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Theor of

Oscillosco e1

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n ro uc on

-

z X axis: Time

z Y axis: Volta e

z Z axis: Intensity or brightness

2

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n ro uc on

z Time and voltage

z Fre uenc and hase

z DC and AC components

z Spectral analysisz se an a t me

z Mathematical analysis

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as c se ng

z attenuation or amplification of signal (volts/div)

z

z The Time base (sec/div)

z Trigger systemz To stabilize a repeating signal and to trigger on a single

event

4

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Architecture of

Oscilloscope

Cathode-Ray Tube (CRT)

5

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The oscilloscope consist of the following major

subsystemsCRT

Sweep Generator

Trigger circuit

Horizontal amplifier

Associated power supplies

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z The CROs are very fast X-Y plotters, displaying an input signal versus

another signal or versus time.

z The stylus of this plotter is a luminous spot which moves over the display area in

response to an input voltage

z The normal form of a CRO uses a horizontal input voltage which is an

internally generated ramp voltage called Time Base

z This moves the luminous spot periodically in a horizontal direction from left to right over

the screen

z The vertical input to the CRO is the voltage under measurement

z This moves the luminous spot up and down in accordance with the voltage

z When the input voltage repeats itself at a fast rate, the trace on the screen

z CROs operate on voltages

z However, it is possible to convert current, strain, acceleration, pressure and other physical

quantities into voltages with the help of transducer, and visualize their behavior

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z A CRO consist of a Cathode ray Rube (CRT), which is the heart of the

tube, and some additional-circuitry to operate the CRT.

z The main part of CRT are:

z

Deflection plate assemblyz Fluorescent screen

z Glass envelop

z Base, through which connections are made to various parts

z

Electron gun assemblyproduces a sharply focused beam of electrons

z This focused beam of electron strikes the fluorescent with sufficient energy to cause a

luminous spot on the screen

,ofElectrostatic deflection plates.

z Voltages applied to theses plates deflect the beam vertically and horizontally

z These two deflections are inde endent to each other thus the beam ma be ositioned an where on the

screen

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Cathode-Ray Tube (CRT) Found in oscilloscopes, and similar devices are used in TV picture tubes

9

Use an electron beam

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Control Grid Focusing

anode

Electron gun

Deflecting plates

10

Main parts of CRT

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Control Grid regulates the number of electrons that reach the anode andhence control the bri htness of the s ot on the screen.

Focusing ensures that electrons leaving the cathode in slightly different

11

at the same spot on the screen.

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lates x latesu e agram

ano e

heatersupply

+-

H.T. supply screen

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lates x latesproduces a beam

of electrons

ano elight produced on

the screen by

electron beam

heatersupply

+-a p.d. across the Y

plates deflects the

a p.d. across the X

plates deflects theH.T. supply

screentrace vertically trace horizontally

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Principle Elements of a CRT

z The cathode, at the left end in the figure, is raised to a high temperature by

the heater, and electrons evaporate from the surface of the cathode.

z The accelerating anode, with a small hole at its center, is maintained at a

high positive potential V1, of the order of 1 to 20kV, relative to the

cathode.

z This potential difference gives rise to an electric field directed from right to

left in the region between the accelerating anode and the cathode.

z Electrons passing through the hole in the anode form a narrow beam and

travel with constant horizontal velocity from the anode to thefluorescent

screen.

z The area where the electrons strike the screen glows brightly.

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Electrostatic Focusing

z The pre-accelerating anode is connected to a high positive

z Hence, it collimates the electron beam which enters it through a smallopening on the left hand side

z The focusing and the accelerating anodes are co-axial with the

pre-accelerating anode

z The pre-accelerating and accelerating anodes are connected to

the same potential while the focusing anode is connected to a

z Due to the difference of potential between the anodes, a non-uniform

field exist on each of the two ends of the focusing anode

- , ,

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Electrostatic Focusing

z The electron beam entering the field at angles other than the

-

the normalz and the beam is thus focused towards the centre of the tube axis

z By changing the voltage of the focusing anode, the refractive

index of the electron lens is changedz an ere ore e oca po n o e eam can e c ange

z The control is brought to the front panel, and is marked Focus

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Electrostatic Deflection

z The parallel plates produces a uniform EF in Y-direction

z -

direction only, and will be accelerated in that directionz No force either in X- or Z-direction

z The loss of Potential Energy (PE) when electron moves from

cathode to anode,PE = eVa

z The gain in Kinetic Energy (KE) by an electron,

KE = mvox2

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Electrostatic Deflection

z Equating above energies, we get

2ox a

z This velocity remains in X-direction, with value remains same throughout

the passage through deflecting plates, as there is no force acting in this

direction

z The EFI in Y-direction,Ey = Vd/d

Force Fy = eEy = eVd/d

a so, y = maythus, ay = Fy/m

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Electrostatic Deflection

z As there is no initial velocity in Y-direction,

2 2 y y

z As the velocity in x-direction is constant,

x = v t

or t = x/vox

hence,y = (eEy/mvox

2)x2 (Equation of parabola)

the slope, dy/dx = (eE /mvox2)x

z Now, for x = ld,

tan = eE /mv 2 l = eV l /mdv 2

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Electrostatic Deflection

z The tangent intersect the X-axis at O given as

dz The apparent origin is the center of deflection plates

, ,

D = Ltan = LldVd/2dVa

z D is proportional to Ed i.e., for a given Ea & a particular dimension of CRT

z Hence, the CRT may be used as a linear indicating device

z D does not depend upon e/m ratioz i.e., ion travel with electron, and are not concentrated at one point

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Deflection Sensitivity:

z defined as the deflection of the screen per unit deflection

voltage

d d a

Deflection Factor:

z e ne as e rec proca o sens v y

G = 1/S = 2dVa/Lld (V/m)

z The sensitivity can be increased by decreasing the value of

accelerating voltage Vaz But, this decreases the luminosit of the s ot

z Also, a high value of Vaproduces a highly accelerated beam and thusproduces a bright spot

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Lissajous Patterns methods(LP) Lee-suh-

z oo

Electronic En ineer use LP to measure radio si nal

frequencyThey do this by analyzing the type of pattern an

unknown signal produces when it is combined with a

signal of a known frequency.

LP is determined by applying sinusoidal wave to

horizontal input(X) (unknown signal) and vertical

in ut Y known si nal . use X-Y mode

LP observed depends on the ratio of the two

frequency ( Horizontal/Vertical or Vertical/Horizontal)

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X Fre uenc = 1.000KHz and Y Fre uenc =1.000kH

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The ur le line leads the reen line b a 90 de ree hase shift.

28

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.

29

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x axis frequency (1kHz) is twice the y axis frequency (2kHz).o

30

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2:1 1:2

3:2

31

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32

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t

Angl

emen

0o or Same hase

has

e

asur

90o

(b)M

180

o

0

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b Phase An le Measurement

3. Phase Measurements or Time Delay, TD (Time/Div : 0.5ms/Div)

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- phase angle in degree Yo-Y axis intercept Ym-maximum vertical deflection

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xamp e

If, in figure above, the distance Yo is 1.8cm and Ym=2.3cm,what is the phase angle?

35

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Solution:

The phase angle can be determined by

using equation

sin = yo/ym

= sin-1(yo/ym)

= sin

-1

(1.8/2.3)= sin-1(0.783)

= 51.50o

36

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d

z d