CATHODE RAY OSCILLOSCOPE (CRO)

Using a CRO , the wave shapes of alternating currents and voltages can be studied.  It can also be used for measuring voltage, current, power, frequency and phase shift.

IntroductionThe cathode Ray Oscilloscope or mostly called as CRO is an electronic device used for giving the visual indication of a signal waveform.It is an extremely useful and the most versatile instrument in the electronic industry.CRO is widely used for trouble shooting radio and television receivers as well as for laboratory research and design.

Block Diagram

Electron Gun Assembly

The electron gun assembly consists of an indirectly heated cathode, a control grid, a focussing anode and an accelerating anode and it is used to produce a focused beam of electrons.

Cathode Ray TubeIt consist of following parts asGlass EnvelopeIt is a conical highly evacuated glass housing which maintains vacuum inside it  and supports various electrodes.The inner wall of CRT between the neck and screen are usually coated with a conducting material known as aquadag. This coating is electrically connected to the accelerating anode so that the electrons which accidentally strike the walls are returned to the anode.  This prevents the walls from charging to a high negative potential

ScreenThe screen is coated with some fluorescent materials such as zinc orthosilicate, zinc oxide etc and is the inside face of the tube.When high velocity electron beam strikes the screen, a spot of light appears at the point of impact. The colour of the spot depends upon the nature of fluorescent material.

Deflection Plate AssemblyIt consists of two sets of deflecting plates within the tube beyond the accelerating anode and is used for the deflection of the beam.One set is called as vertical deflection plates and the other set is called horizontal deflection plates.The vertical deflection plates are mounted horizontally in the tube. On application of proper potential to these plates, the electron beam can be made to move up and down vertically on the screen.The horizontal deflection plates  are mounted vertically in the tube. On application of proper potential to these plates, the electron beam can be made to move right and left horizontally on the screen.

Trigger (Sync.) CircuitA sample of the input waveform is fed to a trigger circuit which produces a trigger pulse at some selected point on the input waveform. This trigger pulse is used to start the time base generator which then starts the horizontal sweep of CRT spot from left hand side of the screen.

Vertical AmplifierThe input signal is applied to vertical amplifier. The gain of this amplifier can be controlled by VOLT/DIV knob. Output of this amplifier is applied to the delay line Delay LineThe delay Line retards the arrival of the input waveform at the vertical deflection plates until the trigger and time base circuits start the sweep of the beam. The delay line produces a delay of 0.25 microsecond so that the leading edge of the input waveform can be viewed even though it was used to trigger the sweep

Power SupplyA high voltage section is used to operate CRT and a low voltage section is used to supply electronic circuit of the oscilloscope.

Time Base (Sweep) GeneratorThis produces a saw – tooth waveform that is used as horizontal deflection voltage of CRT. The rate of rise of positive going part of sawtooth waveform is controlled by TIME/DIV knob. The sawtooth voltage is fed to the horizontal amplifier if the switch is in INTERNAL position. If the switch is in EXT. position, an external horizontal input can be applied to the horizontal amplifier. Horizontal AmplifierThis amplifies the saw – tooth voltage. As it includes a phase inverter two outputs are produced. Positive going sawtooth and negative going sawtooth are applied to right – hand and left – hand horizontal deflection plates of CRT

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