CRO Working Principle | Cathode Ray Oscilloscope


CRO Working | Cathode Ray Oscilloscope: Cathode Ray Oscilloscope i.e. CRO is an instrument used for testing and observing the constantly varying signal voltages. It usually gives a two-dimensional plot of one or more signals parallel as a function of time. CRO device has many applications mostly in electronics and electrical. Cathode Ray Oscilloscope has a fast processing and plots X-Y pattern at a very fast rate

Actually, cathode ray oscilloscope is very fast X-Y plotters that can display an input signal versus time or another signal.

CRO Working Principle

Construction of CRO consists of Five divisions:

  • Electron gun.
  • Deflection plate system.
  • Fluorescent screen.
  • Glass envelope.
  • Base

Look to the below CRO Block diagram to know more about the five divisions.

cathode ray oscilloscope, CRO, CRO working, CRO principle

Let us have some detailed discussion about the different division of CRO i.e. Cathode Ray Oscilloscope.

Electron Gun: Electron Gun as the name suggest is the source which emits electrons at a very high speed. The electrons so emitted are much more energy, acceleration, and focused beam. Electron Gum itself consist of six different parts namely:

  • Cathode
  • Heater
  • Grid
  • Pre-Accelerating anode
  • Focusing Anode
  • Accelerating Anode

A layer of Barium oxide deposited at one end of the cathode is indirectly heated at a moderate temperature so as to gain high emission. The control grid is present next to it, controls the intensity and no of electrons with its negative bias effect. This control grid is made up of nickel. Now after the control grids, these electrons are passed through the pre-accelerating and accelerating anodes to get accelerated. Both the accelerating anodes are connected to 1500 volts. The focusing anode is used to focus the beam of electrons produced.

Deflection plate system: Beyond the accelerating anode there are two sets of deflection plates used to deflect the beam to the desired path.

  • Vertical Deflection Plate: These plates are assembled horizontally in the tube. With this, the electron beam can be shifted up and down on the screen.
  • Horizontal Deflection Plates: These plates are assembled vertically in the tube. With this, the electron beam can be shifted left and right on the screen.

Fluorescent Screen: Due to the bombardment of the high-velocity beam, a spot of light appears on the screen due to the impact. The color of the beam depends on the nature and properties of the fluorescent material.

CRO Working | Cathode Ray Oscilloscope Principle

cro working, cathode ray oscilloscope, cro principle

So as we have discussed the different parts of Cathode Ray Oscilloscope. CRO is mainly used in troubleshooting in video and radio signals. Electron Gun emit electrons as the cathode is given some energy in form of heat. A large number of electrons are produced which passes through the control grid. Control grids as we discussed controls the electron flow. Negative Potential on the control grid lessens the number of electrons and intensity bombarding the screen. Due to this a dim spot will light on the fluorescent screen. If the negative potential is reduced then more electrons will strike on the screen and hence will give bright light.

Result: The intensity of spot can be controlled by managing the negative potential on the control grid.

Now after coming out of the control grid, the electron beam passes from the accelerating and focusing anodes. Due to the high positive potential of these anodes, they produce field of the electrostatic lens to converge the beam at one point on the screen.

After passing from accelerating anode, the beam is influenced by vertical and horizontal deflection plates.

Note: If you want to produce the beam spot at the center of the screen than the voltage applied to the deflection plates should be zero.

  • When the voltage is applied to the VDP i.e vertical deflection plates only then the beam spot will be shifted either upwards or downwards.
  • When the voltage is applied to the HDP i.e horizontal deflection plates only then the beam spot will be shifted either left or right.

cro working, cathode ray oscilloscope, cro principle

Application of CRO

  • Examination of Waveform: Most important application of CRO is to observe the waveform patterns of different signals.
  • Voltage measurement
  • Current measurement

So this was all about CRO Working Principle | Cathode Ray Oscilloscope. We will discuss more Useful Practical applications of Thermal Expansion-Physics Concepts in our next session. If you have any further information then you can share us by commenting in the comment box below. Stay tuned to us at EducationTimes.


  1. after the spot light appearance…how the electrons goes back to the anodes…how this process is going on