Detailed Notes Cathode Ray Oscilloscope - GATE Notes & Videos for Electrical Engineering

What is a Cathode Ray Oscilloscope?

A Cathode Ray Oscilloscope (CRO) is an electronic instrument used in laboratories to display, measure and analyse electrical waveforms. It is effectively a very fast X-Y plotter that shows one electrical quantity (usually voltage) against another (commonly time) on a fluorescent screen. The visible trace is produced by a luminous spot formed when an electron beam strikes the phosphor-coated screen. The beam moves in response to the variations of the input signal, enabling visual observation of rapidly changing instantaneous quantities.

The CRO normally operates with voltages as the measured quantity. Other physical quantities such as current, pressure or acceleration can be observed after conversion to voltage using suitable transducers. This makes the CRO a versatile tool for electrical and electronic measurements.

Construction of a Cathode Ray Oscilloscope

The principal component and the heart of a CRO is the cathode ray tube (CRT). The CRT and associated circuits form a system that generates, focuses, deflects and displays the electron beam. The CRT is normally enclosed in a glass envelope and mounted on a base with electrical connections.

Internal Structure of CRT

Major parts of the cathode ray tube are:

1. Electron gun
2. Deflection plate system
3. Fluorescent screen
4. Glass envelope
5. Base and external connections

Electron gun

The electron gun is the source of the accelerated, energised and focused electron beam. It typically consists of a heater, a cathode, a control grid, a pre-accelerating anode, a focusing anode and an accelerating anode.

• A cathode coated with barium oxide is indirectly heated to emit electrons; the heater provides the required temperature for thermionic emission.
• The emitted electrons pass through a small aperture in the control grid (often made of nickel). The control grid is negatively biased to control the number of electrons passing through and therefore the intensity (brightness) of the spot on the screen.
• Electrons are accelerated by the pre-accelerating and accelerating anodes, typically connected to a common positive potential (examples: of the order of 1500 V for the accelerating system in many CRTs).
• The focusing anode produces the desired beam convergence. Its potential is adjustable (typically a few hundred volts; an example value often used in CRO descriptions is 500 V) to change the effective focal length and obtain a sharp spot on the screen.
• Focusing methods are either electrostatic focusing or electromagnetic focusing. The CRT design often uses electrostatic focusing because of its compactness and ease of voltage control.

Electrostatic focusing

The electrostatic focusing system uses electric fields to act on the beam in a lens-like manner. The force on an electron in an electric field is given by -qE, where q is the electron charge (q = 1.6 × 10^-19 C), and E is the electric field intensity. The negative sign indicates that the force on a negatively charged electron is opposite to the direction of the field.

Two illustrative configurations used to explain electrostatic focusing are given below.

Case 1: Parallel plate focusing

Two plates A and B are held at potentials +E and -E respectively. The electric field is perpendicular to the plate surfaces and directed from A to B. Equipotential surfaces are orthogonal to the field lines. When the electron beam passes through this plate system, it is deflected opposite to the electric field direction. The amount of deflection (and hence the effective focusing) can be varied by changing the plate potentials.

Case 2: Concentric cylinder focusing

A pair of concentric cylinders with a potential difference between them produces curved equipotential surfaces. The curvature gives a focusing action analogous to a lens. Consider a curved equipotential surface S: the potential on one side is +E and on the other side is -E. An electron beam incident at an angle A to the normal will be deflected to angle B after crossing the surface. The tangential component of velocity remains unchanged while the normal component changes due to the normal electric force. Equating tangential components yields V₁ sin(A) = V₂ sin(B), so

sin(A)/sin(B) = V₂ / V₁

This relation shows beam bending across the equipotential surface and explains focusing action in this configuration.

Electrostatic deflection

Deflection plates produce a transverse electric field that deflects the electron beam. The usual arrangement is two parallel plates separated by a distance d with a potential difference across them, producing a field perpendicular to the beam's primary direction (x-axis). The deflection is measured on the phosphor screen placed a distance L beyond the deflection plates.

Electrostatic Deflection 

Refraction of an Electron Beam

Consider an electron moving initially along the x-axis and entering the region between the plates. The electric field between the plates produces a force in the y-direction; there is no force along x. The electron therefore acquires a transverse acceleration and follows a curved trajectory. Using conservation of energy for the axial acceleration and Newton's laws for transverse motion gives the deflection on the screen.

From the accelerating potential we obtain the electron speed v after acceleration.

The transverse acceleration a_y is given by the force divided by mass:

Combining axial motion and transverse motion yields the equation of the trajectory:

The slope (dy/dx) of the trajectory within the deflecting region is:

If l is the length of the deflecting plates and L is the distance from the end of the plates to the screen, the deflection D observed on the screen is:

The final expression for the total deflection D at the screen becomes:

From this expression the deflection sensitivity (deflection per unit deflecting voltage) is:

Fluorescent screen

The inner surface of the CRT screen is coated with a phosphor that emits light when struck by electrons. The persistence and brightness of the phosphor determine how the trace appears (sharpness, intensity, afterglow).

