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1. Introduction
Cathode Ray Tubes (CRTs) utilize deflection systems to control the path of the electron beam, enabling precise positioning on the screen. There are two primary types of deflection systems: electrostatic and magnetic.
Electrostatic Deflection
In electrostatic deflection, pairs of charged plates create electric fields that influence the electron beam’s trajectory. Vertical deflection plates control movement along the y-axis, while horizontal deflection plates manage the x-axis. This method is commonly employed in oscilloscopes and smaller CRT devices due to its rapid response time and simplicity.
Magnetic Deflection
Magnetic deflection utilizes varying magnetic fields generated by coils, known as deflection yokes, to steer the electron beam. This approach is prevalent in television sets and larger CRT displays, as it efficiently handles the broader deflection angles required for larger screens.
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2. Development of Deflection Systems
The concept of deflecting electron beams was integral to the evolution of CRT technology. Early CRTs employed electrostatic deflection, suitable for small displays and instruments like oscilloscopes. As the demand for larger screens grew, particularly in television technology, magnetic deflection became the preferred method due to its effectiveness over larger deflection angles. This transition was driven by the need for more efficient and scalable deflection mechanisms in consumer electronics.
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3. Basic Principles of Deflection Systems
Electrostatic Deflection
When a voltage is applied across the deflection plates, an electric field is established between them. As the electron beam passes through this field, it experiences a force perpendicular to its initial direction, causing it to deflect. The degree of deflection is proportional to the applied voltage, allowing precise control over the beam’s position.
Magnetic Deflection
In magnetic deflection, current flows through coils positioned around the CRT neck, generating magnetic fields. According to the Lorentz force law, a moving electron in a magnetic field experiences a force perpendicular to both its velocity and the magnetic field direction, resulting in beam deflection. Adjusting the current through the coils modulates the magnetic field strength, thereby controlling the beam’s position on the screen.
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4. Calculations: Beam Trajectory & Deflection
Electrostatic Deflection
The deflection angle (θ) of an electron beam in an electrostatic field can be determined using the following relationship:
θ ≈ (e x V x L) / (2 x d x m x v²)
Where,
- e = electron charge
- V = voltage across deflection plates
- L = length of deflection plates
- d = separation between plates
- m = electron mass
- v = initial velocity of electrons
This equation illustrates that increasing the applied voltage (V) or the length of the plates (L) enhances the deflection angle, while a higher initial electron velocity (v) reduces it.
Magnetic Deflection
For magnetic deflection, the radius (r) of the electron’s circular trajectory in a magnetic field is given by:
r = (m x v) / (e x B)
where,
- B = Magnetic field strength (measured in Tesla, T): It represents the intensity of the magnetic field applied by the deflection coils.
- m = Mass of the electron (measured in kilograms, kg): A constant value representing the mass of a single electron. The value of m = 9.109 × 10−31 kg
- v = Velocity of the electron (measured in meters per second, m/s): The speed of the electron as it enters the magnetic field. It depends on the accelerating voltage applied earlier in the electron gun.
- e = Charge of the electron (measured in Coulombs, C): A fundamental constant representing the magnitude of the electric charge of a single electron.
e = 1.602 × 10−19 C - r = Radius of the circular trajectory (measured in meters, m): The distance from the center of the circular path to the electron beam’s trajectory in the magnetic field. It determines the extent of deflection.
The deflection angle depends on the distance the electron travels within the magnetic field and the field’s strength. By controlling the current through the deflection coils, the magnetic field (B) can be adjusted, thereby modulating the beam’s trajectory.
5. Simple Explanation of Deflection Systems in CRT
The electron beam can be deflected using electric or magnetic field. As per the application of CRT, there are two types of deflection systems used in CRT – the electrostatic deflection system and the magnetic deflection system. They are explained as follows:
Electrostatic Deflection System
In this system, the electron beam is deflected with the help of electric field created between the pair deflection plates. There are four deflection plates in CRT: one pair of horizontal deflection plates and the other pair is of vertical deflection plates.
The plates are made up of copper or aluminium. The vertical deflection plates are fitted horizontally and the horizontal deflection plates are fitted vertically, as shown in the article of constructional details of CRT.
The vertical deflection plates are connected to the input signal (i.e. y-input) which we want to observe on the screen and the horizontal deflection plates are connected to internal sawtooth wave.
Due to this, strong electric field is produced between the plates. When electron beam enters in this field, it is deflected towards the plate which is positive. So the path of beam becomes parabolic. Then the beam comes out of this field in straight line and strikes on the screen at a particular point on the screen. The setup of vertical deflection plates is shown in following diagram –
Definition: The deflection sensitivity of CRT is defined as the deflection on screen in meters per volt of the deflection voltage.
S = D / Ed
Where,
D = deflection on screen in meters
Ed = deflection voltage (Volts)
The more is the PD across the plates, the stronger is the field and so more will be the deflection angle θ of the electron beam.
Magnetic deflection system
In this system, electron beam is deflected using external magnetic field. It is particularly used when we want to deflect the electron beam over a considerable distance on the screen i.e. if the screen dimensions are very large, for example, TV screen.
To create alternating magnetic field two pairs of deflection coils, known as yoke, are fitted on the neck of CRT. The pair of vertical deflection coils is connected to y-input of CRO and the pair of horizontal deflection coils is connected to internal sawtooth wave or external signal at x-input.
The setup of magnetic deflection system used in CRT is given in following diagram –
6. Applications
Electrostatic deflection is used in slow systems like in CRO, ECG monitors, EEG machines, etc.
The magnetic deflection system is extremely fast. It can deflect electron beam over a very large distance on screen, in extremely short time. So it is used in fast systems like TV, computer monitors (VDU), RADAR system, earthquake monitoring system, etc.
7. Conclusion
Understanding the deflection systems in CRTs is essential for grasping how these devices control electron beams to render images and waveforms. Both electrostatic and magnetic deflection methods have unique advantages and applications, with their development marking significant milestones in the advancement of electronic display technology.
For a comprehensive visual overview of CRT technology, you might find the following video informative:
