Cathode Ray Oscilloscope

Cathode Ray Oscilloscope (CRO) uses a cathode ray tube to produce visible graphical representations of electrical signals. 





The graphs produced consist of a horizontal axis which is normally a function of time, and a vertical axis which is a function of the input voltage.

Many physical quantities can be converted into a corresponding electric voltage. The oscilloscope is a useful tool in many physics experiments.

The componentd in a cathode ray tube consists of a vacuum glass tube with an electron gun, a deflection system for deflecting the electron beam, and a flourescent coated screen.

Electron gun

In a cathode ray tube, a beam of electrons is produced by heating the filament with a small voltage supply. the power supply can be AC or DC. The electron beam emerging from the electron gun passes between two pairs of deflection plates, i.e. X and Y - plates mounted horizontally and vertically.

Deflection system

CRO has a fluorescent screen. When the screen is struck by a beam of electrons, wave forms will be traced out on the screen. The kinetic energy of the electrons is changed to light energy.

There is a bright spot on the screen when the beam strikes. By changing the vertical gain on the Y-plates, the beam is deflected vertically. The beam can be moved up and down and if it moves fast enough, the dots will appear as a line.

When an AC supply is connected to the Y-plates, the electron beam will move vertically. The amount of vertical movement can be amplified by increasing gain control. The vertical movement of the electron depends on the vertical gain control and it can be adjusted, using the VOLTS/DIV control. The control is adjusted so that the resulting display is neither too small nor too large, but it fits the screen.

The horizontal deflection plates or X-plates produced a left to right movement. The movement is produced by a circuit called the time base inside the oscilloscope. The time base produces a saw tooth wave form. During the rising phase ( the rising line) of the voltage, the spot is driven at a uniform rate from left to right across the screen. During the falling phase, (the straight vertical line downwards) the electron beam returns rapidly from right to left.

Sawtooth wave form


The spot is moving in a very short duration and will not appear on the screen. The time base has a changing voltage across the X-plates so that the spot moves from left to right across the screen again. The speed at which the spot sweeps across the screen horizontally can be controlled by altering the frequency of the time base.