Disclosure to Promote the Right To Information

Transcription

1 इ टरन ट म नक Disclosure to Promote the Right To Information Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public. ज न1 क अ+धक र, ज 1 क अ+धक र Mazdoor Kisan Shakti Sangathan The Right to Information, The Right to Live प0र 1 क छ ड न' 5 तरफ Jawaharlal Nehru Step Out From the Old to the New IS 5627 (1987): Methods of measurements of radar and oscilloscope on cathode-ray display tubes [LITD 4: Electron Tubes and Display Devices]! न $ एक न' भ रत क +नम-ण Satyanarayan Gangaram Pitroda Invent a New India Using Knowledge! न एक ऐस खज न > ज कभ च0र य नहB ज सकत ह ह Bhartṛhari Nītiśatakam Knowledge is such a treasure which cannot be stolen

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4 is : Indian Standard METHODSOFMEASUREMENTOFRADARAND OSCILLOSCOPECATHODE-RAYTUBES ( First Revision ) UDC : : Q Copyright 1988 BUREAU OF INDIAN STANDARDS MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG NEW DELHI Gr 6 October -1988

5 Indian Standard METHODSOFMEASUREMENTOFRADARAND OSCILLOSCOPE CATHODE-RAY TUBES ( First Revision ) 0. FORE 0.1 This Indian Standard ( First Revision j was adopted by the Bureau of Indian Standards on 31 December 1987, after the draft finalized by the Electron Tubes Sectional Committee had been approved by the Electronics and Telecommunication Division Council. 0.2 The object of this standard is to lay down uniform and repeatable procedures for carrying out measurements on basic characteristics of cathode-ray tubes. 0.3 This standard was first published in This revision has been undertaken to update its requirements in the light of the latest IEC Publication. WORD 0.4 While preparing this standard, assistance has been derived from IEC Pub Measurements of the electrical properties of electronic tubes: Part 14 Methods of measurement of radar and oscilloscope cathode-ray tubes, issued by the International Electrotechnical Commission ( IEC ). 0.5 In reporting the results of measurements on cathode-ray tubes made in accordance with this standard, if the final value, observed or calculated, is to be rounded off, it shall be done in accordance with IS : *Rules for rounding off numerical values ( n4srd ). 1. SCOPE 1.1 This standard specifies the methods of measurement of radar and oscilloscope cathode-ray tubes. 2. TERMINOLOGY 2.1 For the purpose of this standard, the definitions given ii1 IS : 1885 ( Part 4/Set 4 )-1970* shall apply. 3. GENERAL MEASURING CONDITIONS 3.0 The measurements described in this standard are not all appropriate to every type of cathoderay tube. They should, therefore, be applied only when specifically stated. 3.1 General Precautions - General electron tube precautions specified in IS : t are applicable in addition to those given below Safety Precautions - Precautions should be taken to protect the operator from high voltage shock X-radiation and tube implosion Insulation - In testing cathode-ray tubes, the insulation and spacing shall be provided in the test equipment to prevent arcing and leakage *Electrotechnical vocabulary: Part 4 Electron tubes, Section 4 Cathode-ray tubes. TMethods of measurements for electron tubes - receiving and transmitting tubes (first revision ). that may change the operating conditions imposed on the tube or give false current readings. In general, cathode-ray tubes are operated so that those electrodes upon which the highest frequency signals are impressed are at or near earth potential. Oscilloscope tubes are generally operated with deflecting electrodes near earth potential Leakage currents and charges on external surface of the tube can produce spot shifts and distortion of the screen patterns. One method of reducing these effects is to test the tube with the screen near ground potential Precautions for Measurement of Optical Characteristics - It may be necessary to control the ambient temperature when making measurements of screen characteristics. Protective shielding against the effects of other sources of radiant energy should be provided. 3J.5 Magnetic Shielding - Stray magnetic fields through a cathode-ray tube during test can deflect and distort the beam sufficiently to give misleading test results. Fields through the tube near its screen will usually only shift the spot position, while fields through the electron-gun assembly can distort or even cut off the beam by decentering it in the focussing fields and the limiting apertures. It is, therefore, desirable to provide magnetic shielding ~for a cathode.ray tube 1

