Subject: Applied Physics 2

Topic: Motion Of Charged Particles In Electric And Magnetic Fields

Difficulty: High

A cathode ray oscilloscope is a very important electronic measuring instrument. It is used to display and measure electrical signals, time intervals phase shifts between two signals etc

CRO basically consists of CRT, Time base circuit, trigger circuit, Horizontal and vertical circuit and high voltage and low voltage power supply.

CRT: The main function of CRT is to generate, focus and accelerate the electron beam towards the screen. It consists of three main parts: electron gun assembly, deflection system and fluorescent screen. Electron gun assembly consists of cathode, grid and three anodes (A1preacclerating-A2 focusing and A3 accelerating). Cathode is a nickel cylinder and encloses a filament. On the passage of electric current through the filament electrons are emitted thermionically. Grid is always maintained at a negative potential and it regulates the passage of electron. Anodes are maintained at positive potential. A1 and A3 at relatively higher potential than A2. As a result, two electrostatic lenses are formed between them which focus or defocus the electron beam. By applying proper potential to the deflecting plates, the electron spot can be moved anywhere on the screen. The inner surface of circular front face is coated with fluorescent screen usually ZnO which glows when high energy electron strikes it.

Time base circuit: If a sine wave is applied to Y plates it is expected that on screen a sine wave is observed. But if there is no signal on X-deflection plate, we will get the image on screen as shown in Fig. below, that is a vertical line.

That means electron beam should be made to move vertically and horizontally at a uniform rate covering equal distance in equal interval of time and after completing one cycle it should return to starting point to repeat the cycle. The ramp voltage fulfills the above requirement.

The ramp voltage rises linearly with time. It starts from some negative maximum value. This sets the beam at left on the screen. With time voltage rises uniformly and move towards right side of the screen. It reaches the maximum which set the beam at right end of screen.i.e spot sweeps from left to right in steps with the cycle of ramp. So often ramp voltage is called as sweep voltage. The ramp voltage enables the x axis to be calibrated in terms of time. So it is called as time base and the circuit which generates ramp voltage is known as time base generator.

Display of signal:

As the signal is applied to Y plate and ramp to X plate, electron beam is subjected to two forces simultaneously acting perpendicular to each other. So the deflection of electron will be resultant of two forces.

At instant 1, signal voltage is zero and ramp is at negative maximum so it sets the beam horizontal but towards left of screen.

At instant 2, signal voltage is positive and ramp is towards lesser negative side that sets up the beam upward in the second quadrant.

At instant 3, signal voltage is zero and ramp voltage is zero that sets the beam at center of screen.

At instant 4, signal voltage is negative and ramp is positive that sets the beam downward in the fourth quadrant.

At instant 5, signal voltage is zero but ramp is positive maximum that sets the beam horizontal but to the right of screen. After this beam returns to position 1 and cycle repeats. Thus, a faithful display of screen is seen on screen.