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Lissajous pattern

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Used for measurement of frequency and phase angle

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Lissajous pattern

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  2. 2. 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 collectorcurrent 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.
  3. 3. 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 phosphorscreen, 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 forabout 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.
  4. 4. It can also be used to store several traces before the first one fades, so as to see how the signal changes with time. There are broadly two types of oscilloscope named as analogue oscilloscope and digital oscilloscope where the difference between both is well ill-starred by their name itself. Analogue oscilloscope involves only analogue signal during the processing of the signal in its various section like amplification, attenuation, filtration, deflection of any other operation some of its examples are sampling oscilloscope, conventional low frequency oscilloscope. But it is to mention here that it does not mean that it is not able to display digital signal as any wave of input signal can be studied by them irrespective of the types of oscilloscope used with some exceptions. On the other hand digital oscilloscope uses digital processing techniques in its various sections of processing like storage, transition between different sections. It is the availability of electronic circuits at relatively low cost that many digital features can be added to the analogue oscilloscopes like their storage. Some of the examples of such digital features involve a trigger after an elapse time or it may also be after a fixed number of pulses have passed. Digital display have some parameters like • Integrated digital voltmeter • Digital counter • Remote control In spite of the digital features embedded in it the basic system still remains an analogue oscilloscope on the basis of its working with some exceptions. Like an analogue oscilloscope CRT is used in it. The difference lies in the fact that it digitizes the analogue signal and as result all the substantial signals occur to be in digital form. As also has been told above that a conventional CRT is used here while storage of signal takes place in digital memory cells. The following figure well describes the working of basic oscilloscope with the help of a block diagram. This is basically a complete digital oscilloscope. The signal that is encountered by the system generally occurs in analogue form which is first digitized at input section and then it is stored in memory in the same form. Being in digital form various types of analysis can be done upon it and many other type of information can be built with this system and this all is possible because of the digital form of the given signal because in digital form an information signal can be easily fed into a computer and analysed by conventional methods. As conventional CRTs work upon analogue signal only so in order to use CRT the signal must be converted back to its analogue form before feeding it to CRT. Therefore, the digital signal is first retrieved from memory cell then converted back to analogue form by using a conventional
  5. 5. digital to analogue converter then is applied to CRT for its reproduction as a display. Block diagram of digital storage type oscilloscope The process of conversion from analogue to digital form is called Digitizing. Digitizing of the analogue signal takes place by use of sampling form. According to sampling theorem the samples of the input wave forms are taken out at fixed intervals of time which is further quantized to give a series of digital number equivalent to the magnitude of those samples. To make sure that no portion of the information signal is going to be lost the sampling rate should be greater than the frequency of the signal. What is sampling frequency? Sampling frequent is the rate at which the samples are taken out from input analogue signal in order to digitize the given information. Sampled input signal and quantized levels
  6. 6. And according to Nyquist rate the sampling frequency should be equal to double of that of the input analogue signal. This rate ensures that no information is lost during digitizing process. This requirement for higher frequency rate implies the digitizer, which is an analogue to digital converter, must have a fast conversion rate. A diagram can illustrate the working of sampling technique in which samples of analogue wave form are being taken at fixed intervals of time denoted by points a,b,c etc. the higher sampling needs expensive flash analogue to digital converter and resolution of such converters falls with increase in sampling rate. This is the basic reason that band width and resolution of the system is kept limited to by the converter section. There is absolution to overcome the need of high performance converter which is the use of an analogue store. The following figure shows one of such a kind. The sampling of input signal is performed and these samples are then stored in a shift register. Block diagram of analogue storage type oscilloscope This shift register is of analogue nature. When it is required to display the data on CRT it can be directly fed to CRT without having need of conversion from analogue to digital form and this saves the time of conversion and this result in fast processing. The speed of this process is about 100 mega samples per second and this has an advantage that now a cheap analogue to digital converter can be utilized. Resolution of the converter does not fall with increase in sampling rate. What are the Limitations of digital storage type oscilloscope? As, all the techniques have its own advantages and disadvantage it also has some limitation that oscilloscope does not take the input signal when the digitizing process is going on. This results in a blind spot in this time period. When sweep speed is less then it is switched out analogue memory and deeds the analogue signal to convector the same time of operation. Many input channels can be used with digital storage oscilloscope. If all the channels are made to share the same store them memory allotted to each channel gets reduced. Sharing of many channel simultaneously is performed with the help of a multiplexer which a sequential circuit that makes
  7. 7. it possible to uses same device by many user. Input channel of such an oscilloscope can reach up to 40 channels per system which is very easily available in commercial market. This type of instrument is not used in small level laboratories but at commercial level laboratories and large research centres. The storage capacity of such a device ranges up tp 25000 dots. A floppy disc can be used in many such advanced oscilloscope as a storage device that leads to increment to the capability of the system as a whole. Floppy disc plays a role of external storage device and the memory stored upon it can be saved by the user.

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