Digital Storage Oscilloscope

There are certain disadvantages of storage cathode ray tube due to which there is need of digital storage oscilloscope. The disadvantages of analog storage cathode ray tube are as follows

i) The waveform can be preserved for finite amount of time only and eventually the waveform will be lost.

ii) As long as image is required to be stored, the power must be supplied to the tube.

iii) The trace obtained from the storage tube is not fine as compared to the conventional oscilloscope tube.

iv) The writing rate of storage tube is less than that of conventional cathode ray tube. This limits the speed of the storage tube

v) The storage cathode ray tube is very much expensive than conventional cathode ray tube.

vi) The storage cathode ray tube requires additional power supplies

Vii) Only one waveform can be stored in storage tube.

viii) The stored waveform cannot be reproduced on the external device like computer.

Block Diagram

The block diagram of digital storage oscilloscope is shown in the Fig.    

• The input signal is applied to the amplifier and attenuator section.

• The oscilloscope uses same type of amplifier and attenuator circuitry as used in the conventional oscilloscopes. 

• The attenuated signal is then applied to the vertical amplifier. 

• To digitize the analog signal, analog to digital (A/D) converter is used.

•  The output of the vertical amplifier is applied to the A/D converter section.

• The successive approximation type of A/D converter is most of tenly used in the digital storage oscilloscopes.

• ‘The sampling rate and memory size are selected depending upon the duration & the waveform to be recorded. 

• Once the input signal is sampled, the A/D converter digitizes it. 

• The signal is then captured in the memory. 

• Once it is stored in the memory, many manipulations are possible as memory can be read out without being erased.

The digital storage oscilloscope has three

    1. Roll mode
    2. Store mode
    3. Hold or save mode.

Advantages

i) It is easier to operate and has more capability.

ii) The storage time is infinite.

iii) The display flexibility is available. The number of traces that can be stored and recalled depends on the size of the memory.

iv) The cursor measurement is possible.

v) The characters can be displayed on screen along with the waveform which can indicate waveform information such as minimum, maximum, frequency, amplitude etc.

vi) The X-Y plots, B-H curve, P-V diagrams can be displayed.

vii) The pretrigger viewing feature allows to display the waveform before trigger pulse.

viii) Keeping the records is possible by transmitting the data to computer system where the further processing is possible.

ix) Signal processing is possible which includes translating the raw data into finished information e.g. computing parameters of a captured signal like r.m.s. value, energy stored etc.

PRINCIPLES OF DESIGN OF WATER SUPPLY

•    Plumbing fixtures and appurtances should be supplied with water in sufficient volume and at pressures adequate to function satisfactorily and without undue noise under all circumstances.
•    The design of pipe should be made so that there is no contact between the lines feeding to the cistern or any such other appliance with those feeding water for human consumption.
•    Pipe network should be completely water tight and also remain undamaged either by traffic loads, vibrations or temperature and any strains of buildings.
•    Pipe network in the premises should be optimum discharge of water is obtained consistent with economy.  The system should be free from water hammer, corrosion and should also look aesthetic. 
•    The pipe network should be laid and fixed that it doe not pass by the side of any sewage line or refuse drain nor does it pass through any field of foul ground where dirt or city have been deposited and manure dumps.
•    The pipe network should be laid and fixed so that it shell be accessible at any time for attending to damages, leakages etc.
•    The pipe network should be of adequate size to give the desired rate of flow
•    The pipe network should be divided into sections to facilitate repairs. zzThese section should be separated by valves in order that a section can be isolated for repairs keeping the rest of the distribution.
•     The methods of joining pipe should be such as to avoid water loses.
•    Whenever the pipes are bent it should be so made that these are not likely to materially diminish or alter cross section.
•    The piping should be so laid that air locks do not occur and it should be possible to flush out the network from time to time.
•    The pipes to carry satisfactory and un satisfactory water should be laid separately .whenever a supply of less satisfactory water and satisfactory water has to mixed, which shall be done only by discharging both the flows in to a cistern and by a pipe discharging in to the air gap above t he top level of the cistern at the height equal to twice its nominal bore and in no case less than 15cm.  It is necessary to maintain a definite air gap in all plumbing used in the water closet.

•    In the building if a provision is required to be made for storage of water on account of

1.    In the interruption of supply

2.    to maintain a reserve supply

3.    to regulate discharge in the mains

4.    to maintain a reserve for fire fighting arrangements, a tank for storage of water should be provided which should be watertite and also should be of sufficient thickness and capacity.  The storage reserve should be dust prove and mosquito-proof.  Each storage tank should be easily accessible and placed in such a position as to facilitate thorough inspection and cleaning.  Stagnation of water within the tank is to be avoided.  The tank should be so arranged as to have periodical cleaning done without seriously interfering with the supply of water.  It is to be understood that water when it is stagnant is likely to become a good breeding ground for various organisms which are very dangerous for human beings.  Therefore water tanks are require to be periodically cleaned and after cleaning properly disinfected.