Graticule

A graticule is a grid of lines used as a scale on the CRT screen to measure amplitudes and time intervals directly from the trace. Common graticule types are:

• Internal graticule: Deposited on the inner surface of the CRT faceplate. It gives negligible parallax error but cannot be changed.
• External graticule: A transparent graticule placed externally on the screen (or on a separate glass) that can be changed as needed. There may be slight parallax error if not aligned precisely.
• Illuminated external graticule: An external graticule with its own illumination to improve visibility under different screen brightness conditions.

Basic Circuit Diagram and Major Systems of a CRO

The basic CRO front-end and supporting circuits are organised into several functional blocks. The principal parts are the vertical deflection system, the horizontal (time-base) deflection system, and the synchronisation and control circuits. Each block contains attenuators, amplifiers and switching networks to condition signals for display.

Vertical deflection system

The vertical deflection system accepts the signal under test (after attenuation and impedance matching) and applies the appropriate voltage to the vertical deflection plates. Key elements are the input attenuator and a chain of vertical amplifiers. These amplifiers:

• Provide gain to display small signals with sufficient amplitude on the screen.
• Maintain low distortion and adequate bandwidth for the frequencies of interest.
• Present the correct input impedance, commonly 1 MΩ or 50 Ω depending on the probe and mode.

Horizontal deflection system and time base

The horizontal deflection plates are driven by the time-base (sweep) generator. The sweep provides a sawtooth (or an equivalent waveform) so that the electron beam moves horizontally at a known constant speed; the vertical input signal is then plotted versus time. The horizontal path includes horizontal amplifiers to drive the plates with sufficient amplitude across a wide range of sweep speeds.

Common sweep types are:

• Free-running (recurrent) sweep: The sawtooth generator runs continuously, repeatedly producing sweeps at a fixed rate.
• Triggered sweep: The sweep is initiated only when a specified condition (trigger) is met in the input signal; useful for non-periodic or single-shot events.
• Driven sweep: The sweep runs continuously but is synchronised (locked) to the input signal to maintain a stable display.
• Non-sawtooth sweeps (special modes): Includes Lissajous or X-Y modes used for phase comparison and frequency measurements where the horizontal input is another signal rather than a time base.

Synchronisation (Triggering)

To obtain a stationary and repeatable waveform on the screen, the sweep must be synchronised (triggered) with the signal under test. Typical synchronisation sources selectable by a synchronisation selector are:

• Internal: Trigger derived from the input signal after the vertical amplifier.
• External: Trigger supplied by an external source connected to the trigger input.
• Line: Trigger derived from the mains power line (useful for signals locked to the power frequency).

Other controls and circuits

• Intensity modulation: The brightness of the trace can be modulated by inserting a modulation signal between the cathode and the ground; this causes local brightening or dimming of the trace.
• Positioning controls: Small DC offsets are applied to the deflection plates (via potentiometers) to shift the display vertically or horizontally and centre the trace on the screen.
• Focus control: The focal length of the focusing electrode is altered by changing its potential; this adjusts spot sharpness and corrects astigmatism.
• Intensity control: Varying the control grid bias changes beam current and hence the brightness of the spot.
• Calibration circuit: An internally generated square-wave calibration voltage of known amplitude is usually provided to check vertical sensitivity and time base accuracy.
• Astigmatism correction: By adjusting focus settings and correcting electrodes, the spot shape can be made circular and sharp, reducing the astigmatism effect.

Practical notes, applications and measurements

Practical use of a CRO requires attention to probes, input impedance, bandwidth, and grounding. Important common measurements and applications include:

• Measuring amplitude, frequency and period of waveforms using the graticule and calibrated time base and vertical sensitivity.
• Observing waveform shape, rise time and distortion for amplifiers and digital circuits.
• Comparing phase between two signals in X-Y mode or using Lissajous patterns for frequency ratio measurements.
• Triggering on single-shot or rare events using triggered sweep modes.
• Using external calibration and probes (1×, 10×) to extend input range and maintain accurate amplitude readings.

Summary

The Cathode Ray Oscilloscope is a fundamental laboratory instrument that converts electrical voltages into a visible trace by controlling and deflecting an electron beam inside a CRT. Understanding the electron gun, focusing systems, deflection mechanisms, sweep (time-base), and synchronisation is essential to use a CRO effectively for measurement and waveform analysis. Correct use of the graticule, calibration signals and probe compensation ensures accurate amplitude and time measurements.