Cathode Ray Oscilloscope

1. Cathode-ray tubes have become part of everyday life.2. They can be found in the screens of television sets and computer monitors.3. In the Physics laboratory, we use the cathode-ray tube in the oscilloscope to study

waveforms.4. In SPM, you need to know

• How cathode ray is produced (Thermionic emission and electron gun)• The characteristics of cathode ray (Study by using the Maltese Cross Tube and Deflection Tube).• the structure of a Cathode Ray Oscilloscope• how to operate a Cathode Ray Oscilloscope• the uses of a Cathode Ray Oscilloscope

Cathode Ray Oscilloscope

(Cathode Ray Tube Television)

Thermionic Emission

1.Thermionic emission is a process of emission of charge particle (known as

thermion) from the surface of a heated metal.

2.The charge particles normally are electrons.

3.The rate of emission (number of electrons emitted in 1 second) is affected by 4 factors, namely

a.the temperature of the heated metal,When the temperature of the metal increase, the emission rate of electron will increase.b.the surface area of the heated metal,When the surface area of the metal increase, the emission rate of electron will increase.c.the types of metalThe rates of thermionic emission are different with regard to different types of metals.d.the coated material on the surface of the metal.If the surface is coated by a layer of barium oxide or strontium oxide, the rate of emission will become higher.

Thermionic Diode

• Thermionic emission is applied in thermionic diode.• A diode is an electrical component that only allowed current flows in one

direction.

• Electrons can only released from the tungsten filament (when it is hot) and move toward the anode which is connected to the positive terminal.

• Electrons are not allowed to move in the opposite direction because no electrons will be released from the anode.

• As such, the electrons can only move from left to right but not the other way round.

The cathode-ray oscilloscope (C.R.O.) consists of the following components:

• The electron gun.• The deflecting plates.• A fluorescent screen.

Parts of Electron Gun Function

Filament To heat the cathode. Cathode Release electrons when heated by filament. Grid The grid is connected to a negative potential. The more negative this potential, the more

electrons will be repelled from the grid and fewer electrons will reach the anode and the screen.

The number of electrons reaching the screen determines the brightness of the light. Hence, the negative potential of the grid can be used as a brightness control.

Focusing Anode and The other feature in the electron gun is the use of the anode. The anode at positive potential accelerates the electrons and the electrons are focused into a

fine beam as they pass through the anode.

Accelerating anode

Part of the

deflecting system Function

Y-plate The Y-plates will cause deflection in the vertical direction when a voltage is

applied across them.

X-plate On the other hand, the X-plates will cause the electron beam to be deflected in

the horizontal direction if a voltage is applied across them. The fluorescent screen• The screen is coated with a fluorescent salt, for example,

zinc sulphide.

• When the electrons hit the screen, it will cause the salt to produce a flash of light and hence a bright spot on the screen.

1. Power switch To switch on and off of the oscilloscope

2. Focus control To control the focus of the spot on the screen.

3. Intensity control

To control the brightness of the spot on the screen.

4. X-offset5. Y-offset

Y-offset moves the whole trace vertically up and down on the screen, while X-offset moves the whole trace from side to side on the screen.

Using CRO

Using CRO

6. Time base control Whenever we switch on the time-base, we are actually

applying a sawtooth voltage to the X-plates.

* This make the electron beam sweep across the screen at a constant speed.* By knowing the period of each cycle, T, we can then know how fast the beam is sweeping across the screen. The time-base is thus a measure of time for the oscilloscope.

Using CRO

7. Y gain control * the "Volts/Div." wheels amplify an input signal so that for a division a given voltage level is in valid. A "division" is a segment, a square on the screen of the oscilloscope.* A setting of ".5" i.e. means, that the height of a single square equals a voltage of 0.5 V. An amplitude of 1 V would have a size of two divisions vertical to the abscissa.

8. d.c./a.c. switch d.c. – d.c. and a.c. voltage displayed.a.c. – only a.c. voltage displayed.

9. X-input and Y-input

Electric input connect to the X-plate and Y-plate.

ExampleTable below shows the sample display of direct current and alternating current when the time base is switched ON and OFF.

Direct Current (Time Base Switched Off) Direct Current (Time Base Switched On)

Alternating Current (Time Base Switched Off) Alternating Current (Time Base Switched On)

Uses of Cathode Ray Oscilloscope

In a laboratory, a cathode ray oscilloscope can used to• display different types of wave form. • measure short time interval• measure potential difference (as a voltmeter)

Displaying Wave Forms• A cathode ray oscilloscope can be used to display different types of waveform by

connecting a power supply to the Y-input. • Figure below shows a few types of waveform displays on an oscilloscope.

Measuring Potential Difference 1. In order to measure the potential difference, we need to move the bright spot to the centre before

the Y-input is connected to any circuit.

2. We also need to set the Y-gain. However, this can be adjusted later so that the signal can be fully

displayed on the screen.

3. The potential to be measured is then applied to the Y-plates via the Y-input terminals.

Uses Of Cathode Ray Oscilloscope - Measuring Potential Difference

Measuring Potential Difference of a Direct Current• The time-base is switched off. When a potential difference is applied to the Y-

input, an electric field is set up between the plates. This will deflect the cathode ray either up or down.

• The deflection of the electron beam by an electric field is proportional to the voltage applied. The reading of the voltage can be determined by referring to the Y-gain.

1. For example, in the figure above, if the Y-gain is set to 2V per division (2V/div), then the reading of

the potential difference is 4V.

1. If the terminal of the direct current is inverted, the bright spot will be deflected to the opposite side,

as shown in the diagram below. The reading of the potential difference will remain the same (4V).

Effect of the Y-gain Figure below shows the display of the CRO when the Y-gain is set to 1V/div and 5V/div respectively for a potential difference of 4V.

Effect of the Time Base 1. The time base move the bright spot across the screen at a

constant speed. 2. Usually, the speed is very high. As a result, we are not able

to see the motion of the bright spot, but a straight line across the screen.

3. Figure below shows the display of a CRO when the time base is ON and OFF.

Measuring Potential Difference of an Alternating Current 1. If an alternating current is connected to the Y-input, a changing

potential difference will be applied between the Y-plates. 2. The changing potential difference will move the bright spot up and

down continuously. 3. As a result, vertical straight line will form on the screen of the CRO.

The reading of the potential difference can be determined by referring to the Y-gain.

4. For example, for the diagram above, if the Y-gain is set to 2V/div, then

the maximum potential difference (peak voltage) is 4V.

Effect of the Time Base 1. If the time base is switched on, it will move the bright spot

across the screen horizontally. 2. The result of the vertical motion caused by the Y-plate and

the horizontal motion caused by the time base is a sinusoidal wave form.

3. Diagram below shows the CRO display when the time base is on and off.

Measuring Short Time Interval 1. A cathode ray oscilloscope can be used to determined the time

interval between 2 pulses, even though the time interval is very small.

2. Figure above shows 2 pulses on the screen of a cathode ray

oscilloscope. 3. If the time base is set to 2 ms/div, the time interval between the 2

pulses can be calculated as follow: t = 6 x 2ms = 12 ms = 0.012s.

Displaying Wave Forms

1. A cathode ray oscilloscope can be used to display different types of waveform by connecting a power supply to the Y-input.

2. Figure below shows a few types of waveform displays on an oscilloscope.