       In the case of underground tanks the contamination of stored water on account of above ground flow and due to seepage of underground water should be avoided.  Whenever underground tanks are required for fire fighting purposes, the same should be approachable easily by fire tenders.  The water which is required for fire fighting is so provided that everyday it gets renewal through an inflow of fresh water supply.

Lead piping should not be utilized anywhere in the domestic water supply system.

Polythene and PVC pipes should not be installed near hot water pipes or near any source of heat.


The dead ends in the pipe lines should be avoided to the extent possible.  The mains should be arranged in a grid formation or network fashion.  Where dead ends are unavoidable, a hydrant should be provided as a washout as the deposition of solids is at a higher rate in the case of dead ends where water stagnates requiring cleaning of pipe lines at frequent intervals.

The wash hydrant should connect to an effectively trapped chamber to avoid contamination.  It should not discharge directly into a sewer line or a manhole or chamber on sewer line.

Air valves should be provided at all summit and washout at low points. The pipeline may follow the general contours of the land. It should be so laid that it generally rises to air walls to a water washout. Care should be maintained positive pressure at every point in the pipe line under normal working conditions.

The mains should be laid at least 90cms below surface under load and atleast 75cms below under a foot path to protect it from various traffic loads coming on it.  Otherwise, there are chances of pipe settling under load and breaking.

Safety demands that water pipe should be separated from electric, telephone and other such cable no piping should be so laid as to pass into or through any sew line or manhole pit, as pit or any material of such nature that it is likely to cause undue deterioration of pipe.

Where laying of any pipe through corrosive soil or environment is inevitable, the piping should be properly protected from contact with such conditions by being carried through
Cast iron tubes or some other suitable means.

Changes in diameter of pipe or changes in direction pipe should be gradual and not abrupt.  Abrupt changes involve avoidable lose of head of water.

No boiler for generating steam or closed boiler of any description or any such machinery should be directly connected with supply pipe.  Every such boiler or machinery should be supplied from a feed tank.

The design of pipe work should be such that there is no possibility back flow from any cistern or appliance whether by siphonage or otherwise. Non return valve should not be relied upon to prevent such back flow.

Where pipe network has to be laid in a recent fill, proper precautions have to be taken to provide continuous and even support.

Cathode Ray Tube (CRT) Display

The device which allows, the amplitude of such signals, to be displayed primarily as a function of time, is called cathode ray oscilloscope. The cathode ray tube (CRT) is the heart of the C.R.O. The CRT generates the electron beam, accelerates the beam, deflects the beam and also has a screen where beam becomes visible as a spot. The main parts of the CRT are

i)  Electron gun
   
ii) Deflection system

iii) Fluorescent screen   

iv) Glass tube or envelope

v) Base

(fig) cathode ray tube

Electron Gun 

• The electron gun section of the cathode ray tube provides a sharply focused, electron beam directed towards the fluorescent-coated screen. 

• This section starts from thermally heated cathode, emitting the electrons. 

• The control grid is given negative potential with respect to cathode. 

• This grid controls the number of electrons in t beam, going to the screen.

• The momentum of the electrons (their number x their speed) determines the intensity, or brightness, of the light emitted from the fluorescent screen due to the electron bombardment. 

• The light emitted is usually of the green colour.

Deflection System

•   When the electron beam is accelerated it passes through the deflection system, 

•   with which beam can be positioned anywhere on the screen.

Fluorescent Screen

•     The light produced by the screen does not disappear immediately when bombardment by electrons ceases, i.e., when the signal becomes zero.

•     The time period for which the trace remains on the screen after the signal becomes zero is known as “persistence or fluorescence”.

•     The persistence may be as short as a few microseconds, or as long as tens of seconds or even minutes.

•     Medium persistence traces are mostly used for general purpose applications.

•     Long persistence traces are used in the study of transients.

•     Long persistence helps in the study of transients since the trace is still seen on the screen after the transient has disappeared.

Glass Tube

•     All the components of a CRT are enclosed in an evacuated glass tube called envelope. 

•     This allows the emitted electrons to move about freely from one end of the tube to the other end.

Base

    The base is provided to the CRT through which the connections are made to the various parts.

X-Y Recorders

 

A strip chart recorder records the variations of a quantity with respect to time while X-Y recorder is an instrument which gives a graphic record of the relationship between two variables.    

    In strip chart recorders, usually self balancing potentiometers are used. These self balancing potentiometers plot the emf as a function of time. The X-Y recorders, an emf is plotted as a function of another emf. This is done by having self balancing potentiometer control the position of the rolls while another self balancing potentiometer controls then position of the recording pen.