6 under test to reduce or eliminate the effect of undesired constant or varying magnetic fields. Shields are preferably made of a high permeability alloy and should be of such a design as to minimize the undesired magnetic fields Demagnetization of Parts - Tube parts made of ferromagnetic materials may become magnetized and produce anomalous effects. As a -general rule, therefore, tubes containing ferromagnetic materials shall be demagnetized prior to testing. 3.2 Pre-heating - Prior to measurement, the cathode of the tube should be preheated, at a suitable heater voltage, for a time sufficient to ensure that the cathode has reached its nominal operating temperature. 3.3 Precautions for Tubes for Use at High Beam Currents - To avoid risk of damage to the screen, the drive voltage required to attain a specified value of beam current should be measured with the spot deflected off the useful screen area or with the screen over-scanned. 3.4 Applied Voltages - The various voltages shall be applied in such a sequence as to prevent the tube damage. In applying the voltages, care should be taken to ensure that the maximum rated voltages between electrodes are not exceeded. -The control electrode voltage should always be of such a value as to prevent screen burning during testing Regulation - High voltage power supplies are often designed with poor regulation. If the regulation of the power supply used is such that any changes occur in the electrode voltages as a result of changes in electrode currents, the electrode voltages shall be readjusted to the desired values Filtering - Undesired alternating components in the source voltages result in errors of measurement. The magnitude of these errors depends upon the relative magnitude of alternating and direct components of the source voltage. 3.5 Focus - Improper focus adjustment of cathode-ray tubes may result in misleading data on such items as luminance and chromaticity. The focussing field should, therefore, be adjusted for the desired focus, depending upon the type of measurement being made. 3.6 External Magnetic Components - Many tubes require the use of one or more external magnetic components. These components include either permanent magnets or electromagnets or both. The strength of field of these magnets is an important part of their characteristics and may be altered by sharp blows, heat, or the presence of other strong magnetic fields. The number and type of external magnetic components used during testing shall be as specified for the particular tube type. The precise location and orientation of these external components with respect to the tube is of the greatest importance in obtaining reproducible test results. In general, the external components shall be located as requird by the specification for the tube type under test. NOTE - The location of external magnetic components can affect tube parameters such as focus, luminance, current distribution among electrodes, colour uniformity, convergence, raster geometry, etc. Diverse tests require different positions or field strengths of the components or both. The distribution and uniformity of the field produced by these components has an important effect on performance of the tube. In comparing the performance of tubes, it is important to know the complete characteristics of these external components. Some examples of external magnetic components are: focussing coils Dr focussing magnets, magnetic deflection yokes, ion-trap magnets, centering devices, beam-aligning magnets, convergence magnets, and colour-purity magnets. 3.7 Deflecting Electrode Potentials - The zero signal potential of the deflecting electrodes shall, unless otherwise specified, be equal to the potential of the electrode through which the electrons pass just before entering the deflecting field. With tubes designed for balanced deflection, care shall be taken to ensure that the ~deflecting signals are balanced in order to minimize distortion. 3.8 Electrode Current - When making certain electrode current measurements, in particular, the zero-bias cathode current, care shall be taken to prevent burning of the screen or damage to limiting apertures as a result of excessive heating. The screen may usually be protected by using a full raster scan or defocussing the tube during measurement. 4. MEASUREMENT OF OPTICAL CHARAC- TERISTICS 4.1 Luminance Characteristics The luminous intensity should be measured on an optimum focussed raster of convenient size using a photoelectric device having an overall response approximating to the standard photometric observer. Prescribed voltage adjustments are made to attain the required luminous intensity and any prescribed electrode voltages or currents are measured. The average luminance of a raster is related to the luminous intensity by the formula: Z=LxA where Z = luminous intensity ( cd ), L = luminance (cd/ma ), and A = luminous area in m viewed by the photoelectric device. 2

