What is Cathode Ray Oscilloscope (CRO)？

What is Cathode Ray Oscilloscope (CRO)？

Definition

A cathode ray oscilloscope (CRO) is an electrical instrument for measuring, analyzing and visualizing waveforms and other electronic/electrical phenomena. As a high - speed X - Y plotter, it shows an input signal against another signal or time. Capable of analyzing waveforms, transient phenomena and time - varying quantities across a wide frequency range (from very low to radio frequencies), it mainly operates on voltage. Other physical quantities (current, strain, etc.) can be converted to voltage via transducers for display.

Key Operation

A luminous spot (from an electron beam hitting a fluorescent screen) moves on the display per input voltages. A standard CRO uses an internal horizontal ramp voltage ("time base") for left - to - right horizontal movement, with vertical movement controlled by the voltage under test, enabling stationary viewing of fast - varying signals.

Construction

Main components:

• Cathode Ray Tube (CRT)：A vacuum tube converting electrical to visual signals, with an electron gun and electrostatic deflection plates (vertical/horizontal). The gun produces a focused, high - speed electron beam; plates move the beam independently for screen positioning.

• Electronic Gun Assembly：Emits and shapes the electron beam, including heater, cathode (barium/strontium - coated for electron emission), control grid (regulates beam intensity via negative potential), and anodes (for acceleration at ~1500V). Focusing uses electrostatic methods (standard in CROs).

• Deflecting Plates：Two pairs: “Y plates” (vertical) and “X plates” (horizontal) control beam movement.

• Fluorescent Screen：CRT faceplate (flat for small, curved for large screens) coated with phosphor. Electrons striking it cause fluorescence (light emission).

• Glass Envelope：Highly evacuated, cone - shaped. Inner surface has aquadag (conductive coating connected to the accelerating anode) to aid electron focusing.

Working Principle

Electrons from the cathode pass through the control grid (negative potential adjusts intensity). Accelerated by anodes, focused, and deflected by plates per input voltages, they hit the screen, creating a visible spot to trace waveforms.

After passing through the control grid, the electron beam travels through the focusing and accelerating anodes. The accelerating anodes, at a high positive potential, converge the beam to a point on the screen.

Emerging from the accelerating anode, the beam then comes under the influence of the deflecting plates. With zero potential on the deflecting plates, the beam forms a spot at the center of the screen. Applying a voltage to the vertical deflecting plates deflects the electron beam upward; applying a voltage to the horizontal deflecting plates deflects the light spot horizontally.