CATHODE RAY OSCILLOSCOPE

Cathode Ray Oscilloscope (CRO) is an electronic equipment which gives a visual representation of electrical quantities such as voltage and current waveforms in an electrical circuit. In this device, the cathode rays are deflected by an electric and magnetic field and of producing scintillations on a fluorescent screen. Since the inertia of cathode rays (electron beam) is very small, they can follow the alterations of very high frequency fields and thus electron beam serves as a practically inertia-less pointer.

When a varying potential difference is established across two plates between which the beam is passing, it is deflected and moves in accordance with the variation of potential difference. When this electron beam impinges upon a fluorescent screen, a bright luminous spot is produced there which shows and follows faithfully the variation of potential difference without any time lag.

A cathode ray oscilloscope consists of the following main constituents:

  1. Cathode ray tube
  2. Time base circuits
  3. Power supply
  4. Deflection voltage amplifiers.
Fig. 1: A general sketch of Cathode ray Oscilloscope

Cathode Ray Tube:

It converts a varying voltage into a visible waveform. It has three basic parts:

  1. Electron gun, which is an arrangement for producing and focusing an electron beam.
  2. Deflecting plate, which is an arrangement for deflecting the beam either horizontally or vertically.
  3. Fluorescent screen, upon which the electron beam is focussed to create a well-defined spot.

The essential parts of a typical cathode ray tube are shown in Fig. 1.

  1. The Electron Gun:

The function of the electron gun is to produce, accelerate and focus the election beam to give a fine spot on the fluorescent screen. It consists of the following parts:

  1. Thermionic cathode for emission of electrons.
  2. Control grid for varying the electron current density.
  3. Accelerating electrode for attracting the electrons.
  4. Focussing electrode of first anode for focussing the electron beam into a fine spot.
  5. Second or final anode to provide further acceleration and focussing.

The construction of an electron gun is shown in Fig. 1. The electrons are produced by an indirectly heated cathode of cylindrical shape. The control grid which surrounds it has a small aperture in the centre of end surface. It is given a negative potential with respect to cathode. Next to control grid is the accelerating electrode (first anode) having one or more apertures. It is given a high positive potential and the apertures eliminate electrons which diverge from the beam. As this narrow electron beam travels further from the accelerating electrode, it tends to spread because of mutual repulsion between electrons. Hence a focusing electrode (second anode) is placed next to the accelerating electrode. The positive voltage applied on this first anode focusses the beam of electrons. The second anode which is given a very high positive potential of the order of 2000 volt or more, accelerates the electron beam. The first and second anode together provide necessary focusing to the electron beam so that it produces a high intensity light spot on the screen.

Focussing:

The electron beam from accelerating electrode tends to diverge due to mutual repulsion between electrons. To bring it to a sharp focus at the screen electrostatic focussing is employed. The first and second anode of the electron gun provides an electron lens system which focusses the beam into a fine spot on the screen. Both these anodes are kept highly positive with respect to the cathode but the potential of the first one is usually lower than that of the second anode. The second anode also has an aperture to permit a well-defined beam to pass through it.

  • Deflection System:

The electron beam in a cathode-ray tube can be deflected from its normal position along the axis of the tube by either electrostatic or magnetic means. In Fig. 1, two pairs of deflecting plates are shown, which provide electrostatic deflection and are kept next to the focusing system. One pair of these parallel plates is kept vertical while the other pair is kept horizontal. If deflection in the horizontal direction is desired, voltage is applied to the plates kept vertical and the beam is attracted towards the positive plate. On leaving the electrostatic field of the plates, the electron beam travels along a straight line at an angle with the axis and hits the screen. Accordingly, these plates, mounted in the vertical plane, are called horizontal deflection plates or X-plates. The second pair of plates is kept horizontal but produces deflection in the vertical direction and, is therefore, called the vertical deflection plates or Y-plates.

Thus, the voltage applied simultaneously to X and Y plates, give control of the beam and the spot of light moves in both, i.e., X and Y directions.

