It is very versatile electronic device. It is in fact a high speed graph plotting device. It displays the size or shape of an electrical signal on the screen. It provides accurate time and amplitude measurements of voltage signals over a wide range of frequencies. It provides visual signals images of varying electrical quantities. It provides accurate time and amplitude measurements of voltage signals over a wide range of frequencies. Also, it provides visual signals images of varying electrical quantities. It is very versatile electronic device. It is in fact a high speed graph plotting device. It displays the size or shape of an electrical signal on the screen.
Principle
When a charged particle moves in an electrical field, a force acts on it.
Construction
Cathode rRay oscilloscope consists of ;
- Electron Gun
- Deflection system
- Display system
1. Electron Gun
The eelectron gun produces a beem of electrons. It consists of ;
- Filament
- Cathode
- Grid
- Accelerating Anode
- Focusing Anode
1. Filament
When current passed through the filament, it flows and heat from it heats the cathode.
2. Grid
It is at negative potential relate to cathode. It controls the number of electrons reaching the screen and thus controls the brightness of the spot on the screen. It is at negative potential relate to cathode. It controls the number of electrons reaching the screen and thus controls the brightness of the spot on the screen.
3. Cathode
When cathode is heated, it emits electrons by thermionic emission.
4. Accelerating Anode
It is at zero potential. Its main function is to accelerate the electrons.
5. Focusing Anode
IIt is at positive potential relate to cathode. The electrons that pass through this Anode are focused into a fine beam.
2. Display System
The sscreen of CRO is coated with a fluorescent material (zinc sulphide). The zinc sulphide gives a glow of light when electrons collide with it.
3. Deflection System
TThe beam of electrons from anodes firstly passes through a pair of horizontal deflecting plates called x-plates which deflect the beam horizontally on the screen i.e. parallel to x-axis and then between two vertical plates called Y-plates which deflect the beam vertically on the screen i.e. along the y-axis.
Working of Cathode Ray Oscilloscope
Time base generator
It’s output wave form is saw-tooth. The voltage increases linearly with time for period T and then drop to zero. As this voltage is applied across x-plates, the spot moves along x-axis for a time T. As after time T, the saw-tooth voltage becomes zero, so the spot rapidly reaches to its initial position at the end of each period T. If the time period is very small, the spot moves so quickly that we see just a bright line on the screen. As this voltage is applied across x-plates, the spot moves along x-axis for a time T. As after time T, the saw-tooth voltage becomes zero, so the spot rapidly reaches to its initial position at the end of each period T.
Voltage across x-plates
The voltage that is applied across the x-plates is usually provided by a circuit that is built in CRO (known as time base generator).
Voltage across y-plates
Of a sinusoidal is applied across y-plates with time base generator off, the spot moves vertically up and down at the frequency of applied AC voltage. This produces a vertical straight line on the screen whose length represents the peak to peak value of applied voltage. If sinusoidal voltage is applied y-plates time with base generator On, the vertical straight line now spread out and appears as sinusoidal trace on the screen.
The pattern will appear stationary only if the time T is equal to or some multiple of the time of one cycle of the voltage on y-plates. It is thus necessary to synchronize the frequency of the time base generator with the frequency of the voltage at the y-plates. This is possible by adjusting the synchronization controls provided on the front panel of the CRO.
Uses of CRO
CRO is used to
- Measure the time period of AC signals.
- Measure the instantaneous value, peak value.
- Display the wave form of a given voltage.
- Measure the phase difference between two voltages.
- Measure the voltage, frequency and phase of the input