7 4.1.2 Measurements of intensity of a particular colour omitted by the screen should be made using a suitable colour filter in addition to standard photometric observer. This filter/photocell combination is calibrated against a light source of known characteristic. The procedure given in is then applied using the new calibration. Colour filters and filter/photocell combinations may also be used for screen build-up and persistence measurements ( see 4.6 ). Full details of titers and filter/photocell combinations should be stated The luminous intensity or the beam currents a function of bias voltage should be measured by varying the bias voltage from cut-off to the value corresponding to the required luminous intensity or to the working beam current. 4.2 Stray Illumination - With the specified heater voltage applied and with all other voltages at zero, the luminance of screen due to light from the cathode assembly, is measured. Since this luminance will usually be at a very low level, the ambient illumination should be virtually zero. 4.3 Stray Emission The tube being measured should be placed in a given circuit, with stated voltages including a cut-off voltage and deflection voltage applied. The ambient illumination measured at the screen of the tube should not exceed 5 lx. The observer should have accommodated his eyes to the ambient illumination before viewing the screen of the tube It shall be noted whether any luminance is visible within a given time. 4.4 Flasharc Method A -The tube should be placed in a given circuit with stated voltages applied, The number of flasharcs should be observed on the screen of the tube during a given time Method B -The tube should be placed in a given circuit with stated voltages applied. This circuit should include a defined impedance in the cathode lead and a counting device suitable for counting the voltage pulses which develop over the cathode impedance as a result of the flasharc. The characteristics of the counting device ( input impedance, sensitivity and time discrimination between successive pulses ) should be stated The number of flashes should be counted during a given time. 4.5 Measurement of Cut-Off Voltage - The cut-off voltage should be measured at the threshold of visibility of an undeflected-focussed spot. The light intensity ( room illuminations) falling on the screen should be at a low level. Alternatively, the voltage should be measured for a stated low-beam current ( typically 0.1 PA ); allowance being made for leakage currents. 4.6 Measurement of Screen Build-Up and Persistence Method A -The tube is operated under stated conditions of electrode voltages. No deflection is applied. The beam current is pulsed at a stated repetition frequency, pulse length and amplitude. The period between pulses must be much longer than the persistence of the screen being measured unless special conditions apply for the measurement of build-up. The light output is received by a photomuhipber, the output of which is fed to a suitable recording instrument. For the measurement of tubes intended for visual applications, the characteristics of response of the photomultiplier and its associated apparatus should be adjusted by means of suitable filters to correspond to that of standard photometric observer. When tubes are intended for other applications, for example, for use in photographic applications, other filters may be used. The recording instrument must have a response time sufficienlty short to reproduce faithfully the build-up and persistence being measured. For the measurement of buildup and for short persistence screens, an oscilloscope is suitable: for very long persistence, it may be sufficient fo use a light-meter and a stopwatch Method B - The tube is operated under stated conditions of electrode voltages and beam current. The spot is scanned by means of a suitable deflecting system at a known sweep speed along a line. Flyback suppression should be used if the conditions of flyback are likely to cause excitation of the screen. A mask with a slit of known width is placed in front of the screen so that the slit is at right angles to the scanned line, and only a small portion of the line is visible. The width of the slit and the sweep speed must be such that the time taken for the spot to traverse the slit is short compared with the persistence being measured. The light emitted by this small portion of the screen is received by a photomultiplier and fed to a suitable recording instrument as described in Method A. The time of excitation of the screen can be calculated from the knowledge of the spot diameter and sweep speed. The spot diameter must be measured under the same operating conditions ( see 4.7 ) Method C- This method may be used for tubes having a long persistence and where light output is low. The tube is operated under stated conditions of electrode voltages, beam current and ambient temperature. Using a suitable deflecting system, the beam is formed 3

