A study on CRO
2016
Introduction
It stands for Cathode Ray Oscilloscope. Thecathode ray oscilloscopeis an extremely useful and versatile laboratory instrument used for studying wave shapes of alternating currents and voltages as well as for measurement of voltage, current, power and frequency, in fact, almost any quantity that involves amplitude and waveform. It allows the user to see the amplitude of electrical signals as a function of time on the screen. It is widely used for trouble shooting radio and TV receivers as well as laboratory work involving research and” design. It can also be employed for studying the wave shape of a signal with respect to amplitude distortion and deviation from the normal. In true sense the cathode ray oscilloscope has been one of the most important tools in the design and development of modern electronic circuits.
The instrument employs a cathode ray tube (CRT), which is the heart of the oscilloscope. It generates the electron beam, accelerates the beam to a high velocity, deflects the beam to create the image, and contains a phosphor screen where the electron beam eventually becomes visible. For accomplishing these tasks various electrical signals and voltages are required, which are provided by the power supply circuit of the oscilloscope. Low voltage supply is required for the heater of the electron gun for gen-eration of electron beam and high voltage, of the order of few thousand volts, is required for cathode ray tube to accelerate the beam. Normal voltage supply, say a few hundred volts, is required for other control circuits of the oscilloscope.
Horizontal and vertical deflection plates are fitted between electron gun and screen to deflect the beam according to input signal. Electron beam strikes the screen and creates a visible spot. This spot is deflected on the screen in horizontal direction (X-axis) with constant time dependent rate. This is accomplished by a time base circuit provided in the oscilloscope. The signal to be viewed is supplied to the vertical deflection plates through the vertical amplifier, which raises the potential of the input signal to a level that will provide usable deflection of the electron beam. Now electron beam deflects in two directions, horizontal on X-axis and vertical on Y-axis. A triggering circuit is provided for synchronizing two types of deflections so that horizontal deflection starts at the same point of the input vertical signal each time it sweeps.
A study on CRO
2016
A study on CRO
2016
Fig-2 Block Diagram of a CRO.
Components of a CRO
Cathode ray tube.
Deflection plates.
Phosphor coated screen.
Other circuitry.
A study on CRO
2016
Cathode Ray Tube
Cathode ray tube essentially consists of an electron gun for producing a stream of electrons, focusing and accelerating anodes for producing a narrow and sharply focused electron beam, horizontal and vertical deflection plates for controlling the beam path and an evacuated glass envelope with phosphorescent screen giving bright spot when struck by a high velocity electron beam.
Fig-2 Block Diagram of CRT
Electron Gun Assembly.
The electron gun assembly consists of an indirectly heated cathode, a control grid surrounding the cathode, a focusing anode and an accelerating anode. The sole function of the electron gun assembly is to provide a focused electron beam which is accelerated towards the phosphor screen. The cathode is a nickel cylinder coated with an oxide coating and emits plenty of electrons, when heated. The emitting surface of the cathode should be as small as possible, theoretically a point. Rate of emission of electrons or say the intensity of electron beam depends on the cathode current, which can be controlled by the control grid |n a manner similar to a conventional vacuum tube. The control grid is a metal cylinder covered at one end but with a small hole in the cover. The grid is kept at negative potential (variable) with respect to cathode and its function is to vary the electron emission and so the brilliancy of the spot on the phosphor screen. The hole in the grid is provided to allow passage for electrons through it and concentrate the beam of electrons along the axis of tube. Electron beam comes out from the control grid through a small hole in it and enters a pre-accelerating anode, which is a hollow cylinder in shape and is at a potential of few hundred volts more positive than the cathode so as to
accelerate the electron beam in the electric field. This accelerated beam would be scattered now because of variations in energy and would produce a broad ill-defined spot on the screen. This electron beam is focused on the screen by an electrostatic lens consisting of two more cylindrical
A study on CRO
2016
anodes called the focusing anode and accelerating anode apart from the pre-accelerating anode. The focusing and accelerating anodes may be open or close at both ends and if covered, holes must be provided in the anode cover for the passage of electrons. The function of these anodes is to concentrate and focus the beam on the screen and also to accelerate the speed ofelectrons.
Electrostatic Focusing System
Fig-3 Electrostatic Focusing System
An electrostatic focusing system is shown in figure. Electrostatic lens consists of three anodes, with the middle anode at a lower potential than the other two electrodes.
In figure two anodes and its electrostatic lines and equipotential surfaces are shown. A pd is kept between these two electrodes so that an electric field is generated between them. Spreading of electric field is caused because of repulsion between electric lines. If equipotential lines are drawn, as shown in figure, they would bulge at the center of the two anodes. As we know that electrons move in a direction opposite to that of electric field lines and equipotential surfaces are perpendicular to the electric field lines so force on the electron is exerted in the direction normal to the equipotential surface.
