What is a Cathode Ray Oscilloscope

A Cathode Ray Oscilloscope or most commonly known as a ‘CRO’ is a device used for the analysis and measurement of electric wave forms by displaying them on a fluorescent screen. Along with being a very popular instrument in electronics and electrical laboratories, it is also used on a wide scale in many industries of a similar field for testing purposes.

A CRO consists of the following parts:-

An electron gun assembly consisting of a cathode, pre accelerating anode, focusing anode and an accelerating anode. A pair of deflecting plates in the horizontal plane and another pair of deflecting plates in the vertical plane A Fluorescent Screen A glass tube that envelops the whole of the apparatus described above.

The working of a CRO is very easy to understand. A high negative voltage is applied to the cathode element causing it to heat up and therefore emit a stream of electrons. The pre accelerating and accelerating anodes connected to a positive potential causes this stream to accelerate away from the cathode and towards the screen. The focusing anode is used to convert this stream of electrons in to a highly focused beam. Thus the function of the whole electron gun assembly here is to provide a highly accelerated and focused beam of electrons.

This beam of electrons then passes through the deflecting plates and strikes the screen causing a luminous spot to form there. Now if a time sweep voltage in the form of a saw tooth wave is applied across the pair of horizontal plates, then due to the low inertia of the electron beam it will get deflected by the electric field developed because of the sweep voltage on the horizontal plates and thus follow changes in it. Since the waveform is saw tooth, it will cause the luminous spot on the screen to move in a horizontal direction to create a time base.

Now the waveform to be displayed and analyzed is applied across the vertical plates. This voltage now causes the electron beam to deflect in the Y direction along with its deflection in the X direction by the time sweep voltage. Thus the waveform is displayed on the screen by the movement of the luminous spot on the screen in both X and Y directions simultaneously. This is how a CRO works. These days sophisticated CRO’s that can respond to waveforms of frequency as high as 1 GHz and as low as 20 Hz are available. Also these can be used for analyzing more than one wave form at a time by using multiple beam tubes or by using beam splitting technologies.

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