8 into a raster. The held time for the raster must b,: short compared with the persistence being measured. The screen is then excited for a stated number of fields and the light output from the whole area is received by a photomultiplier, the output of which is recorded as described in Method A. It should be noted that the excitation time is not the field scan time, but is the spot diameter divided by the sweep speed. 4.7 Measurement of Focus Quality - The focus quality may be determined by a measurement of the width of a line using one of the following methods. To prevent screen burning, it may be necessary to pulse the grid positively from cut-off voltage with pulses of specified duration and repetition frequency. Tubes utilizing magnetic focus and/or deflection should be measured in a defined focus/deflection unit Methods of Examination by Microscope i Expanded raster - A raster formed by a stated number of lines at a given field frequency is applied about the centre of the screen, and the grid voltage is adjusted to obtain the stated light intensity or beam current. The length of the line should be stated and kept constant. The pattern is then expanded to make the line structure clearly visible and to include the required positions of measurement. The focus should be adjusted to the optimum at the centre of the raster, and the line width is measured by a microscope as stated in 4.7. This procedure is repeated, without adjustment of focus, with the scanning voltages interchanged and the raster size adjusted to give the same line lengths. For electrostatic deflection tubes, symmetrical deflection voltages should be used Elliptical or circular trace - An elliptical or circular trace having axes of stated lengths and frequency is used, and the grid voltage is adjusted to attain a stated light intensity or beam current. The focus is adjusted to optimum and the width of the trace measured at the point of poorest definition Pulsed line - A line of prescribed repetition frequency and length is used and the grid voltage is adjusted to a value equivalent to that for the required light intensity or beam current on a raster. The grid may be pulsed positively from cut-off with suitable pulses at a given repetition frequency to obtain the equivalent peak-beam current or light intensity conditions. The focus should be adjusted to optimum and the line width measured at the centre of the trace Shrinking Raster Method - The tube is operated under stated conditions which include a linear scanning raster whose frequencies in both directions are stated. The focus should be adjusted to optimum. The amplitude of the pattern is first increased until the line structure is clearly visible, and then reduced to the condition in which the edges of adjacent lines have merged and the display is of uniform luminance. The reduced dimension of the pattern is then measured and divided by the number of lines in the display. The quotient is a measure of the line width. It will be noted that the defurition of line width for the purpose of the above measurement is significantly different from that for a measurement of the width of a single line or spot. The relationship between the two types of measurement depends on the energy distribution within the line or spot but it has been found that in many cases, the line width as obtained by the shrinking raster method is approximately one-half of that obtained by measurement of a single line or spot. Whilst this method is simple and requires a minimum of additional equipment, the accuracy of the measurement depends on the linearity of the field scan Narrow Slit Method - With the tube operated under stated conditions of electrode voltages and beam current, the beam is scanned into a line. Light from this line is focussed by a suitable optical system to produce a magnified real image in some known plane. A narrow slit is mounted in this plane. The width of the slit must be small compared with the width of the image of the line being measured. It is important also that the image and the slit are accurately parrallel to each other. The light passing through the slit is then focussed on to the photocathode of a photomultiplier which is coupled to a suitable recording instrument. By measuring the light intensity at several points across the width of the line, a distribution curve may be plotted; from a knowledge of the magnification of the optical system, the line width may be determined. Measurements may be carried out by: a) moving the slit across the image of the line, OP b) moving the scanned line on the tube in such a way that the image moves across the slit. This method produces information on the complete light intensity distribution across the width of the line. When this information is available, any arbitrary definition for line width (for example, that it is limited to 20 percent of peak intensity ) may be applied. 4