  • Fluorescent Screen:

The electron gun and the deflecting plates are placed in a highly evacuated glass envelope, as shown in Fig. 1. The front portion of the glass envelope, where the electron beam strikes, is termed as screen. The screen is coated from inside with a fluorescent material which emits light when bombarded by electron. These materials are commonly known as phosphors. The conical surface of the glass envelope is coated with a conducting coating of carbon particles. The conductive coating is connected to the second anode. As the second anode is usually grounded, this connection provides a return path for the electrons to complete the electrical circuit. Consequently, the screen remains at a positive potential with respect to the second anode.

  • Power Supply:

The power supply is a source of d.c. voltage for cathode ray tube and other parts of C.R.O. In general, the final anode voltage in a C.R.O, is of the order of 1000-2000 volt but in special cases such as in radar and in television, much higher anode voltages are required. This high voltage is obtained by specially designed power supplies or by a voltage doubler circuit.

In practice, the final anode is often kept at earth potential and a negative voltage is applied to the other electrodes of CR tube. It avoids the danger of an electric shock which may come from a high anode voltage.

Horizontal and vertical amplifiers are used in a C.R.O. between the applied voltage and the X and Y-plates. It is necessary to get the full-scale deflection from applied voltages. These amplifiers are well designed to transmit all necessary harmonic components of the highest frequencies used without distortions of waveform or change in phase angles.

Block Diagram and Panel Controls of CRO:

The figure given below depicts the block diagram of a C.R.O.

The signal to be examined is fed to the input of the vertical amplifier. After amplification, it is fed to the vertical deflection plates or Y-plates. Simultaneously, a saw tooth wave is applied from a time-base circuit to the X-plates through a horizontal amplifier. The time-base circuit is connected to an external synchronizing circuit. A power supply is included in the circuit as a source of d.c. voltage for the CR tube and also for the amplifiers and sweep circuits.

As the voltage at the X-plates rises linearly with time, the movement of the spot along X-axis becomes proportional to time. The resultant pattern on the screen is thus a plot of magnitude of input signal versus time.

Front Panel Controls: Following front panel controls are provided in a C.R.O.:

(1) Intensity Control: It controls the intensity of light spot by changing the negative bias on the control grid. When control grid is made more negative, the number of electrons reaching the screen decreases. Hence the intensity of spot also decreases. The intensity of spot can be increased by making control grid more positive.

(2) Focus Control: This control focuses the electron beam into a fine spot on the fluorescent screen by varying the voltage on the focusing anode.

(3) Vertical Shift: It controls the d.c. voltage applied between the Y-plates. The spot can be manually varied in the vertical direction by the adjustment of this control.

(4) Horizontal Shift: This control is similar to the vertical shift control. An adjustable d.c. voltage can be applied to the X-plates by its help and hence the initial position of the spot on the screen can be adjusted.

(5) Sweep Frequency Range Switch: This switch is provided to set the frequency of time-base oscillator to any desired range.

(6) Sweep Frequency Control: Fine adjustment of sweep frequencies within a given frequency range can be made by the help of this control.

(7) Vertical Gain: This control adjusts the gain of vertical amplifier by setting the potential divider provided in the circuit. It helps to increase the magnitude of weak signals.

(8) Horizontal Gain: This gain control is provided for the horizontal amplifier. Saw-tooth voltage can be adjusted by it to cause light spot movement from edge to edge.

(9) Synchronization Control: For stationary pattern on the screen, the sweep frequency must be either equal to or a submultiple of the input signal frequency. Hence, it is essential that the saw-tooth generator be synchronized to the signal frequency. The synchronization control is provided to lock the two frequencies together.

Applications of CRO:

  1. Measurement of a.c. and d.c. voltage.
  2. Measurement of a.c. and d.c. current.
  3. Study of waveforms.
  4. Measurement of frequency.
  5. Measurement of phase.