Cathode ray Oscilloscope , digital storage oscilloscope

Cathode ray Oscilloscope

1. Electrical and Electronics measurements Dr. Sk Babar Ali Associate Professor Department of ECE Aliah University, New Town, Kolkata

2. Cathode Ray Oscilloscope (CRO) The Cathode Ray Oscilloscope ( CRO) is an electronic device containing a cathode ray tube(CRT) that generates an electron beam that is used to produce visible patterns, graphs on a phosphorescent screen. CRO is a very useful and versatile laboratory instrument used for display, measurement and analysis of waveform and other phenomena in electrical and electronic circuits. The ‘stylus’ of this ‘plotter’ is a luminous spot which moves over the display area in response to an input voltage. The luminous spot is produced by a beam of electrons striking a fluorescent screen. CRO uses a horizontal input voltage which is an internally generated ramp voltage called ‘time base’. The horizontal voltage moves the luminous spot periodically in a horizontal direction from left to right over the display area or screen. The vertical input to the CRO is the voltage under investigation. The vertical input voltage moves the luminous spot up and down in accordance with the instantaneous value of the voltage. The luminous spot thus traces the waveform of the input voltage with respect to time.

3. External View of CRO Types of CRO Analog CRO: It is the simplest and earliest type of oscilloscope comprises of a vertical amplifier, a cathode ray tube, time base, a power supply and a horizontal amplifier. These are commonly known as analog CRO. Digital CRO: It is a complex electronic device composed of various software and electronic hardware modules that work together to capture, process, display and store data that represents the signals of interest of operator.

4. Basic Block Diagram Of CRO

5. Cathode Ray Tube (CRT)

6. Cathode Ray Tube (CRT) The main part of the CRO is Cathode Ray Tube (CRT). 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. The phosphor screen is coated with ‘aquadag’ to collect the secondary emitted electrons. Low voltage supply is required for the heater of the electron gun for generation of electron beam and high voltage, of the order of few thousand volts, is required for cathode ray tube to accelerate the beam. Horizontal and vertical deflecting plates are fitted between the electron gun and screen to deflect the beam according to the input signal. The electron beam strikes the screen and creates a visible spot. This spot is deflected on the screen in the 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.

7. The CRT is composed of two main parts: Electron Gun: Deflection System : Electron gun: It provides a sharply focused electron beam directed toward the fluorescent-coated screen. The thermally heated cathode emits electrons in many directions. The control grid provides an axial direction for the electron beam and controls the number and speed of electrons in the beam. The momentum of the electrons determines the intensity, or brightness, of the light emitted from the fluorescent coating due to the electron bombardment. Because electrons are negatively charged, a repulsion force is created by applying a negative voltage to the control grid, to adjust their number and speed. A more negative voltage results in less number of electrons in the beam and hence decreased brightness of the beam spot. Since the electron beam consists of many electrons, the beam tends to diverge. This is because the similar (negative) charges on the electrons repulse each other. To compensate for such repulsion forces, an adjustable electrostatic field is created between two cylindrical anodes, called the focusing anodes. The variable positive voltage on the second anode cylinder is therefore used to adjust the focus or sharpness of the bright spot.

8. Cathode Ray Tube (CRT)

9. The Deflection System The deflection system consists of two pairs of parallel plates, referred to as the vertical and horizontal deflection plates. One of the plates in each set is permanently connected to the ground (zero volt), whereas the other plate of each set is connected to input signals or triggering signal of the CRO.

10. Y-Gain Amplifies the Y-deflection. Small input voltages are amplified by built-in amplifiers before applying to the Y-plates. Y- Gain = 0.5 V/div • 0.5 volt will cause a vertical deflection of 1 division Time Base It is a saw-tooth voltage applied internally across the X-plates. It controls the speed at which the spot sweeps across the screen horizontally from left to right.

11. It helps to display the actual waveform of any a.c. applied across the Y-plates • Normally calibrated in – s/cm – ms /cm ,µs/cm • It gives the time required for the spot to sweep 1 cm horizontally across the screen.