9 4.8 Determination of Display - Tube Resolution by the Spatial Frequency Method The following principles of measurement may be applied to all types of display tubes but the exact techniques used vary according to the persistence of the sensitive layer and the magnitude ( high or low value ) of beam current flowing. The theoretical details arc covered in Appendix A Measurement Principle -A spot or tract, as relevant, is displayed on the tube and is imaged through a microscope objective on to a grating such as that shown in Fig. 1. The opaque and transparent strips have dimensions so as to provide, in conjunction with the microscope objective magnification, the required spatial test frequency. The direction of relative movement of the spot and the grating is across the alternating strips, as indicated in Fig. 1. The transmitted light is collected by a photomultiplier having a defined spectral response, through a suitable field lens which is focuses the aperture of the microscope objective on to the photomultiplier cathode. The output from the photomultiplier is fed directly to a display device (for example, oscilloscope or pen recorder ) whose time base is synchronized to the speed of the relative movement. The indicated amplitude on the display device, which is a m:asure of the transmitted light through the grating, is measured for both the single large spacing and the series of close spacings. The amplituda measured for the close spacing? is th?n com?arzd to that obtained for the single large spacing, which rzpres=nts the amplitude obtainabl.: at z?ro frequency. The ratio of the two amplitud:s is expressed a s a p:rc-stage amplitude for th: defined spatial frequency Procedure - The measurement of tubes using magnetic focus and/or deflection should be mlqde using a defined focus/deflection coil unit. Thl: tube is operated under given conditions of electrode voltages and beam current in a system arrangement as given in Fig. 2. The method of measurement chosen should be in accordance with the persistence of the sensitive layer on the screen and with -the magnitude of the beam current; the significance of beam current will dzp:,.d upon the ease with which the sensitive layer burns in use. The following methods take care of these factors: a) Method A (for tubes havirzg a low-beam curretzt and nzedium to short persistence sensitive layers ) - The beam is scanned across the tube screen, at a low frequency, to produce a short line. The speed is made sufficiently low to ensure that the energy in the sensitive layer is dissipated before the subsequent line is traced. The direction of the scan _is across the strips of the grating ( set Fig. I ). PRECAUTION - Cu order to minimizs the phosphor screen noise effects, a small amount of high-frequency spot deflection may be introduced at right angles to the line of scan. b) Method B (for tube having a low-beam current aiztl long persistence sefzsitive layers ) - A stationary spot is displayed on the tube screen and the grating is moved across the image of the spot in a direction perpendicular to the strips of the grating. The means of moving the grating should be free from vibration and mechanical noise. PRECAUTION - Because of the ease with which the longer persistence phosphors, particularly flourides, may be burnt, it is recommended that the small amount of high-frequency spot deflection be introduced in a direction parallel to the strips of the grating. This deflection reduces the local loading of the phosphor and also minimizes the phosphor screen noise effects. DIRECTION b OF SCAN FIG. 1 TYPICAL GRATWG ( NOT TO SCALE ) 5

