Variation of this current will change the field, and so deflect the spot.Įlectrostatic deflection is, however, achieved by means normally contained within the tube. To achieve this, coils are placed outside the tube envelope, and a current passed through them. Variation of this field will, therefore, vary the position of the spot on the screen. This deflection of the beam from the straight path it was originally following causes the spot to take up a new position on the screen, because the electrons strike the screen at a different point. If a magnetic field is brought near to the electron beam after it has left the final anode, the beam will bend. The operation of magnetic deflection is quite simple. Focusing may be achieved magnetically, if required, and is frequently used in TV receivers. It is worth noting that the focusing system described above is electrostatic, that is, it is controlled by the potential of the second anode. As we are concerned with oscilloscopes here, magnetic deflection will be dealt with very briefly. Magnetic deflection is seldom used in oscilloscopes, but is almost invariably used in television receivers. There are two main methods of deflecting the spot: (a) Magnetic and (b) Electrostatic.Ī small oscilloscope CRT of the type used in 1955 when this article was written Deflection A small magnetically deflected CRT of the type found in the 1980's for portable monitors So much for the production of the spot - now lets see how it may be moved around the screen. The intensity of the light is controlled by varying the grid voltage as previously mentioned. A small spot of light, therefore, appears on the screen. The inside face of the screen is covered with a material which fluoresces when the electrons strike it. The thin pencil beam of electrons is travelling at a high velocity when it leaves the final anode, and is therefore capable of traversing the space between the final anode and the screen. The second anode is, therefore, called the 'focusing anode'. The potential of the second anode is normally made variable, so that focusing of the electrons into the narrowest possible beam may be achieved. These are of essentially the same mechanical form as the first anode, but they are connected to progressively higher potentials.īy the time the electrons have passed through these anodes, they are concentrated into a narrow beam. One or two further anodes may be situated beyond the first anode. The electrons are attracted toward this electrode and stream through the hole at the centre. This anode is often in the form of a disc with a small hole at its centre. Beyond this electrode is an anode positively charged with respect to cathode. By biasing this electrode relative to the cathode, a variation in the number of electrons passing through the hole may be achieved, as in the case of a normal valve. Between the cathode and the anode there is a 'grid' consisting of a tube with a hole in one end, as shown above right. This cathode therefore emits electrons, which are attracted away from it by the anode A1. The left hand diagram above shows an electrostatically deflected tube.Īt the base of the tube there is a cathode heated in the same way as in an ordinary valve. This is the device which produces the picture, and therefore all the circuitry is designed around it. The heart of the oscilloscope is the cathode ray tube. The vertical axis would normally represent voltage. If the frequency of the signal were 25 Hz, and the horizontal scale were 0.5 second, 12.5 cycles would be displayed. If, for example, we were to connect our oscilloscope to a point in a circuit where a sinusoidal signal existed, a picture of the sinusoid should appear on the screen of the oscilloscope. The purpose of the oscilloscope is to draw what is virtually a graph of regularly varying electrical quantities, such as voltage or current, against a horizontal scale of time. In this 1955 article the CRT is discussed with specific application to the oscilloscope. The Cathode-Ray Tube is used in instrumentation and picture presentation. Cathode-Ray Tube Cathode-Ray Tubes CRT Power Network
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