The CRO stands for a cathode ray oscilloscope. It is typically divided into four sections which are display, vertical controllers, horizontal controllers, and Triggers. Most oscilloscopes are used probes and they are used for the input of any instrument. The waveform can be analysed by plotting amplitude along with the x-axis and y-axis. The applications of CRO are mainly involved in radio, and TV receivers, also in laboratory work involving research and design. In modern electronics, the CRO plays an important role in electronic circuits.
Block Diagram of CRO
The following block diagram shows the general-purpose CRO contraction. The CRO recruits the cathode ray tube and acts as a heat of the oscilloscope. In an oscilloscope, the CRT produces the electron beam which is accelerated to a high velocity and brought to the focal point on a fluorescent screen.
Thus, the screen produces a visible spot where the electron beam strikes with it. By detecting the beam above the screen in reply to the electrical signal, the electrons can act as an electrical pencil of light
which produces a light where it strikes.CRO Block Diagram.
To complete this task we need various electrical signals and voltages. This provides the power supply circuit of the oscilloscope. Here we will use high voltage and low voltage. The low voltage is used for the heater of the electron gun to generate the electron beam. A high voltage is required for the cathode ray tube to speed up the beam. The normal voltage supply is necessary for other control units of the oscilloscope.
The horizontal and vertical plates are placed between the electron gun and the screen, thus it can detect the beam according to the input signal. Just before detecting the electron beam on the screen in the horizontal direction which is in X-axis a constant time-dependent rate, a time base generator is given by the oscillator. The signals are passed from the vertical deflection plate through the vertical amplifier. Thus, it can amplify the signal to a level that will be provided the deflection of the electron beam.
If the electron beam is detected in the X-axis and the Y-axis a trigger circuit is given for synchronizing these two types of detections. Hence the horizontal deflection starts at the same point as the input signal.
Working Principle
The CRO working principle depends on the electron ray movement because of the electrostatic force. Once an electron ray hits a phosphor face, then it makes a bright spot on it. A Cathode Ray Oscilloscope applies the electrostatic energy on the electron ray from two vertical ways. The spot on the phosphor monitor turns due to the effect of these two electrostatic forces which are mutually perpendicular. It moves to make the necessary waveform of the input signal.
Construction of Cathode Ray Oscilloscope
The construction of CRO includes the following.
Cathode Ray Tube
Electronic Gun Assembly
Deflecting Plate
Fluorescent Screen For CRT
Glass Envelop
Cathode Ray Tube
The CRO is the vacuum tube and the main function of this device is to change the signal from electrical to visual. This tube includes the electron gun as well as the electrostatic deflection plates. The main function of this electron gun is used to generate a focused electronic ray that speeds up to high frequency.
The vertical deflection plate will turn the ray up & down whereas the horizontal ray moved the electrons beams from the left side to the right side. These actions are autonomous from each other and thus the ray may be located any place on the monitor.
Electronic Gun Assembly
The main function of the electron gun is to emit electrons to form them into a ray. This gun mainly includes a heater, a grid, a cathode, and anodes like accelerating, pre-accelerating & focusing. At the cathode end, the strontium & barium layers are deposited to obtain the high electrons emission of electrons at a moderate temperature, the layers of barium, and are deposited at the end of the cathode.
Once the electrons are generated from the cathode grid, they it flows throughout the control grid which is generally a nickel cylinder through a centrally situated co-axial by the axis of CRT. So, it controls the strength of the generated electrons from the cathode.
When electrons flow throughout the control grid they it accelerates with the help of a high positive potential which is applied to the pre-accelerating or accelerating nodes. The electron ray is concentrated on electrodes to flow throughout the deflection plates like horizontal and vertical & supplies on to the fluorescent lamp.
The anodes like accelerating & pre-accelerating are connected to 1500v & the focusing electrode can be connected to 500v. The electron ray can be focused on using two techniques like Electrostatic & Electromagnetic focusing. Here, a cathode ray oscilloscope utilizes an electrostatic focusing tube.
Deflecting Plate
Once the electron ray leaves the electron gun then this ray will pass through the two sets of the deflecting plate. This set will generate the vertical deflection that is known as the Y plate’s otherwise vertical deflecting plate. The set of the plate is used for a horizontal deflection which is known as X plate’s otherwise horizontal deflection.
Fluorescent Screen of CRT
In the CRT, the front face is known as the faceplate, For the CRT screen, it is flat and its size is about 100mm×100mm. The CRT screen is somewhat bent for bigger displays and the formation of faceplate can be done by pressing the molten glass into a form & after that heating it.
The inner face of the faceplate is covered by using phosphor crystal to change the energy from electrical to light. Once an electronic ray hits the phosphor crystal, the energy level can be enhanced & thus light is generated throughout phosphorous crystallization, so this occurrence is known as fluorescence.
Glass Envelope
It is an extremely evacuated conical form of construction. The inside faces of the CRT among the neck as well as the display are covered through the aquadag. This is a conducting material that acts like a high-voltage electrode. The surface of the coating is connected electrically toward the accelerating anode to help the electron to be the centre.
Phase Measurement
We can measure the phase difference θ between two waveforms with the help of CRO. For this we want to use both input terminals i.e. y-input and x-input of CRO.
To measure the phase difference between two waveforms, their frequencies must be same. Connect the two waveforms randomly at x-input and y-input of CRO. Keep time/div knob in x-y mode. Now a Lissajou’s pattern of an ellipse is obtained as shown below.
Lissajou’s pattern of an ellipse is obtained on the CRO screen
Measure the displacement “a” and “b” and then use the following equation to calculate the phase difference between two waveforms –
θ = sin-1(a/b)
Special Cases
The special cases obtained while measuring phase difference are given below –
θ = 0° or 360°θ = 90° or 270°θ = 180°
Frequency Measurement
We can measure frequency with the help of CRO, by using two different methods. The first method is called direct method and the second method is called relative frequency measurement method.
Direct Method:In this method, the unknown frequency AC signal is connected to y-input of CRO. A waveform is obtained on the screen as shown below.
Now measure the number of divisions covered by one complete cycle of the wave. Note down the time/div knob setting. Then using the formula we can calculate the frequency –
Time (t) required for one cycle = number of div. × time/div
Unknown frequency, f = 1 / t
In above diagram, the frequency will be 10kHz.
Relative Frequency Measurement Method: We can measure the unknown frequency fx by comparing it with a known frequency value, say fy.
To measure the frequency using this method, the phase difference between the two waveforms must be zero.
Connect unknown frequency signal at x-input and known frequency signal at y-input. Keep time/div knob in x-y mode. Now a Lissajou’s pattern of loops is obtained as shown below.
Count the number of loops cut by x-axis and then y-axis and use the formula to calculate the unknown frequency fx, as follows –
fy / fx = (number of loops cut by x-axis) / (number of loops cut by y-axis)
In above example, suppose the known frequency value is fy= 300Hz, then the unknown frequency will be fx= 100Hz.
Exercise
Calculate the phase difference between two waveforms A and B, if a Lissajou’s pattern of an ellipse is obtained on the screen, such that a = 1cm and b = 1.414cm. (Ans: 45° )
A known frequency signal of frequency 500 Hz is connected to x-input of CRO. The unknown frequency signal is connected to y-input. A Lissajou’s pattern of an “8” is obtained on the screen. Then calculate the value of unknown frequency. (Ans: 250Hz)
What will be the value of unknown frequency connected at x-input of CRO, if a single loop is obtained on the screen? The known frequency at y-input is 1kHz. (Ans: 1 KHz)
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