10 MiCROSCOPE OBJECTIVE PHOTOMULTIPLIER MEAN AMPLITUDE AT TEST FREQUENCY REFSRENCE AMPLITL~~ AT LOW FREQUENCY OSClLLOSCOPE Fan. 2 MEASURISENT OF SPATIAL FREQUENCY RESPONSE -c) Method C (for tubes having a high-beam current) - When the mean current density at any point on the tube screen is sufficiently high to cause burning of the phosphor within the time taken to complete a measurement, the measurement procedure must be modified to reduce the mean current to a safe level. Either of the two following methods may be applied individually, or both simultaneously: 1) A repetitives scan is applied to the spot in a direction at right angles to the direction of movement of the grating. The amplitude of this scan should be sufficient to reduce the mean current density at any point of the screen to a safe level. The scan rate should approximate to that used in normal operation. 2) The spot is pulsed between full-brightness and cut-off levels by means of an appropriate signal applied between grid and cathode. The on-to-off ratio of the modulation signal should be sufficiently low to reduce the mean current density at any point of -the screen to a safe level, and the transition time between the two levels should be short compared to the on time. The relative movement of trace image and grating must be such that an adequate number of pulses ( for example, 20 ) occur within one cycle of the spatial frequency of the grating. 3) In extreme cases, these two methods may be combined: a repetitive scan is applied as in (1) with the tube held beyond cut-off and the trace is periodically pulsed as in (2) to full brightness for the duration of one complete scan. The repetition rate of the brightening pulse will be dependent upon the required degree of reduction in the mean current density at any point of the screen. Again, at least 20 pulses are required within one cycle of the spatial frequency of the grating Measuring Equipment Requirements - The microscope objective, grating, field lens and photomultiplier are housed in a light-proof container which is mounted so as to permit movement relative to the face of the cathode-ray tube being measured. The distances between the microscope objective, the tube and the grating must be adjustable to allow fineadjustmentof the magnification during calibration. The field lens is fitted immediately behind the grating ( see Fig. 2 ) and the photomultiplier cathode is positioned preferably at approximately the same distance from the field lens as the microscope objective %rating - The grating pattern shown in Fig. 1 contains one cycle at a very low frequency, and the remainder at a defined single high measuring frequency. The dimensions of the spacings, together with the appropriate objective magnification, provide the required spatial frequency. For example, where the -width of the spacing is 0,016 7 cm with an objective magnification of five times, the square-wave spatial frequency of the measurement is 150 cycles per centimetre. 0,016 7x2 + 5 The grating may be obtained by photographic reduction from a master. To maintain the accuracy of the master on the final grating, great care shall be taken during the photographic process Photomultiplier - The photomultiplier may be any suitable tube but a convenient type is one having an end window. The spectral response and characteristics should be appropriate to the measurement. 6

11 4.8.6 Oscilloscope - The deflection sensitivity should be of that value which gives an adequate deflection on a display tube having a persistence which provides ease of amplitude measurement. It may be convenient to use the oscilloscope time base saw-tooth waveform to provide an input to a scan amplifier for the purpose of deflecting the spot on the cathode-ray tube being measured Calibration Method A -Calibration of the system may be carried out by removing the photomultiplier and placing a suitable lamp in the plane of the photocathode. The projected image of the grating is observed through a microscope assembly fitted with a calibrated stage micrometer. The distance of the microscope objective from the grating is adjusted until the appropriate number of black and white areas occupy a known length on the stage micrometer. The position of the objective is then fixed and the photomultiplier replaced Method B - An optical beam splitter and viewing microscope are placed in the light path between the grating and the photomultiplier, permitting an observer to view the grating ( as shown in Fig. 3 ). When the viewing microscope has been focussed on the grating, it is locked in position and a stage micrometer is placed in the object plane in place of the tube being measured. The positions of the objective of the measuring system and the stage micrometer are then adjusted so that the aerial image of the stage micrometer coincides with the grating and both may be viewed simultaneously through the microscope. The viewing microscope may also be used to aid the setting-up of the electrical focus of the tube being measured, the focussing of the trace image at the plane of the grating and the precise alignment of the trace image with the grating strips. The grating should be enclosed in a lightproof container. 