What is Cathode Ray Oscilloscope | Cathode Ray Oscilloscope Block diagram | CRO Tube | CRO Applications
Table of Contents
What is Cathode Ray Oscilloscope
Cathode ray oscilloscope is an electronic device that is widely used in laboratories. It has an X‑Y plotter. In which the luminous point acts as a ‘stylus’. The luminous point moves over the display area when applying input voltages
Block Diagram Of CRO

- Cathode Ray Tube
- Vertical Amplifier
- Horizontal Amplifier
- Time Base Generator
- Trigger Circuit
- Delay Line
- Power Supply
1.Cathode Ray Tube
Cathode Ray tube is an important part of the CRO. It generates an electron beam and accelerates the beam to a high velocity. This electron beam moves from the electron gun to the screen of the CRT.
The inside of the screen is coated with a fluorescent material. As soon as the electron beam reaches the screen, a bright light spot appears at that point
2.Vertical Amplifier
We call this the Y amplifier. It is used to control the voltage signal y amplitude. It has a calibrated alternator. You can adjust the amplification of the input signal by modifying this alternator. The rate of deflection in votIs / cm is obtained by the calibrated altemator switch
Horizontal Amplifier
This is called the X amplifier. It generates saw tooth voltage on the time base generator. This voltage is applied to the horozontal amplifier. This amplifier is used to control the time relationship and the frequency of the voltage
Time Base Generator
The A.C voltage is applied to the vertical deflection plates. Positive and negative supply of plates is available in succession. Thus the electronic spot will be given up and down. Moves on the screen according to the voltage. Thus a continuous vertical line is obtained. This creates a linear saw tooth wave from
Trigger Circuit
This creates a trigger pulse to start the horizontal sweep. For a steady waveform to appear on the CRO screen, the vertical input signal frequency must be equal to the time base generator frequency
The trigger circuit does this job. The trigger circuit produces the trigger pulse corresponding to the input waveform at a selected point. This trigger pulse is used to start the time base generator
Delay Line
The leading edge of the input waveform is used to initiate the trigger generator. It also starts the sweep generator. But sweep can be initiated only after 0.15ms. But before this 0.15 ms the leading edge of the input signal will pass.
Thus the wave form does not become a leading edge display. To avoid this the input wave form should be delayed. The delay line is used for this. The Delay line gives a delay of 0.25 ms to the vertical plates
Power Supply
Power supply The power supply includes a power transformer filter and stabilizers. This includes high voltage (HV) Selection and low voltage (LV) sections. The low voltage supply required for the CR tube is available from power transformers LT Secondary.
Similarly the H.T supply required for the accelerating anode is available from the HT Secondary. Thus different voltages in the power supply are applied to the electrode and the filament
Cathode Ray Tube

- Electron Gun Assembly
Electrongun is the name given to the system of electrodes that produce the electron beam. It has an indirectly heated cathode, control grid, focusing anode and OLD accelerating anodes. The negative supply to the control grid depends on the cathode. Similarly possib supply is given to both anodes
The cathode is a nickel cylinder. It is coated with oxide around it. Thus producing more electrons. The control grid is located around the cathode. It is a metal cylinder. There is a small hole in the middle of it. Through this comes the electron beam
The focusing anode transmits the electron beam‑m focus to the sharp point. This is because the focusing anode is given a positive potential. The accelerating anode is given a high positive potential. So it pulls the electrons to its side and converts it into a narrow beam
Deflecting Plate Assembly
The electron beam leaving the electron gun is passed through two set plates inside the tube. One set is called vertical plates and the other set is called horizontal plates. The vertical plates are arranged horizontally inside the tube. The beam became when the required potential was given to these plates
Moves up and down on the screen. Horizontal plates are arranged vertically inside the tube. Given the required potential, the electron beam deflects to the left and right sides of the screen
Screen
The screen is found on the front of the tube. It is coated with furescent material. zinc orthosilicate, zinc oxide etc. are flurescent materials. When electron beams hit the screen, a light spot appears. Its color coated with depends on the flurencent material
Envelope
It is a conical glass tube that is completely vacuumed. Its interior is vacuum and bears different electrodes. This coating is applied to the inner wall of the CRT, ie the area between the strip and the screen, which is lined with aquadag material — accelerating anode. This creates a secondary emission screen where the electrons that collide with the wall end up causing some accidents on the wall
Because electron bombardment is happening. To prevent this, negative potential must be given
Applications
1.Measurement Of Frequency
This system uses a lissajous pattern to compare two frequencies. It uses a sinsoidal oscillator that can adjust the frequency. The output of this oscillator is taken and given between a set of deflecting plates. have to give. The unknown frequency can be measured from the available lissajous pattern

Fv=Number of times tangent touches top or bottom/Number of times tangent touches either side
In this we have to draw two lines. One should be horizontal and the other vertical and should not cut any part
Phase Measurement

To measure the phase angle of two sinusoidal voltages of equal amplitude and frequency we need to give them to the X and Y plates of the cro
An ellipse is available in which we need to find the value of A, B from this ellipse. We can calculate the phase angle α by the following expression
Sinα=A/B
Study of waveforms
We need to give the signal we want to study to the vertical input terminal of the CRO. Give the saw tooth wave to the horizontal input. Adjust the controls and bring the screen to the correct waveform
Voltage measurement
Give the signal to the vertical deflection plates. A vertical line is visible on the screen. The height of the vertical line depends on the peak to peak magnitude of the voltage we have given. Multiply this height by the deflection sensitivity to get the peak voltage value
Current Measurement
The signal must be transmitted through a known resistor. The resulting voltage should be measured with CRO. Then find the current to be measured by ohm’s law
Time Base Generator

When AC voltage is applied to vertical deflection plates, the plates alternately become postive and negative potential. The resulting electron beam moves up and down at the frequency of the applied voltage. Now a vertical line is visible on the screen due to the visual stability
It is necessary to apply a sweep voltage or time base voltage to the horizontal deflection plates to display the waveform of a given voltage on the screen
This sweep voltage is periodic. Such a characteristic voltage is called the ramp voltage. Also this voltage is called ramp voltage. Also this voltage increases slowly from zero and decreases rapidly to zero after reaching a certain value. Hence it is called saw tooth voltage. The circuits that make up saw tooth waveforms are called time base circuits
During sweep time (Ts) the beam moves from left to right on the screen. By increasing the amplitude of the ramp voltage, the beam moves to the right side of the screen. During retrace or flyback time (Tr) the beam rotates rapidly from right to left of the screen.
Thus a thin line is visible on the screen when turned on. To avoid this, too much negative voltage is applied to the control grid. This blankouts the electron beam from the electron gun during retrace time