Introduction
1. The cathode ray tube (CRT) is an electron tube in which electrons emitted from a cathode are formed into a narrow beam, accelerated to high speed and directed to a screen or face coated with a fluorescent material. This material glows at the point of impact producing a visible dot. A changing field, either electrostatic or electromagnetic, between the source of electrons and the screen, causes the dot of light to move in accordance with the field variations.
2. There are two basic types of CRT, the electrostatic CRT and the electromagnetic CRT. In the electrostatic CRT, focusing and deflection are done with electronic fields while in the electromagnetic CRT, magnetic fields do these jobs.
The Electrostatic CRT (ESCRT)
3. Fig 12-1 shows the main parts of an electrostatic CRT. These parts can be divided into three groups:
(a) The electron gun which produces a stream of fast-moving electrons and focuses them into a narrow beam. Sometimes the electron gun refers only to the cathode and grid, the anodes are then called the focusing system.
(b) The deflecting plates which enable the beam of electrons to be moved up and down and from side to side.
(c) The fluorescent screen which shows the movement of the beam by producing a spot of light.
All the electrodes are enclosed in an evacuated glass envelope.
4. The cathode is a small tube, coated at the end with an oxide which emits electrons when heated. The grid is a hollow cylinder surrounding the cathode, with a central hole through which the electrons pass. The grid is made negative with respect to the cathode and by varying this voltage the number of electrons in the beam, and so the brilliance of the spot of light on the CRT screen, is varied. The brilliance control thus alters the voltage on the grid of the CRT. If the grid is made
sufficiently negative the electron beam will be completely cut off and the spot on the screen will be blanked out.
5. The first and third anodes are circular plates with holes through their centres. They are held at a high positive voltage relative to the cathode (several hundred or even thousand volts), and so they accelerate the electrons to a high speed. The third anode voltage is higher than the first anode voltage.
6. The second anode is a hollow cylinder mounted between the first and third anodes. Its purpose is to focus the electrons into a narrow beam and for this reason it is made negative with respect to the other anodes, This voltage can be varied to adjust the focusing of the beam, hence the focus control varies the voltage on the second anode.
7. In practice the third anode is earthed and the other electrodes are made negative with respect to it. Typical EHT (extra high tension) voltages used in CRTs are: cathode -4 kV, grid -4.02 kV (variable), first anode -2 kV and second anode -3 kV (variable). For third anode and screen, EHT voltages are needed in order to give the electrons enough speed to produce light on the fluorescent screen.
8. Two sets of deflecting plates are mounted after the third anode as shown in Fig 12-1.
By applying varying voltages to these plates the focused beam can be swung in any direction.
The plates nearer the third anode (Y plates) are used to move the beam vertically and those nearer the screen (X plates) deflect the beam horizontally. The plates are often flared at the ends to provide the required amount of deflection without fouling the beam. When a voltage is applied to the plates the electron beam will be attracted or repelled, depending on the polarity.
Fig 12-1: The Electrostatic CRT (ESCRT)
9. For horizontal deflection a sawtooth voltage is applied to the X plates. This voltage increases from minimum to maximum values at a uniform rate (the sweep) and then returns rapidly to minimum (the fly-back). The wave-form is repetitive and causes the beam to move from the left-hand to the right-hand side of the screen and then to return quickly to the left-hand side to start another sweep.
The fly-back may be ‘blanked out’ by applying either a negative pulse to the grid or a positive pulse to the cathode during this period.
10. This type of wave-form applied to the X plates produces a horizontal line is on the screen.
When a voltage pulse is applied to the Y plates the beam is vertically deflected for the duration of the pulse and a “blip” appears on the screen.
11. The fluorescent screen is a chemical coating on the inside end of the glass tube. When fast-moving electrons hit the screen they cause it to glow with a colour which depends upon the type of chemical used. The spot of light remains for a time after the electron beam has moved away, this effect is after-glow and enables a complete steady picture to be seen.
12. When electrons hit the screen, secondary electrons are emitted. These are conducted via a powdered graphite coating, called the aquadag, to the third anode (earth). The coating prevents the screen becoming negatively charged.
13. In some types of CRT an extra anode is used. It is made of a conducting ring of powdered graphite held at about twice the voltage of the third anode with respect to the cathode. This enables small input signal voltages to produce large displays on the screen, i.e. the deflection sensitivity of the
tube is increased (Fig 12-2). Fig 12-2: Post-Deflection Acceleration
The Electromagnetic CRT (EMCRT)
14. The construction of an electromagnetic CRT, employing magnetic focusing and deflections is shown in Fig 12-3. The heater, cathode and grid are the same as for an electrostatic CRT, but there is usually only one anode. This may be the aquadag coating, connected to a suitable voltage.
Deflecting and focusing currents are passed through coils mounted on the outside of the neck of the tube. The electron beam is acted upon by a sideways force as it passes through the magnetic field around the coil, in much the same way as a current-carrying conductor has a force exerted
on it in an electric motor. Fig 12-3: The Electromagnetic CRT (EMCRT)
15. Focusing is done by passing DC through a specially shaped coil. The position of the coil and amount of current control the focusing of the beam. The focus control is a rheostat which varies the current through the coil.
