Circuit Concepts Circuit Concepts - :2 n CATHODE-RAY TUBES BY CHUCK DeVERE Significant Contributions by BOB ORWILER CIRCUIT CONCEPTS SECOND EDITION FOURTH PRINTING JULY 1969 062-0852-01 PRICE $1.00 © 1967, TEKTRONIX, INC. BEAVERTON, OREGON ALL RIGHTS RESERVED CONTENTS 1 INTRODUCTION 1 2 ELECTRON OPTICS 3 3 THE TRIODE SECTION 9 4 UNBLANKING 17 5 FOCUS AND ASTIGMATISM LENS 25 6 THE DEFLECTION SYSTEM 27 7 ACCELERATION SCHEMES 47 8 SPACE CHARGE REPULSION EFFECT AND TRACE WIDTH 55 9 PHOSPHORS 61 10 HUMAN EYE RESPONSE 63 11 LUMINANCE CHARACTERISTICS OF PHOSPHORS 67 12 SPECTRAL RESPONSE 71 13 WRITING SPEED 73 14 PHOSPHOR BURNING 77 15 ALUMINIZED TUBES 79 16 LIGHT FILTERS 81 17 GRATICULES 83 18 TRACE ALIGNMENT 87 19 DUAL BEAM 89 20 DUAL GUN 91 INDEX 95 CIRCUIT CONCEPTS BY INSTRUMENT 97 ... ... ~~i .-... VERTICAL .... .... AMPLIFIER '~~'·'·'·~~"'.I • .. .•: POWER SUPPLY .... ..... + ~Ir ~, .. ~l. TRIGGER .... GENSWEREAETPO R .......... . AHOMRPILZIOFNIETRA L 1 INTRODUCTION The cathode-ray tube (CRT) is the output or display section of an oscilloscope and any study of oscillography must surely include the CRT. When trouble-shooting a malfunctioning oscilloscope, an understanding of the inner workings of the CRT will aid in isolating the section of the oscilloscope causing the problem. The design of the circuitry that interfaces with the CRT is dependent upon the requirements of the CRT and before these circuits can be fully analyzed the requirements of the CRT must be known. The proper operation of the various controls and adjustments directly associated with the display requires an understanding of the probable effect upon the CRT. The specifications, limitations, and capability of the CRT must be considered when selecting an oscilloscope for a particular application. A CRT can be divided into five sections as shown in Fig. 1-1. The triode section furnishes a controllable source of electrons which the focus section forms into an electron beam. This beam is deflected vertically and horizontally in the deflection section and may be accelerated in the acceleration section. The beam strikes the phosphor-covered screen of the tube and light is produced. Each of these sections and their interrelationships will be covered in the following discussion. I I I I I I TRIODE FOCUS DEFLECTION ACCELERATION OR DRIFT REGION SCREEN I I 1 I I I I 1 1 1 I ::l rlr - II .-J- rilll I ~ LfL - --111 LJ I I I I I I I I 1 1 1 I Fig. 1-1. CRT sections. 2 TYPICAL CATHODE RAY TUBE ELECTRON GUN Fig. 1-2. CRT electron gun. The first three sections of a CRT are shown in Fig. 1-2. A review of electron optics is needed in order to fully understand the operation of these CRT sections. 3 ELECTRON OPTICS The electrons emitted by the heated cathode are acted upon by the electric field between the cathode, grid, and first anode. This field can be represented by lines showing where the potential of the field is constant. These lines are called equipotential equipotential lines and their effect on an electron lines passing through them depends on their shape, the voltage difference between, their spacing, the velocity of the electron, and whether the electron is traveling in the direction of increasing or decreasing potential. The CRT axis is used as a reference for describing the path of an electron. This axis is the center line of the tube or, in other words, the shortest s'traight line from the cathode to the face plate. An electron's velocity can be separated into two axial and components, axial velocity and radial velocity. radial Fig. 2-1 shows an electron with its velocity velocity separated into these two components. Both components are referenced to the CRT axis. The radial component is perpendicular to the CRT axis and the axial component is parallel to the axis. action of The action of an electrostatic electron lens is electron based on the fact that the force acting upon an lens electron traveling in the field is in a direction perpendicular or normal to the lines of equal potential and in the direction of increasing potential. RAD I Al COMPONENT OF VElOCITY~~-----------' : I elECTRON VElOC ITY VECTOR • I : I : I .J--------- ____ __ 1.~ ........ AX I Al COMPONENT OF VELOCITY Fig. 2-1. Velocity components. 4 IOV ZOV 30V 40V 50V CRT AXIS --------- -- -- - ------- -----1.~ ... ~ ... ~ ... ~ ... ~ ...~ ... ENTRANCE VELOC I TY FORCE EXIT VELOCITY VECTOR OF VECTOR OF ELECTRONS ELECTRON EQU I POTENT I AL LINES Fig. 2-2. Normal equipotential lines. Fig. 2-2 shows an electron passing through equipotential lines. The electron is traveling in the direction of increasing potential. The electron entrance velocity of the electron is normal to the velocity equipotential lines and the force acting on the electron is normal to the lines in the same direction. Therefore, the velocity vectors add and the axial velocity of the electron is increased. The electron has no radial velocity before entering the field and the field imparts none. If the field in Fig. 2-2 were reversed so that the decreasing electron was traveling in the direction of decreasing velocity potential, the force acting on the electron would still be normal to the equipotential lines but in the opposite direction than is shown. The exit velocity in such a case would be less than the entrance velocity. Notice that there is no change in the direction of the electron in Fig. 2-2, only the velocity of the electron is changed. This will be true for all fields where an electron passes through an equipotential line normal to the line. The electron's velocity will be affected but not its direction. Fig. 2-3 shows an electron passing through a field with equipotential lines parallel to the CRT axis. The.force acting on the electron is normal to the line and therefore perpendicular to the electron's radial entrance-velocity vector. These two vectors are component shown in Fig. 2-3 and the final velocity vector is shown. The axial velocity of the electron has not changed, but a radial component has been added.