Electrons entering at the center line of the two anodes experience no force but electrons
displaced from the center line experience a force normal to the direction of equipotential surface and deflect, as shown in figure. An equipotential surface is shown, in which an electron with velocity V1 and at an angle θ. to the normal of equipotential surface enters and experiences a force in a direction normal to the equipotential surface. Thus the velocity of the electron increases to V2. This force on the electron is exerted in the direction normal to equipotential surface so only the normal component of electron velocity V1N increases to V2N and the tangential component of velocity V1T remains the same.
A study on CRO
2016
Deflection Plates
Electron beam, after leaving the electron gun, passes through the two pairs of deflection pates. One pair of deflection plates is mounted vertically and deflects the beam in horizontal or X-direction and so called the horizontal plates and the other pair is mounted horizontally and deflects the beam in vertical or Y-direction and called the vertical or Y-plates. These plates are to deflect the beam according to the voltage applied across them. For example if a constant pd is applied to the set of Y-plates, the electron beam will be deflected upward if the upper plate is positive. In case the lower plate is positive then the beam will be deflected downward. Similarly if a constant pd is applied to the set of X-plates, the electron beam will be deflected to the left or right of the tube axis according to the condition whether the left or right plate is positive. When a sinusoidal voltage is applied to Y-plates, the beam will be moved up and down according to the variation of plate potential. If the frequency of variation is more than 16 Hz the deflection will be a vertical line in the center of the screen. In case the sinusoidal voltage is applied to X-plates and frequency of variation is more than 16 Hz the deflection will be a horizontal line. If potentials are applied to both sets of plates simul-taneously, the deflection will be an oblique line. The amount of deflection is in proportion to the voltage applied to the pair of plates.
Screen For CRT.
As we know that some crystalline materials, such a phosphor, have property of emitting light when exposed to radiation. This is called the fluorescence characteristic of the materials. These fluorescent materials continue to emit light even after radiation exposure is cut off. This is called the
phosphorescence characteristic of the materials. The length of time during which phosphorescence occurs is called the persistence of the phosphor. The end wall of the CRT, called the screen, is coated with phosphor. When electron beam strikes the CRT screen, a spot of light is produced on the screen. The phosphor absorbs the kinetic energy of the bombarding electrons and emits energy at a lower frequency in a visual spectrum. Among the fluorescent materials used are zinc orthosilicate giving a green trace very suitable for visual observations; calcium tungstate giving blue and ultra-violet radiations very suitable for photography and zinc supplied with other materials giving a white light suitable for TV. Zinc phosphate gives a pronounced afterglow and is useful when studying transient phenomena because the trace persists for short while after the transient has disappeared.
Glass Body And Base.
The whole assembly is protected in conical highly evacuated glass housing through suitable supports. The inner walls of CRT between neck and screen are usually coated with a conducting material known as aquadag and this coating is electrically connected to the accelerating anode. The coating is provided in order to accelerate the electron beam after passing between the deflecting plates and to collect the electrons produced by secondary emission when electron beam strikes the screen. Thus the coating
prevents the formation of – ve charge on the screen and state of equilibrium of screen is maintained. Horizontal and vertical marks are marked on the screen of the CRT to provide user a correct measurement. These marks, usually in rectangular form, are called graticule.
A study on CRO
2016
Basic Controls of CRO
Vertical deflection system provides an amplified signal to the vertical deflection plates.
Horizontal deflection system provides an amplified signal to the horizontal deflection plates.
Position control, controls the vertical and the horizontal movement of the trace.
Intensity control, determines how bright the trace is.
Focus control, used for narrowing the trace, so as to get an accurate observation.
Astigmatism control, used for getting a focused beam at all regions of the screen.
Vertical Deflection System.
Fig-4 Linear Time Base
The function of vertical deflection system is to provide an amplified signal of the proper level to drive the vertical deflection plates without introducing any appreciable distortion into the system.
The input sensitivity of Many CROs is of the order of a few milli-volts per division and the voltage required for deflecting the electron beam varies from approximately 100 V (peak to peak) to 500 V depending on the accelerating voltage and the construction of the i tube. Thus the vertical amplifier is required to provide this desired gain from milli-volt input to several hundred volt (peak to peak) output. Also the vertical amplifier should not distort the input waveform and should have good response for entire band of frequencies to be measured. The deflection plates of CRO act as plates of a capacitor and when the input signal frequency exceeds over 1 MHz, the current required for charging and discharging of the capacitor formed by the deflection plates increases. So the vertical amplifier should be capable of supplying current enough to charge and discharge the deflection plate capacitor.
A study on CRO
2016
As we know that electrical signal is delayed by a certain amount of time when transmitted through an electronic circuitry. In CRO, output signal voltage of the vertical amplifier is fed to the vertical plates of CRT and some of its portion is used for triggering the time base
generator circuit, whose output is supplied to the horizontal deflection plates through horizontal amplifier. The whole process, which includes generating and shaping of a trigger pulse and starting of a time-base generator and then its amplification, takes time of the order of 100 ns or so. So the input signal of the vertical deflection plates of a CRT is to be delayed by at least the same or little more amount of time to allow the operator to see the leading edge of the signal waveform under study on the screen. For this purpose, delay line circuit is introduced between vertical amplifier and the plates of CRT, as shown in figure.
