Chapter 4: ELECTRONICS 4.1 Understanding the uses of the Cathode Ray Oscilloscope (C.R.O.)

Thermionic emission

the process of emission of electrons from a heated metals surface.

Electrons emmited are accelerated towards the anode by the high potential difference between the cathode and anode.

A beam of electrons moving at high speed in a vacuum is known as a cathode ray.

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Factors that influence the rate of

thermionic emission Temperature of the cathode When the temperature of the cathode increases, the

rate of thermionic emission increases.

Surface area of the cathode larger surface area of the cathode increases the rate

of thermonic emission.

Potential difference between the anode and cathode. rate of thermionic emission is unchanged, when the

potential difference increases, but the emitted electrons accelerate faster towards the anode.

Properties of cathode ray

They are negatively charge particles

They travel in straight lines

They posses momentum and kinetic energy.

Can cause fluorescence. (A process where the kinetic energy of the electrons is converted into light energy)

They are deflected by magnetic and electric field.

CRT

Energy Change in A Cathode Ray

By using the principle of conservation of energy,

Maximum velocity of electron,

v = velocity of electron

V = Potential difference between Anode and Cathode

e = Charge on 1 electron = 1.6 x 10 -19 C

m = mass of 1 electron = 9 x 10 -31 kg

Example:

In a cathode ray tube, an electron with kinetic energy of 1.32 10-14 J is accelerated.

Calculate the potential difference, V between the cathode and the accelerating anode.

[ e = 1.6 x 10 -19 C]

Maltese cross cathode ray tube

Properties of Maltese cross tube in magnetic field

Step Observation

Connect only the 6.3 V power supply to the filament

A dark shadow of the Maltese Cross is formed on the screen .

Connect the 6.3 V and EHT to the electrodes

A darker shadow of the Maltese Cross is seen on the screen. The shadow is surrounded by green light.

Bring a pole of a bar magnet near to the neck of the tube.

Two shadow are seen on the screen. The light shadow remains at the centre of screen while the dark one is shifted.

Reverse the pole of the bar magnet The light shadow remains at the centre of screen while the dark one is shifted to the opposite direction.

Properties of Maltese cross tube in electric field

Step Observation

No voltage connected to the deflecting plates

No deflection

Top plate is connected to EHT(+) and lower plate is connected to EHT(-)

The electron beam will deflect upward

Top plate is connected to EHT(-) and lower plate is connected to EHT(+)

The electron beam will deflect downward

Cathode Ray Oscilloscope

Uses a cathode ray tube that converts electronic and electrical signals to a visual display.

graph produced

horizontal axis -- function of time,

vertical axis -- function of the input voltage.

Components in a cathode ray tube:

vacuum glass tube with an electron gun

deflection system for deflecting the electron beam

a fluorescent coated screen.

Structure of the Cathode Ray Oscilloscope

Electron gun produce a narrow beam of electrons

Filament

heat up the cathode

Cathode

emits electrons through the process of thermionic emissions

Control grid

Control the number of electrons in the electron beams.

The more negative the grid, the fewer the electrons are emitted from the electron gun and the less the brightness of the bright spot on the screen

Focusing anode

focus the electrons into a beam and to attract electrons from the area of the control grid

Accelerating anode

accelerate the electron beam towards the screen.

Fluorescent Screen

The fluorescent screen is coated on the inside surface with some fluorescent material such as phosphor or zinc sulfide.

When electron beam strikes the screen, the material becomes glows. This enables a bright spot to appear whenever an electron beam strikes the screen.

The moving electrons have kinetic energy. When this electrons strikes the screen, the fluorescent coating on the screen converts the kinetic energy of the electrons into light energy.

Deflection System allows the electron beam to be deflected from its straight-line path

when it leaves the electron gun.

Working principle of CRO

Control knob Function

Power switch Control the power supply

Focus Control the sharpness of the bright spot Connected to the focusing anode The sharpness of the bright spot is also affected by the brightness.

Brightness To control brightness or intensity of the bright spot Connected to the control grid. Brightness level should be set as low as possible to obtain a clear and sharp trace.

X-shift To adjust the horizontal position of the bright spot on the screen. Connected to the X-plate

Y-shift To adjust the vertical position of the bright spot or the trace displayed. Connected to the Y-plate

Control knob Function

Y gain (volts / div)

To control the magnitude of the vertical deflection of the bright spot or the trace displayed on the screen by adjusting amplitude. Connected to the Y-plate.

Time-base (time/div)

To control the magnitude of the horizontal deflection of the bright spot or the trace displayed on the screen by adjusting amplitude. Connected to the X-plate.

X-input A terminal to connect the voltage to the X-plate.

Y-input A terminal to connect the voltage to the Y-plate.

AC/DC switch To select the type of input received. When switch DC position, both DC and AC voltage will be displayed. When switch AC position, only A.C voltage will be displayed. Any signals of D.C voltage will be blocked by capacitor in CRO.

Earth disconnect input voltage at the Y-input and to earth the input terminal

Application of CRO

1. Measuring potential difference (AC or DC)

2. Measuring short intervals

3. displaying wave forms

To measure a D.C voltage The unknown voltage, V = (Y-gain) h To measure a A.C voltage Peak-to-peak voltage, Vpp = (Y-gains) h

Peak voltage, Vp = Ygains

()

Effective voltage, .. =

Short time interval, t = no. of divisions between two

pulses time-base value

Example:

If the CRO in figure uses Y-gains of 1.5 Vcm-1, calculate the value of Vpp.

When two claps are made close to a microphone which is connected to the Y-input and earth terminals, both pulses will be displayed on the screen at a short interval apart as shown in figure below. Measure the time lapse between the two claps.

Length between two pulses = 5 divs

Time taken, t = 5 divs 10 ms/div

= 50 ms

Time interval = 0.05 s

An ultrasound signal is transmitted vertically down to the sea bed. Transmitted and reflected signals are input into an oscilloscope with a time base setting of 150 ms cm-1. The diagram shows the trace of the two signals on the screen of the oscilloscope. The speed of sound in water is 1200 ms-1. What is the depth of the sea?

Time taken for ultrasonic waves to travel

through a distance of 2 d time = between P and Q