Oscilloscope

The main purposes of the oscilloscope are:

•Can view the input waveform voltage or current.

•Can measure instantaneous values of the input waveform ( amplitude – time – duration – frequency – modulation index, etc.).

To CRT

CRT

Input Signal

HV supply

LV supply

Vertical Amplifier

Delay Line

Trigger Circuit Time Base Generator

Horizontal Amplifier

H-D

V-D

To All Circuit

Volts / div

Time / div

Oscilloscope block diagram

Major subsystems of oscilloscope:1-Cathode Ray Tube (CRT):

The CRT generates the electron beam, accelerates the beam to high speed and deflects the beam to create the image on the phosphor screen.2-Time base generator (sweep generator):

It generates the ramp voltage to supply the horizontal deflection plats to deflect the illuminated spot horizontally in X-direction with constant speed 3-Vertical amplifier:

The signal to be viewed or displayed is fed through the vertical amplifier which increases the potential of the input signal to a level that makes a viewed deflection of the electron beam ( enough voltage to make a viewed deflection).

4-Horizontal amplifier:Amplify the output from the sweep generator so that

it reaches the level required to the horizontal deflection plate.5-Power supply:

It provides the voltages required by the CRT to generate the electron beam and accelerate it as well as to supply the required operating voltages for the other circuits of the oscilloscope.6-Trigger circuit:

Used to synchronize horizontal deflection with the vertical input such that the horizontal deflection starts at the same time of the vertical input signal each time it sweeps. i.e. the trigger circuits are the link between vertical input and horizontal time base.

7-Delay line : It is required to appear the input wave form in proper position on the time base so that delay line can delay the input signal by the required value to appear on the time base for example in the middle so you can measure the signal parameters easily.Notes:•electrons strikes the screen with sufficient energy.•Deflection plate assembly consists of a pair of electrostatic deflection plates: i/p voltage applied to the vertical deflection plates that moves the electron beam Fluorescent screen will give a luminous spot when a sharply focused beam of vertically up or down and sweep voltage applied to the horizontal deflection plates that moves the beam horizontally from one side to another. Both voltages are independent of each other.

•Inside the CRT there exists the electrostatic focusing system that focus the electron beam to be centered on the axial CRT to obtain good luminous spot. this system consists of three anodes A1, A2, A3. the voltage difference between A1, A2 generate electron lens and also between A2, A3 generate another electron lens that focus the electron beam.•The high voltage in order of thousands of voltage is applied on metal layer inside the screen to extrap the electrons that are generate from secondary emission due to the strike of electron beam with the screen.

Electrostatic deflection :

Consider two parallel plates, deflecting plates, are placed a distance d apart and are connected to a source of potential difference Ed , so that an electric field E exists between the plated as shown in the Fig.the intensity of the electric field E is given by :

(1)

An electron entering the electric field E with an initial velocity Vox is deflected towards the positive plate following a parabolic path as follow:

d

VE d

The force on the electron ymaE e- F

where ay is the acceleration in y- direction .Then

m

eEay (2)

The velocity component in y- direction t

0yy dtaV

tm

eEVy

The displacement of the electron in y- axes

t

0y dtVY 2

t

0t

m2

eEdtt

m

eEY

(3)

Because of the applied force of є on the electron is in y- direction only thus the electron moves in X- direction with constant velocity (Vox), i-e the acceleration ax= 0.The displacement in X- direction is given by:

tVdtVX ox

t

0ox

oxV

Xt (4)

form equation 3 and 4 we get

22ox

X)mV2

eE(Y

(5)

Equation 5 shows that the path of an electron travelling through an electric field of uniform intensity E and entering at right angle to the flax lines is parabolic in X-Y plane. From equations 1,5 we get

22ox

d X)Vdm2

Ee(Y

(6)

When the electron leave the region of the deflecting plate , the deflecting force no longer exists and the electron travels in a straight line towards the point p(point of the fluorescent screen ) . The slope of the parabola at distance x= la, where the electron leaves the influence of the electric field is defined by slope = tanθ = dy/dx at x=ld From equation 6,7 we get

2ox

ddd2

ox

d

mdV

leEl2

mdV2

eEtan

(7)

The straight line of travel of the electron is tangent to the parabola at x=ld and this tangent intersects the x-axis at apparent point o . The location of the apparent point o is given by:

2

l

mdV

leE

lmdV2

eE

tan

yox d

2xo

dd

2d2

ox

d

dlx

(8)

Thus the apparent point o is therefore at the center of the deflection plates . the deflection of the screen is given by

2ox

dd

mdV

lleEtanLD (9)

The kinetic energy of the electron entering the area between the deflection plates with an initial velocity Vox is

2oxa mV

2

1eEE.K

Where Ea is the accelerating voltage in the electron gun

m

eE2V a2

ox (10)

From 9 and 10 we get

a

dd

a

dd

Ed2

ElL

m

eE2dm

leELD (11)

whereD= deflection on the screen (meter)L= distance from the center of deflection plates to the

screenld = effective length of the deflection plates in meters d = distance between the two deflecting plates Ed = deflection voltage (volts)Ea = accelerating voltage (volts)

Equation 11 indicates that for a given accelerating voltage Ea and for the particular dimensions of the CRT, the deflection of the electron beam on the screen is directly proportional to the deflection voltage Ed . This direct proportionality indicates that CRT may be used as a linear voltage indicating device . if the deflection voltage is varying with time thus the image of the screen follow the variations of the deflecting voltage in a linear manner according to equation 11.The deflection sensitivity S of a CRT is defined as the deflection on the screen (in meters) per volt of deflection voltage .i.e

a

d

d Ed2

lL

E

DS (12)

The deflection factor G of the CRT of the reciprocal of the sensitivity S is independent of the deflection voltage but varies linearly with the accelerating voltage Ea (inversely linear proportional). Therefore high accelerating voltage produce an electron beam that requires a high deflection potential for a given position of the spot on the screenEX:What is the minimum distance L that will allow full deflection of 4 cm at the oscilloscope screen with deflection factor G=100 v/cm and with accelerating potential of 2000v ?

d

a

lG

Ed2L

To obtain maximum deflection, the value of y at x=d is approximately slightly less than d/2 i.e y≈d/2 therefore we have two triangles as follow:

D

d

L

ld 2/2/ d

l

D

L d

m016.010

10*2*104*2

G2

DE2L

4

32a2

Thus if the distance from the center of the deflection plates to the oscilloscope screen is reduced from 12-6 cm then the accelerating voltage Ea. is increased to 8000 v and G remains the same , if the length of the oscilloscope is increased to 25.2 cm lower accelerating voltage will be required