Horizontal Deflection System.
External signal is applied to horizontal deflection plates through the horizontal amplifier at the sweep selector switch in EXT position, as shown in figure. The horizontal amplifier, similar to the vertical amplifier, increases the amplitude of the input signal to the level required by the horizontal deflection plates of CRT. When the function of time is required to be displayed on the screen of CRT, INT position of sweep selector switch is used. Before going further we should make ourselves clear first about the linear time base pattern. Assume that we supply an ideal saw-tooth signal voltage to the horizontal deflection plates, keeping vertical deflection plates at zero potential, as shown in figure.
At the starting point A in time, signal voltage is maximum but negative so the spot on the screen of CRO is at the extreme left position. Further at point B in time, signal voltage applied to the horizontal plates is zero so the spot is in the center position on the screen. Now when voltage increases in + ve direction and becomes maximum just before the point C, the spot on the screen is at the extreme right side of the screen. Just after the point C, next cycle of saw-tooth voltage signal starts and again voltage becomes maximum negative so the spot goes back to the extreme left position of the screen from right position in no time.
From the above discussions we may conclude that
(i) The spot moves from left to right over the same path again for every cycle of saw tooth voltage applied to the horizontal deflection plates, so a horizontal line appears on the screen of CRO. (it) The spot moves from left to right on the screen with the uniform speed. Thus it produces a linear time base to display function of time on the screen of CRO.
For making idea of time base clearer let us discuss an application. Suppose a sine-wave voltage signal y of time period T is applied to the vertical deflection plates and a saw-tooth voltage signal vh of time
period T is applied to horizontal deflection plates, as shown in figure.
At zero time, the spot is at extreme left vertically central position on the screen. Because of zero value of VV and maximum negative values of Vh. At time T/4, the spot is at one-fourth way on the
screen in horizontal direction and at maximum positive deflection above the center line in vertical direction because of maximum positive value of VV. At time T/2, values of both VV and Vh are zero, so
the spot is at the central position of the screen. At time 3T/4, the spot is the three-fourth way on the screen in horizontal direction and at the maximum negative deflection in vertical direction. Finally, at the end of time T, the spot is at extreme right vertically central position of the screen and then it
A study on CRO
2016
system appears on the screen. If the period of sine-wave is reduced to half then two sine-wave cycles appears on the screen.From the above discussion, it is obvious that the following conditions are to be satisfied in order to have a waveform of the input signal applied to vertical deflection system as a stationary pattern on the screen of CRO.
(i) Both horizontal and vertical signals must start at the same instant.
(ii) Ratio of frequency of horizontal and vertical signals should be a rational or fractional number. For satisfying the above conditions, saw tooth-wave is generated and synchronized with the vertical input signal by the trigger circuit and time base generator, as shown in figure and explained above. In the INT position of sweep selector switch, horizontal amplifier receives an input from the time base generator, which provides a time base, and controls the rate at which the beam is scanned across the face of the CRT. Time base generator is triggered or initiated by a trigger circuit which ensures that the horizontal sweep starts at the same point of the vertical input signal. As explained earlier, it is necessary to synchronize the sweep with the signal under measurement in order to obtain stationary pattern. Ratio of frequency of the time base and of the signal under measurement should be a rational number; otherwise pattern on the screen will not be stationary.
Position Controls.
There are two knobs — one for controlling the horizontal position and another for controlling the vertical position. The spot can be moved to left or right i.e. horizontally with the help of a knob, which regulates the dc potential applied to the horizontal deflection plates, in addition to the usual saw tooth-wave. Similarly the spot can be moved up and down i.e. vertically with the help of another knob, which regulates the dc potential applied to the vertical deflection plates in addition to the signal.
Intensity Control.
The potential of the control grid with respect to cathode is controlled with the help of potentiometer in order to control the intensity of brightness of the spot.
A study on CRO
2016
Focus Control.
In the electron gun of a CRT, middle anode is kept at lower potential with respect to other two anodes and it acts like an electrostatic lens and focal length of this lens can be varied by varying the potential of the middle anode with respect to other two anodes. So focusing of an electron beam is done by varying the potential of middle anode with the help of a potentiometer, as shown in figure. By increasing the positive potential applied to the focusing anode the electron beam can be narrowed and the spot on the screen can be made a pin point.
Astigmatism.
This is an additional focusing control and is analogous to astigmatism in optical lenses. A beam that is focused at the center of the screen would be defocused at the edges of the screen because the lengths of the electron paths are different for the center and the edges. Adjustment of this control gives a sharp focus over the entire screen. This control is affected by varying the potential of deflection plates and accelerating anodes.