In some XY recorder, one self balancing potentiometer circuit moves a recording pen in the X direction while another self balancing potentiometer circuit moves the recording pen in the Y direction at right angles to the X directions, while the paper remains stationary.

They are many variations of XY recorders. The emf, for operation of XY recorders, may not necessarily measure only voltages. The measured emf may be the output of a transducer that may measure displacement force, pressure, strain, light intensity or any other physical quantity. Thus with the help of XY recorders and appropriate transducers, a physical quantity may be plotted against another physical quantity.

Hence an XY recorder consists of a pai of serve system, driving a recording pen in two axes through a proper sliding pen and moving arm arrangement, with reference to a stationary paper chart.

An signal enters each of the two channels. The signal are attenuated to the inherent full scale range of the recorder, the signal then passes to a balance circuit where it is compared with an internal reference voltage.
 

Strip Chart Recorder

It records one or more variables with respect to time. It is a X-t recorder.

A strip chart recorder consists of:

1.    A long roll of graph paper moving vertically.

2.    A system for driving a paper at some selected speed. A speed selector switch is generally provided. Chart speed of 1-100 m/s are usually used.

3.    A stylus driving system which moves the stylus in a near exact replica or analog of the quantity being recorded.

A range selector switch is used so that input to the recorder drive system is with in the acceptable level.

A. Paper drive system: The paper system should move the paper at a uniform speed. A spring would may be used but in most of the recorder a synchronous motor is used for driving the paper.

B. Marking Mechanism: There are many types of mechanism used for making marks on the paper. The most commonly used ones are:

1.    Marking with ink filled stylus. The stylus is filled with ink by gravity or capillary actions. This requires that ihe pointer shall support an ink reservoir and a pen, or capillary connection between the pen and a pen reservoir. In general red ink is used but other colours are available and in instrumentation display a colour code can be adopted.

2.    Marking with headed stylus. Some recorders use a heated stylus which writes on a special paper. This methods overcomes the difficulties encountered in ink writing systems.

3.    Chopper Bar. If a chart made from a pressure sensitive paper is used a simple recording process is possible. A V-shaped pointer is passed under a chopper bar which presses the pen into the paper once per second thus making a series on the special paper. In fact this system is not purely continuous and hence is suitable for recording some varying quantities.

4.    Electric stylus marking. This method employs a paper with a special coating which a sensitive to current. When current is conducted from the stylus to the paper, a trace appears on the paper. It is clear that the electric stylus marking method has a wide range of marking speeds, has low stylus friction and a long stylus life. The disadvantage is that the cost of paper is very high.

C. Tracing system. There are two types of tracing system used for producing graphic representation.

1.    Curvilinear system. In the curvilinear system, the stylus is mounted on a central pivot and moves through an are which allows a full width chart marking. If the stylus makes a full range recording, the line drawn across the chart  will be curved and the time intervals will be along the curved segments.

2.    Rectilinear system. It is notices that a line of constant time is perpendicular to the time axis and therefore this system produces a straight line across the width of the chart. Hence the stylus is actuated by a drive cord over pulleys to produce the forward and reverse motion as determined by the drive mechanism. The stylus may be actuated by a self-balancing potentiometer system, a photoelectric deflection system, a photoelectric potentiometer system, or a bridge balance system. This system is usually used with thermal or electric wiring.

Tape Recorders

It is frequently desirable and in many cases necessary, to record data in such a way that they can be received reproduced in electrical form again. The most common and most useful way of achieving this through the use of magnetic tape recorder.

Advantages:

1.    Magnetic tape recorders have a wide range from dc to several MHz.
2.    They have a low distortion.
3.    They have a wide dynamic range which exceeds 50db. This permits the linear recording from full scale level to approximately 0.3% of full scale.
4.    The magnitude of the electrical input signal is stored in magnetic memory and this signal can be reproduced whenever desired.

Basic components of Tape recorder:

1.    Recording heat
2.    Magnetic tape
3.    Reproducing head
4.    Tape transport mechanism
5.    Conditioning device.

Principles of Tape recorder: When a magnetic tape is passed through a recording head, any signal recorded on the tape appears as magnetic pattern dispersed in space along the tape, similar to the original coil current variation with time.

The same type when passes through a reproduce or playback head produces variations in the reluctance of the winding thereby inducing a voltage in the winding dependent upon the direction of magnetization and its magnitude on the magnetic tape. The induced voltage is proportional to rate of change of flux linkages, therefore the emf induced in the winding of reproducing head is proportional to rate of change of the level of magnetizing on the tape, e_rep aN df/dt, where N is the number of turns of the winding put on the reproduce head. Since the voltage in the reproduce head is proportional to df/dt, the reproduce head acts as a differentiator.