16. Deflection is produced by two pairs of coils mounted at 90° to each other round the neck of the tube. In Fig 12-3, only one pair is shown for clarity. The amount of deflection depends on the strength of current in the coils. If the current is reversed, the direction of deflection reverses. To produce a horizontal time base a sawtooth wave-form of current must be passed through the horizontal deflection coils. Vertical deflection can be obtained by passing a signal current through the vertical deflection coils but it is more usual to show signals on an electromagnetic CRT by using intensity modulation. In this case a positive-going signal voltage is applied to the grid of the CRT.
The number of electrons in the beam increases for the duration of the signal and a bright spot appears on the time base.
The ESCRT and EMCRT Combined
17. Combinations of the two CRT systems may be used, the most popular being that using electrostatic focusing and electromagnetic deflection. There are various advantages and disadvantages of the two types, and until recently, the greatest advantage of the electromagnetic tube has been its higher resolution, i.e. ability to reduce the spot size to very small proportions. However, recent research has improved the resolution of the electrostatic CRT, although it has increased the complexity of the tube and necessitated the use of low power magnetic correcting coils. Present day electromagnetic tubes have spot sizes of about 0.25 mm.
CRT Displays
18. Time Base Production. All CRT displays need some form of time base and a large number of displays use the A-scope time base. With this type the spot moves linearly across the face of the screen, usually from left to right, then returns rapidly to its starting point. During the rapid return, known as flyback, the spot of light is normally extinguished by application of a suitable blackout wave-form to the grid of the tube. The ESCRT uses a voltage wave-wave-form whilst the EMCRT uses a current wave-form, as follows:
(a) The ESCRT Wave-form. Fig 12-4 shows the voltage wave-form required to produce a type-A time base.
The frequency of the wave-form is equal to the pulse recurrence frequency of the radar. A circular time base, rotating with
angular velocity w can be obtained by Fig 12-4: A-Scope Wave-Form
applying voltages proportional to sin wt and cosine wt to the X and Y plates respectively.
(b) The EMCRT Wave-form. As the coils of the EMCRT are subject to inductance, current modulations must be used to produce the sawtooth wave-form. A PPI display can be produced by physically rotating the coils.
19. Calibration Markers. For accurate measurement of range the time base of a CRT must be set up against some accurate standard. This is done by a calibrator unit which is synchronized with the transmitter pulse. The calibration markers appear as pips or bright rings.
20. Types of Display. The CRT in a radar receiver can be used to present one, two or three-dimensional information concerning the target. If only one-three-dimensional information is presented the trace is deflection-modulated, the spot being deflected from its normal path to indicate the presence of an echo signal from a target. Examples of this type of display are the A-scope, I-scope and J-scope.
For two and three-dimensional displays the trace is intensity-modulated, the brilliance of the trace being varied by an echo signal from the target. Examples of two-dimensional displays are the PPI, B-scope, C-B-scope, E-scope and the RHI-scope. Three-dimensional information is displayed in a variety of ways. The H-scope repeats the target echo to indicate target elevation in addition to azimuth and range. The I-scope shows target range, displacement from the axis and angle-off error. The C-scope shows elevation and azimuth error and superimposes a range indication on the target echo. In all types of display the spot moves in some pre-determined manner on the screen, this movement being termed the time base.
21. Strobes. In some radar equipments it is necessary to expand a section of the main time base on either side of the echo blip in order to measure the range more accurately. To do this a strobe pulse is generated at some definite time after the start of the main time base, the delay being controlled by the operator. The strobe pulse is made to trigger a strobe time base circuit, with its own calibration on a larger scale.
Image Generation
22. We saw that when an electron gun produces a beam of electrons which are accelerated along the tube by an extra-high tension (EHT) voltage and when they strike the phosphor coating of the screen at very high velocity, the phosphor glows at a specific frequency or frequency range (colour), and for a specific length of time. We also saw that the electron beam can be focused or deflected, to move the spot of light around the screen, and its intensity can be altered to control brightness in order to form an image.
23. In most airborne applications a short tube length is desirable so that the device can be more easily accommodated. However, to achieve this it is necessary to provide a higher deflection power,
and to accept a lower resolution, as well as a greater difference between the resolution at the centre and edges of the display. Extra brightness, which may be needed in a cockpit, also carries the penalty of lower resolution, and a slower writing speed.
24. Image Quality. The total image in a CRT display is built up by the rapid movement of the spot of light traced by the electron beam, and the eye's integration of the image. There are two methods by which a total image is produced i.e. the raster scan technique which is that used in domestic television, and the cursive technique which is that used in, for example, oscilloscopes. Each of these techniques will be described in more detail later. The size of the spot is normally fixed by the focusing and will be dictated by a number of design factors, however, the resolution of the final image can never be finer than the spot size. The persistence of the spot is a measure of its decay time and is defined by the length of time it takes for the brightness to fall to 10% of its peak brightness following the electron beam's cessation or movement. The persistence of the phosphor is utilized together with the eye's persistence of vision to build up a complete image from what is in reality a moving spot and the correct choice of persistence is therefore very important. A phosphor's persistence is described as the time taken for the brightness to decay to 10% of final brightness or the time taken for the brightness to reach 90% of steady state brightness. If this time is + 1 sec then it is called very long persistence while if it is 1 to 100 ms it is called medium and if its 1 to 10 ms it is called short and for <
1 ms, it is called very short persistence. The colour of the spot is determined by the choice of phosphor and some examples are shown in Table 12-1.
