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The Cathode ray Oscilloscope

The cathode ray oscilloscope (CRO) is a very useful instrument that has many applications in science.
A beam of electrons is shot down an evacuated tube by an electron gun. This beam passes between two sets of deflecting plates and then strikes a screen that glows when the beam hits it (fluorescence).

The most important parts of the CR0 are:
(a) electron gun - this produces the electron beam. The more electrons there are in the beam the brighter the spot on the screen;
(b) deflecting plates - by putting a voltage on each pair of plates the beam can be moved up and down or side to side.
Since the electron beam is negative it will be attracted towards a positive plate;
(c) fluorescent screen - this glows when the electron beam hits it.

You will find various controls on the front of an oscilloscope, this is what they do:
On/Off- obvious
Brightness - makes the beam brighter, this is sometimes linked to the on/off control
X shift - moves the spot across the screen
Y shift - moves the spot up and down the screen
X gain - magnifies movement in the X direction
Y gain - magnifies movement in the Y direction
Focus - obvious
Time base - makes the spot move from one side of the screen to the other at various speeds. Usually from 1 second to 1 millionth of a second for one crossing.

There are other controls but we will not deal with them here, but the time base is worth looking at more closely.

A voltage is applied to the X plates that pulls the spot across the screen, allows it to fly back again by changing sign and then pulls it across again, and so on. The voltage is called a SAW TOOTH wave and is shown in the diagram below. The bigger the voltage the further the spot gets pulled across, and the greater the frequency the faster it moves.

The input voltage is usually connected to the Y plates and this pulls the spot up and down as the time base moves it across the screen.

The following diagram shows you various patterns that can be made on the screen with two different inputs.

The speed of the time base will change what we see on the screen even if the input signal is kept the same. The following four diagrams show this.

Because the deflection of the spot depends on the voltage connected to the Y plates the CR0 can be used as an accurate voltmeter. The oscilloscope is also used in hospitals to look at heart beat or brain waves, as computer monitors, radar screens and is also the basis of the television receiver.

The cathode ray tube screen showing various inputs

(a) time base off (the small circles have been added to help you see the spot)

(b) time base on

© Keith Gibbs 2013