Simple treatment of cathode-ray oscilloscope

Deflecting the electron beam

The beam of electrons behaves like an electric current but without the wire. It means it can be deflected by electric field. In CRO the Y-plates are charged by the input signal provided from the source. When the alternating voltage is connected at the Y-plates, they attract the electrons towards the positive terminal and repel at the negative terminal. The continuous change of potential at the Y-plates moves the beam up and down.
The X – plates are called time-base circuit. It moves the beam horizontally at constant speed. Repeating this process means a horizontal line appear on the screen.
With the time-base circuit switched on and a variable signal connected to Y-plates, CRO will display the wave form picture on the screen.


Measuring potential difference (or voltage)
Oscilloscope measures time-varying voltages and gives you a graph of voltage (y-axis) vs. time (x-axis). In terms of connection in the circuit they are exactly like voltmeters. It has two terminals which are connected across the two points where you want to measure the voltage. However, what you get from an oscilloscope is not what you get from a voltmeter. When you measure a signal with an oscilloscope, you get a scaled picture of the voltage-time function on the screen. That picture might look like this one if you were measuring a sinusoidal voltage, in which the amplitude of a waveform expresses the voltage and width expresses time period (T).


A CRO has a circuit – the time-base circuit – that moves the dot across the screen, from left to right, at a constant speed and then returns the dot very quickly to the start. Repeating this process quickly means the dot appears as a line across the screen.

A CRO can be used as a very high resistance voltmeter. If an unknown voltage is connected to the y-plates, the deflection produced in the beam can be compared to reference measurements and a value for the voltage obtained.

With the time-base circuit switched on and a variable signal connected to the y-plates, a CRG can be used to show how a waveform varies. This can be used to measure the frequency of a signal.

There are two settings on the oscilloscope
-volts/cm or volts/div: this setting represents the vertical axis or y-axis of the screen. It is used to measure the voltage of the source applied.
-Time base in ms/div or μs: this setting represents the horizontal axis or x-axis of the screen. It is used to measure the time period of the source wave applied. From the time period the frequency of the oscillation can be calculated by the formula T= 1/f
ms and μs are the time period in millisecond and microsecond of the wave.