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9.1.4 Working Principle of CRO

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The cathode-ray oscilloscope (C.R.O.) consists of the following components:

  1. The electron gun.
  2. The deflecting plates.
  3. A fluorescent screen.

The Electron Gun

Parts of Electron Gun Function
 Filament  To heat the cathode.
 Cathode  Release electrons when heated by filament.
 Grid
  •  The grid is connected to a negative potential. The more negative this potential, the more electrons will be repelled from the grid and fewer electrons will reach the anode and the screen.
  • The number of electrons reaching the screen determines the brightness of the light. Hence, the negative potential of the grid can be used as a brightness control.
 Focusing Anode and 
  •  The other feature in the electron gun is the use of the anode.
  • The anode at positive potential accelerates the electrons and the electrons are focused into a fine beam as they pass through the anode.
 Accelerating anode


The Deflecting Plates

Part of the deflecting system Function 
 Y-plate The Y-plates will cause deflection in the vertical direction when a voltage is applied across them.
  X-plate  On the other hand, the X-plates will cause the electron beam to be deflected in the horizontal direction if a voltage is applied across them.


The Fluorescent Screen

  1. The screen is coated with a fluorescent salt, for example, zinc sulphide.
  2. When the electrons hit the screen, it will cause the salt to produce a flash of light and hence a bright spot on the screen.

Using CRO


Function
 1. Power switch  To switch on and off of the oscilloscope
2. Focus control To control the focus of the spot on the screen.
3. Intensity control To control the brightness of the spot on the screen.
4. X-offset
5. Y-offset		 Y-offset moves the whole trace vertically up and down on the screen, while X-offset moves the whole trace from side to side on the screen.
6. Time base control Whenever we switch on the time-base, we are actually applying a sawtooth voltage to the X-plates (Figure below).




* This make the electron beam sweep across the screen at a constant speed.
* By knowing the period of each cycle, T, we can then know how fast the beam is sweeping across the screen. The time-base is thus a measure of time for the oscilloscope.
7. Y gain control * the "Volts/Div." wheels amplify an input signal so that for a division a given voltage level is in valid. A "division" is a segment, a square on the screen of the oscilloscope.
* A setting of ".5" i.e. means, that the height of a single square equals a voltage of 0.5 V. An amplitude of 1 V would have a size of two divisions vertical to the abscissa.
8. d.c./a.c. switch d.c. – d.c. and a.c. voltage displayed.
a.c. – only a.c. voltage displayed.
9. X-input and Y-input Electric input connect to the X-plate and Y-plate.


Example
Table below shows the sample display of direct current and alternating current when the time base is switched ON and OFF.
 Direct Current (Time Base Switched Off)  Direct Current (Time Base Switched On)
 Alternating Current (Time Base Switched Off)  Alternating Current (Time Base Switched On)