Emission Colour Phosphor Fluorescence
(Initial Glow)
Phosphorescence (After Glow)
Persistence Applications
P1 Yellow-green Yellow-green Medium Oscillographs, Radars, HUD
P4 White White Medium to Med-short Monochrome TVs
P12 Orange Orange Long Radars
P43 Green Green Medium Rare Earth Phosphor
used for HUDs
Table 12-1: Characteristics of Typical Phosphors
Raster Scan
25. In a raster scan system the spot is moved over the whole area of the screen in a regular pattern, normally a series of parallel horizontal lines. The image is built up by varying the brightness of the spot in synchronization with the raster. The whole process is repeated at the refresh rate to give an apparently continuous and dynamic image, and the effect is enhanced by the choice of a phosphor with an appropriate persistence. A commonly used variation, known as interlacing, involves producing two or more images in rapid succession by interleaving the horizontal lines of the scan.
The effect is to reduce the true refresh rate without flicker becoming apparent. Each separate picture is known as a field and the total picture, i.e. the sum of the individual fields, is called a frame.
26. The vertical resolution of a raster scan system is set by the number of lines used. Thus for example, in a domestic television with 625 lines, of which 585 are used for imaging, the smallest theoretically resolvable detail is equal to the line width which is 1/585 of the picture height. In practice this figure is degraded by other factors.
27. The way in which a raster scan forms an image is shown in Fig 12-5. Fig 12-5a shows the display which consists of a black and a white letter 'T' on a grey background. For simplicity the scan has only 12 lines of which 10 are used for imaging, and there is no interlacing. Fig 12-5b shows the line time base which has a sawtooth form and represents the 'x' deflection of the spot. The 'x' value increases within each line causing the spot to move across the screen. At the end of each line the spot returns to the left hand end (minimum 'x') as represented by the vertical part of the waveform.
During this 'flyback' the brightness of the spot is normally reduced to zero. Fig 12-5c shows the 'y' deflection of the spot as it moves down the screen. It will be seen that there is 'y' movement coincident with 'x' movement thus accounting for the slope of the lines evident in Fig 12-5a. When the spot reaches the end of any line the accrued 'y' movement ensures that after the flyback the spot is at the start of the next line down. After a time equivalent to scanning the ten lines, the spot returns to the top of the display. This frame flyback coincides with the time occupied by the non-imaging lines 11 and 12. Finally, Fig 12-5d shows the variation of the video signal, i.e. the variation in brightness of the spot from black through grey to white with time. It will be seen that during the period of line 1 the video signal shows grey throughout. During line 2 the video starts at grey, moves to black to begin
Fig 12-5: Formation of Raster Scan Image
forming the top of the black letter 'T', back to grey to form the space between the letters, then to white to start to form the white letter 'T', and finally back to grey at the end of the letter. Similar variations can be detected within the other lines. In addition to the image forming waveforms there will be additional pulses, not shown in Fig 12-5, to ensure correct synchronization of the time bases.
Cursive Writing
Fig 12-6: Example of Production of a Simple Cursive Image
28. Although the term cursive writing may be used to describe any non-raster imaging technique, it is used here to describe the way in which the electron beam is used to draw line symbology, in a similar way to a pencil drawing on paper. Because of the limited persistence of the phosphors any line has to be re-drawn at regular intervals (the refresh rate). The spot position is set by 'x' and 'y' signals and the intensity by a 'z' signal, which in most applications is either on or off. Because only the specific symbol required is drawn, the display can be much brighter than with a raster scan, in which all of the screen must be covered within each refresh period. The disadvantages of the cursive technique are that three signals are
required to produce the image, and it is in practice impossible to produce realistic dynamic images with shades of grey. The production of symbology can often be simplified by storing specific images such as lines and circles in computer memory and recalling and positioning them as required. The way in which a cursive system can be made to form a letter 'R' by simultaneously varying the 'x', 'y' and 'z' parameters is shown in Fig 12-6.
Colour Displays
29. Although a monochromatic display is quite suitable for many airborne applications, there are some uses for which a multi-colour display would be desirable or necessary. There are three practical colour tubes available, the shadowmask CRT, the beam index CRT and the penetration CRT.
30. Shadow-Mask CRT. The shadowmask tube (in which a mask behind the screen carries the colour apertures through which 3 electron beams must be aimed to build up a chromatic image) is the type used in most domestic television receivers. Although it has airborne applications it is sensitive to
30. Shadow-Mask CRT. The shadowmask tube (in which a mask behind the screen carries the colour apertures through which 3 electron beams must be aimed to build up a chromatic image) is the type used in most domestic television receivers. Although it has airborne applications it is sensitive to