12. ELECTROSTATIC DEFLECTION Two parallel plates with a potential applied between them and produce a uniform electrostatic filed in the Y direction. Thus any electron entering the field will experience a force in the Y direction and will be accelerate in that direction. There is no force either in X direction or Z direction and hence there will be no acceleration of electrons in these directions. Let, Ea = voltage of pre-accelerating anode; (volt) e =charge of an electron; (Coulomb) m =mass of electron; (kg) θ =deflection angle of the electron beam

13. Vox = velocity of electron when entering the field of deflecting plates; (m/s) Ed =potential difference between deflecting plates; (volt) d =distance between deflecting plates; (m) ld =length of deflecting plates; (m) L =distance between screen and the center of the deflecting plates; (m) y =displacement of the electron beam from the horizontal axis at time t and D = deflection of the electron beam on the screen in Y direction; (m) The loss of potential energy (PE ) when the electron moves from cathode to accelerating anode This is the velocity of the electron in the X direction when it enters the deflecting plates. The velocity in the X direction remains same throughout the passage of electrons through the deflecting plates as there is no force acting in the direction.

14. Suppose a y is the acceleration of the electron in the Y direction, therefore As there is no initial velocity in the Y direction, the displacement y at any instant t in the Y direction is As the velocity in the X direction is constant, the displacement in X direction is given by Substituting the above value of t, we have This is the equation of a parabola

15. Putting x = ld , we get the value of tan θ. After leaving the deflection plates, the electrons travel in a straight line. The straight line of travel of electron is tangent to the parabola at x = ld and this tangent intersects the X axis at point O’. The location of this point is given by The apparent origin is thus the center of the deflecting plates, the deflection D on the screen is given by

16. For a given accelerating voltage Ea, and for particular dimensions of CRT, the deflection of the electron beam is directly proportional to the deflecting voltage. This means that the CRT may be used as a linear indicating device. The deflection sensitivity of a CRT is defined as the deflection of the screen per unit deflection voltage. The deflection factor of a CRT is defined as the reciprocal of sensitivity It is clear that the sensitivity can be increased by decreasing the value of accelerating voltage Ea. but this has a disadvantage as the luminosity of the spot is decreased with decrease in Ea. On the other hand a high value of Ea, produced a highly accelerated beam and thus produces a bright spot. However, a high accelerating voltage (Ea) requires a high deflection potential (Ed) for a given deflection. Highly accelerated beam is more difficult to deflect and is sometimes called hard beam.

17. Digital Storage Oscilloscope It uses both of digital-to-Analog and Analog-to-Digital (DACs and ADCs) for digitizing, storing and displaying analog waveforms. The overall operation is controlled and synchronized by the control circuits. Which usually have microprocessor executing a control program stored in Read-Only Memory (ROM). The data acquisition portion of the system contains a sample-and-hold (S/H) and a analog-to-digital converter that repetitively samples and digitized the input signal at a rate determined by the sample clock, and transmits the digitized data to memory for storage.

18. The control circuit makes sure that successive data points are stored in successive memory locations by continually updating the memory’s address counter. Data acquisition and the storage process continue until the control circuit receives a trigger signal from either the input waveform (internal trigger) or an external trigger source. When the triggering occurs, the system stops acquiring data further and enters the display mode of operation, in which all or part of the memory data is repetitively displayed on the Cathode Ray Tube (CRT). In display operation two DACs are employed for providing the vertical and horizontal deflecting voltages for the cathode ray tube. Data from memory produce the vertical deflection of the electron beam, while the time base counter provides the horizontal deflection in the form of a staircase sweep signal. The control circuits synchronize the display operation by incrementing the memory address counter and the time base counter at the same time so that each horizontal step of the electron beam is accompanied by a new data value from the memory to the vertical DAC. The counters are continuously recycled so that the stored data points are repetitively re-plotted on the screen of the CRT.