, to refer back to some of the earlier diagrams showing the development of waves of various types by means of spot position.

As has been stated earlier in this chapter, linear sweep circuits, used in modern cathode-ray oscillographs, are of the variable frequency type. As a result of the controls available upon the panel, it is possible to select any one frequency within the range of the device and to produce a sweep voltage, or timing axis voltage, of the pre-determined frequency. In as much as these sweep circuits are only partly calibrated, a rough and fine frequency adjustment is available. The band selection is made by means of the rough adjustment and the final adjustment to the required frequency is made by means of the fine control, which, as has been shown, is a resistance of one type of another, which controls the rate of charge in one type of sweep circuit and the rate of discharge in another type of sweep circuit. The process of distinguishing whether the charge or discharge portion of the cycle is being used for the sweep, is nothing more than an examination of the circuit being used. As a matter of fact, knowledge of this type is not essential to the proper application of the instrument.

The Sweep Frequency

The frequency selected for the sweep circuit usually is a sub-multiple of the frequency of the voltage wave to be observed. In other words, the frequency of sweep circuit is less than the frequency of the voltage wave to be examined. The result is that several cycles of the wave to be observed are developed on the screen. Of course, it is not imperative that several cycles be developed upon the screen. One cycle would do, but since better judgment of the true shape of the wave under examination is possible when several cycles, say two or three or three or four, are upon the screen, the sweep frequency is varied until this number of cycles appear upon the screen. Irrespective of what the frequency of the wave to be observed, providing that it is within the limits of the cathode-ray oscillograph, which means about 100,000 cycles with today's equipment, the number of cycles which appear upon the screen vary directly in proportion with the integral ratio between the frequency of the wave being observed and the sweep frequency. If the wave being observed is at 5000 cycles and the sweep fre quency is 1000 cycles, five cycles will appear upon the screen. If the sweep frequency is 2500 cycles, two cycles appear upon the screen. If the sweep frequency is 500 cycles, ten cycles will appear upon the screen. If the sweep frequency is 5000 cycles, one cycle will appear upon the screen.

Now, it is important to understand that some form of pattern will exist upon the screen for all ratios between the sweep frequency and the frequency of the wave to be examined. However, the nature of these patterns is too complicated to be of any value and, as a general rule, the patterns are moving. To establish the waveshape of the voltage applied to the vertical deflection plates, it is necessary that the pattern appearing upon the screen be the simplest showing the wave shape. Such a pattern is established when the relation between the sweep frequency and the frequency of the voltage under observation, is a simple integral ratio - and not a fractional ratio. When this simple ratio exists, such as 2-1, 3-1, 4-1, 5-1, etc. and the lower frequency is that of the sweep circuit, the maze of lines moving across the screen will develop into a simple waveform. Whether or not this wave will be sine or distorted, depends upon a number of factors, which need not be discussed at this time. At any rate, the pattern desired is that which has a single line waveform of one or more cycles.

The greater the ratio between the sweep frequency and the frequency of the voltage under observation, the greater the number of lines which appear in the pattern, during the time that a fractional ratio exists between the two frequencies. However, as the sweep frequency is changed and approaches that value which provides the simple integral ratio, the number of lines in the pattern becomes less and less, until a single line waveform pattern appears. This adjustment is reached by manipulating the fine frequency control.

The Synchronizing Control

Before showing you examples of various patterns, we want to remind you of the synchronizing control, which is also available on the majority of cathode-ray oscillographs. This part of the instrument is the means whereby the image upon the screen is kept stationary for observation. What is accomplished, as a result of the synchronization adjustment, is that the sweep frequency is "locked in step" with the frequency of the signal being observed. This is done by feeding a small portion of the voltage to be observed, which has been applied to the vertical plates, to the input circuit of the timing axis or sweep voltage oscillator tube. These pulses will maintain the freauency of the sweep oscillator at the setting made to develop the proper image. As stated before, one of the properties of a relaxation oscillator, such as is used to produce the timing axis, is to keep step with a frequency which is applied to the input circuit of the oscillator. The sweep oscillator will keep step with this synchronizing pulse, be the pulse a sub-multiple or a multiple of the frequency of the sweep oscillator. This locks the image and it stands stationary upon the screen.

