CATHODE RAY OSCILLOSCOPE

The cathode ray oscilloscope [CRO] is an electronic device, which is capable of giving a visual indication of a signal waveform. It is widely used for trouble shooting radio and television receivers as well as laboratory work involving research and design. In addition the oscilloscope can also be used for measuring voltage, frequency and phase shift.

Cathode Ray Tube

A cathode ray tube is the heart of the oscilloscope. It is a vacuum tube of special geometrical shape and converts an electrical signal into visual one. A cathode ray tube makes available plenty of electrons. These electrons are accelerated to high velocity and are brought to focus on a fluorescent screen. The electron beam produces a spot of light wherever it strikes. The electron beam is deflected on its journey in response to the electrical signal under study. The result is that electrical signal waveform is displayed visually.

·         Electron Gun Assembly- The arrangement of electrodes which produce a focused beam of electrons is called the electron gun. It essentially consists of an indirectly heated cathode, control grid, a focusing anode, and an accelerating anode. The control grid is held at negative potential with respect to cathode whereas the two anodes are maintained at high potential with respect to cathode.

·         The cathode consists of a nickel cylinder coated with oxide coating and provides plenty of electrons. The focusing anode focuses the electron beam into a sharp pin –point by controlling the positive potential on it. The positive potential ( about 10,000 V) on the accelerating anode is much higher than on the focusing anode. Therefore this anode accelerates the narrow beam to a high velocity.

·         Deflection plate assembly-

1.      Vertical deflection plates

2.      Horizontal deflection plates

The vertical deflection plates are mounted horizontally in the tube. By applying proper potential to these plates, the electron beam can be made to move up and down vertically on the fluorescent screen. An appropriate potential on horizontal plates can cause the electron beam to move right and left horizontally on the screen.

·         Screen-The screen is the inside face of the tube and is coated with some fluorescent   material such as Zinc Orthosilicate, Zinc oxide etc. When high velocity electron beam strikes the screen, a spot of light is produced at the point of impact.

Action of CRT

 

• When the cathode is heated, it emits plenty of electrons. The control grid influences the amount of current flow. As the electron beam leaves the control grid, it comes under the influence of focusing and accelerating anode. As the two anodes are maintained at high potential, therefore they produce a field which acts as an electrostatic lens to converge the electron beam at a point on the screen.

• As the electron beam leaves the accelerating anode, it comes under the influence of vertical and horizontal deflection plates. If no voltage is applied to the deflection plates, the electron will produce spot of light at the center (point O ) of the screen. If the voltage is applied to vertical plates only, the electron beam and hence the spot of light will be deflected upwards (point O₁ ). The spot of light will be deflected downwards (O₂) of the portential on the plate is reversed. Similarly the spot of light can be moved horizontally by applying voltage across the horizontal plates.

Signal Pattern on Screen

 

• If the signal voltage is applied to the vertical plates and saw tooth wave to the horizontal plates, we get the exact pattern of the signal as shown in figure.

• When the signal is at instant 1, its amplitude is zero. But at this instant, maximum voltage is applied to the horizontal plates. The result is that the beam is at the extreme left on the screen as shown. When the signal is at instant 2, its amplitude is maximum. However the –ve voltage on he horizontal plate is decreased. Therefore the beam is deflected upwards by the signal and towards the right by the saw tooth wave. The result is that the beam now strikes the  screen at point 2. On similar reasoning, the beam strikes the screen at points 3,4 and 5. Therefore exact signal pattern appears on the screen.

Various controls on CRO

In order to facilitate the proper functioning of CRO, various controls are provided on the front panel of the CRO.

1. Intensity Control-The knob of intensity control regulates the bias on the control grid and affects the electron beam intensity.If the negative bias on the grid is increased, the intensity of electron beam is decreased, thus reducing the brightness of the spot.
2. Focus Contrl- It regulates the positive potential on the focusing anode. If the positive potential on this anode is increased, the electron beam becomes quite narrow and the spot on the screen is a pin-point.
3. Vertical position control- The knob of vertical position control regulates the amplitude of d.c. potential which is applied to the vertical deflection plates in addition to the signal. By adjusting this control, the image can be moved up or down as required.

Applications of CRO

1.      Examination of waveforms

2.      Voltage measurements

3.      Frequency measurements