5. i%!easuremen L OF DEFLECTION AND SPOT DISPLACEMENT 5.1 Deflection Sensitivity and Coefficient Defection Sensitivity (5 ) - A symmetrical ( or asymmetrical, if specified ) deflection covering 75 percent of the relevant useful screen dimension is applied to each axis successively. The quotient of the deflection, in millimetres, and the instantaneous deflection voltage should be measured for each axis Deflection CoeJicient - This is the reciprocal of the deflection sensitivity. A symmetrical ( or asymmetrical, if specified ) deflection covering 75 percent of the relevant useful screen dimension is applied to each axis successively. The quotient of the deflection voltage and the corresponding deflection, in millimetres, should be measured for each axis. 5.2 Deflection Uniformity Factor - Electrostatic ~Deflection ( P ) - Using the method of measurement described in 5.1.1, the deflection sensitivity S is measured for two specified points of deflection along each axis. The results S, and S, are usually expressed in terms of millimetres per volt and the deflection uniformity factor (F) for each axis, expressed as a percentage, is determined from the formula: J (x) zz _?!$- x Since the deflection sensitivtiy varies with the amount of deflection, the complete determination of deflection -uniformity factor can only be obtained from a curve of the deflection uniformity factor as a function of the amount of deflection, measured from the screen centre, if not otherwise specified. 5.3 Deflection Distortion - Electrostatic Deflection Pattern Distortion - With the required screen area scanned by symmetrical ( or asymmetrical, if specified deflection voltages, the edges SPLITTER FIG. 3 SYSTEM ARRANGEMENT FORCALIBRATION (METHOD B) 7

12 IS : of the raster should lie between concentric tangles of stated dimensions. rec- 54 Mechanical Spot Displacement - The spot position relative to a stated reference point on the screen is measured without any deflection field, by eliminating any effects caused by external electrostatic or magnetic fields or taking them into account. For ekctrical deflection tubes, the deflecting plates shall be directly connected to the relevant electrode. For electrostatically focussed tubes, the spot should be adjusted to optimum focus. For the magnetically focussed tubes, no focussing field should be present, and the bias voltage shall be ad_iusted in order not to damage the screen. 5.5 Electrical Spot Displacement Leakage E cts - With the spot adjusted to optimum focus and each deflecting electrode connected to the appropriate electrode, the deflection of the spot, caused by the insertion of a stated resistance in series with each deflection electrode in turn, is measured. The bias voltage must be adjusted not to damage the screen. in order Beam Current Efsects - With stated resistors in all the deflecting electrode circuits and the deflecting electrodes connected symmetrically the spot, line or raster displacement should be measured when the grid voltage is changed from cut-off to the voltage required to obtain the stated light intensity. When necessary, the grid may be pulsed to prevent damage to the screen. 6. MEASUREMENT OF INTER-ELECI RODE CAPACITANCE 6.1 Provision of 4.10 of IS : * shall apply. *Methods of measurements for electron tubes -- receiving and transmitting tubes (first revision ). APPENDIX A ( Clause ) THEORY FOR SPATIAL FREQUENCY METHOD A-l. A cathode-ray tube reproduces the desired pattern only if the pattern is of a coarser nature than the illuminating spot which is tracing it. If the pattern is of a fine structure, the trace contrast for a given modulation tends to reduce and in the extreme pattern will, be lost. A-2. If sufficiently small sinusoidal signals are applied to a tube to produce intensity modulation, the characteristic between trace intensity and the amplitude of the applied modulation signal is predominantly linear. Therefore, the degree of resolution of a tube may be obtained by observation of the trace in wobbulation for a given input signal -against the spatial frequency of the pattern where the spatial frequency is defined as the number of sinusoidal cycles per unit length of the trace. Hence, a measurement of the spatial frequency response of a cathode-ray tube will provide a means of assessing the performance of the tube when used in intensity-modulated displays, as well as the more familiar spot size or line width measurement. A-3. For convenience, the spatial frequency response of a tube is