Q. Describe in detail the construction and working of analog type storage oscilloscope. Explain the principle of secondary emission.
Analog Storage Oscilloscopes: Storage oscilloscopes, capable of high persistence phosphors, have many applications. Example of these is the capture and storage of transients and the steady display of a very low frequency signal. Two techniques are used to store signals in an oscilloscope and these are called analog and digital storage. Analog storage is capable of higher speeds, but is less versatile than digital storage.
Principle of Secondary Emission: shows a simple electrode arrangement to illustrate this principle. When a beam of electrons from the electron gun strikes the target it emits secondary electrons, which are gathered by the collector. The collector is at positive potential of the target is a at a potential of which can be varied and the electron gun is at ground potential.
The value of the current in the primary electron beam, coming from the electron gun and is the electron current emitted from the target and collected by the collector, the ratio is called the secondary emission ratio.
The value of this ratio depends on the primary electron velocity and intensity and on the chemical composition of the target.
(b) will show the variation of the secondary emission ratio with target voltage, for the arrangement of when the target voltage is much grater than the collector voltage all secondary electrons emitted from the target are attracted back to it.
Therefore the collector current or secondary current is zero and so also is the secondary emission ratio. The operating point is now well to the right of In.
If the target voltage is slightly negative, as at point f, then al the electrons from the gun are deflected on to the collector before they reach the target. Therefore although there is no secondary emission the collector current equals the beam current and the secondary emission ratio is unity. This point is known as the lower stable point. As the target volute increases from these point electrons are attracted away from the collector, but they do not have enough energy to release secondary electrons from the largest. Therefore the secondary emission ratio falls to a minimum at A. Beyond the minimum point secondary emission from the target starts to occur and these electrons are accumulated by the collector, so increasing the secondary emission ratio.
The secondary emission ratio increase though the crossover point E until it reaches a peak at C. Beyond this point secondary electrons emitted from the target are attracted back in numbers greater than those which reach the collector, so that the secondary emission ratio decreases sharply. The curve reaches the upper stable point at G, where the primary and secondary currents are equal and then decreases to zero. The lower and upper stable points represent the erased and written conditions of the CRT screen and in the absence of a target voltage the can remain in one of these two stages only.
Variable Persistence Storage: It is an analog storage oscilloscope is also known as halftone storage or mesh storage.
Construction: Given in illustrates the construction of a CRT using this storage technique. There are two screens, a storage mesh which retains the image traced on it by the wiring gun and the phosphor screen, which is very similar to that used in a conventional CRT. Dielectric material, consisting of a thin layer of material such as magnesium fluoride, is deposited on the storage mesh and this acts as the storage target. The writing gun is at a high negative voltage, the flood gun at a few volts negative, the collector mesh at about 100 V positive and the storage mesh at ground potential or a few volts negative. The collimator consists of a conductive coating on the inside surface of the CRT. It is biased so as to distribute the flood gun electrons evenly over the surface of the target and causes the electron to be perpendicular to the storage mesh.
Working: When the writing gun is aimed at the storage target it causes areas where it strikes to be charged to a positive potential, due to secondary emission.
These areas are maintained at their upper stable point, even after the writing gun is switched off, due to the action of the flood gun. Electrons from the flood gun also pass through those areas which are positively charged causing the phosphor beyond to glow and displaying the original signal traced by the writing gun.
The pattern stored on the storage target lasts for about one hour, but it can display a bright image for about one minute. The stored pattern fades due to electrons from the flood gun charging other parts of the storage surface, giving an impression that the whole screen has been written. This is known as fading positive. To erase an image which has been stored the storage mesh is momentarily raised to the same positive potential as the collector mesh.
Application: Such as the storage of an entire waveform of a slow moving signal, which then fades the next trace is written.
It can also be used to store several traces before the first one fades, so as to see how the signal changes with time.