Hence, after the preliminary adjustment of the sweep frequency to produce the required pattern, the synchronizing control is manipulated so as to keep the image stationary upon the screen. For operation of the type being described, the synchronizing selector switch would be set to the "internal" position. Just what is meant by this internal position can be gleaned by examination of the RCA, National Union or Clough-Brengle oscillograph circuits. In the first unit you will find that a portion of the signal voltage, developed across the vertical deflection plate amplifier, is fed to the synchronizing circuit of the sweep oscillator. In the case of the other two units, you will find that a portion of the signal voltage fed to the free vertical deflection plate is also fed to the synchronizing circuit of the sweep oscillator. At times, a slight readjustment of the fine frequency control is required after the synchronizing control is manipulated. This is so because of the change in frequency of oscillations created in the sweep circuit, as a result of the application of the synchronizing pulse. The greater the amount of synchronization voltage, as determined by the setting of the control, which usually is a potentiometer, the greater the effect upon the frequency of the sweep circuit. At times, as shall be shown, the degree of synchronization also influences the shape of the pattern.

It is necessary that you understand the operation of the synchronizing adjustment, otherwise you will find it extremely difficult to produce a stationary pattern upon the screen. The frequency adjustment of the sweep circuit, when correct, will stop the image, but the image will not stand stationary, because of slight drift in the source of the voltage being observed and in the relaxation oscillator. But when the sweep has been synchronized with the voltage under observation, the pattern will remain stationary despite slight changes in frequency of the voltage being observed, because these changes in frequency, being transmitted to the relaxation oscillator tube, also tend to change slightly the relaxation oscillation, and thus keep the sweep circuit in step with the frequency being observed.

Let us now assume the application of a 700-cycle sine wave voltage to the vertical plates. This figure is mentioned because it happened to be the frequency used to make the photographs which follow. With the synchronization control advanced somewhat beyond its minimum setting and the rough adjustment of the linear sweep frequency set to around 700 and then followed by the manipulation of the fine adjustment until a single line pattern appears, we develop an image such as

Fig. 67. The sine-wave voltage impressed on the vertical plates and the sweep voltage are both 700 cycles, resulting in the appearance of a single cycle on the screen. In A and B, the sweep amplitude is insufficient to give a true image; whereas in C the wave is spread out enough to present a substantially undistorted image.

Fig. 67. The sine-wave voltage impressed on the vertical plates and the sweep voltage are both 700 cycles, resulting in the appearance of a single cycle on the screen. In A and B, the sweep amplitude is insufficient to give a true image; whereas in C the wave is spread out enough to present a substantially undistorted image.

in figure 67-A. This is a single cycle of the 700-cycle voltage applied to the vertical plates. The sweep and signal frequencies are the same. Obviously, it is not the most perfect looking wave. The image stands still. There are several reasons why a perfectly symmetrical wave is not upon the screen. One of these is the amplitude of the sweep. The second is that the sweep is not absolutely linear and tends to distort a single cycle. Also there exists too much synchronization; that is, the synchronization control is advanced too far. Also there is imperfect phase relation between sweep and signal. With this single cycle upon the screen, we can experiment with the action of the sweep amplitude We increase the sweep amplitude control. In the unit we were using, the sweep amplitude control was the horizontal deflection plate amplifier. The actual sweep voltage developed by the thyratron was fixed in amplitude. The amplitude variation was accomplished by the amplifier gain control. The amplitude is increased and the image shown in figure 67-A now looks like 67-B. The fact that the return trace is not in the middle of the pattern is due to the phase relation between the sweep and signal voltages. The phase can be altered by slowly adjusting the fine frequency control.

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