expressed as a percentage of the response of the tube at low frequencies where the resolution is accepted as being the maximum. For ease of measurement, however, the spatial frequency response of the tube may be assessed by maintaining the trace intensity constant and applying an external standard pattern which simulates the black and white areas provided by a modulated trace. In this case, it should be noted that the use of such an external standard pattern provides a modulation waveform equivalent to that of a square wave. A-4. The two characteristics, which show the relationships (a) percentage modulation depth versus the spatial frequency response with sinewave modulation, and (b) percentage modulation depth versu.7 the spatial frequency response with square-wave modulation, are different. The difference should, therefore, be taken into account and this may be done using curves similar to those shown in Fig. 4, 5, 6 and 7. These curves indicate the relationship between the percentage modulation depth and the spatial frequency which is given in terms of that spatial frequency which provides a modulation depth of 60 percent, assuming that the spot intensity follows a Gaussian distribution. The 60 percent modulation depth level is taken in this particular example as being the acceptable level of measurement for the tube. NOTE - It is recommended that the 60 percent modulation depth be adopted as the standard reference level. 8

13 t 70 ; 60 E lx 50 ke ;40 0 F I 2.5 RATIONALIZED TO SPATIAL FREQUENCY (c/cm) PROVIDING 60% MODULATION DEPTH FIG. 4 COMPARISON OF MODULATION DEPTHS OBTAINED WITH SQUARE AND SINE-WAVE MODULATION Multiply sine-wave frequency byrratio to obtain square wave. f, Divide square-wave frequency by ratio to obtain sine-wave. I 0 I FO n SO SPATIAL FREQUENCY RATIO SQUARE I SINE FIG. 5 CURVE RELATING SQUARE/SINE-WAVE SPATIAL FREQUENCIES FOR DIFFEXENT MODULATION DEPTHS

14 80 t 70 E W 60 u 5 a 50 e 0 = LO ;I 4 30 I3 Example : Given a response of 48 % at 167 c/cm, find the spatial frequency at 60%. Look up ratio for 48% and divide the given frequency by the ratio. Frequency at 60% is : 167 c/cm 1.15 =_ 145 c/cm 0 I 2o FIG :2 1.,:6 1:s &i SQUARE-WAVE SPATIAL FREQUENCY RATIO 6 CURVE RELATING SQUARE-WAVE SPATIAL FREQUENCY AND MODULATION A-4.1 If it is assumed that the spot intensity follows a Gaussian distribution, the following formulae may be used: M _ e-2rr2f s2 ; foe ( sin) = -& where M = modulation depth at the spatial frequency f, fgo ( sin ) = sinusoidal spatial frequency at which modulation depth is 60 percent, and 2% = spot width at 60.6 percent height ( see Fig. 7 ). From _- Fig. 4 and 5, it can be seen that for a 60 percent modulation depth, the ratio of (a) the spatial frequency response with squarewave modulation to (b) the spatial frequency response with sinusoidal modulation is given by: where ho ( =I 1 = 1.21 ( line AA ) 1.fso ( sin f,, (sq) = square-wave spatial frequency at which modulation depth is 60 percent, and fco( sin ) = sine-wave spatial frequency at 60 percent modulation depth. Thus using the above formula, the relationship between spot size and square-wave response is given by : h-0 (sq) = -+q 71 A-4.2 Alternatively, the difference arising from the modulation waveforms may be expressed as a difference in percentage modulation depth at a given spatial frequency value. For this measurement, a trace is displayed on the cathode-ray tube and projected by a microscope objective on to the standard pattern which takes the form of a defined grating consisting of alternate opaque and transparent strips, that is, at right angles to the strip length, as shown in Fig. 1, the amount of light transmitted through the grating will depend on the speed of the scan and the relative sizes of the strip widths and the illuminating spot. For example, if the spot is larger than the strip widths, part of the light emitted will Abe masked off by the opaque sections when the spot centre is in a transparent strip, and fringe lighting will occur when the spot is immediately behind an opaque strip ( see Fig. 8 ). In addition, as the light emitted depends energy in the sensitive layer particles, too fast a scan will not provide sufficient energy to a particle to produce maximum intensity, and/or may not permit the energy from a previous scan to be dissipated. The effect of the speed of scan can be cancelled by a suitable choice of speed. Observation of the varying intensity of light transmitted through a grating, having strip widths equivalent to a desired pattern structure, will provide a measure of the resolution capabilities of the tube. 10

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