Os Illo Scope

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1Assignment 03 OSILLOSCOPE

Introduction:

The oscilloscope is basically a graph- displaying device. The C.R.O gives the visual

representation of the time varying signals. Basically it operates on voltages, it is possible to

convert current, pressure, strain, acceleration and other physical quantities into the voltages

using transducers and obtain their visual representation on the C.R.O.

Block Diagram of CRO

Construction:

The major parts of CRO

I. CRT II. Vertical Amplifier III. Delay line

IV. Time base V. Horizontal Amplifier VI. Trigger Circuit

VII. Power Supply

An analog scope has two major signal paths. The 1st

is the vertical signal path for deflecting the

CRT beam vertically in response to input signal. The second is the horizontal. It triggers the

scope and moves the beam from left to right across the screen.

The attenuators match the high input impedance of the scope probes to low impedance of the

vertical preamplifiers. The amount of attenuation and preamp gain is set by the vertical

sensitivity knob provided on the front panel.

The triggering portion tells the horizontal section when to start moving the beam from the left

of the CRT to the right.


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Cathode ray tube (CRT)

The cathode ray tube (CRT) is the heart of the C.R.O. The CRT generates the electron beam.

The main parts of the CRT are

Electron gun ( It provides a sharply focused electron beam directed towards the

fluorescent coated screen)

Deflection system ( it position the electron beam anywhere on the screen)

Fluorescent screen (on striking electrons it glows, enable the tube to visually present

electronic signals)

Glass tube

Base

CRT circuit

Trigger Circuit

The horizontal deflection starts at the same point of the input vertical signal, each time it

sweeps. Hence to synchronize the horizontal deflection with the vertical deflection a triggering

or synchronizing circuit is used. It converts the incoming signal into the trigger pulses, which are

used for synchronization.

Vertical Amplifier

This is used to amplify the signal in the vertical section.


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Time Base Generator

The time base generator is used to generate the saw tooth voltage that require deflecting the

beam in the horizontal section. This voltage deflects the spot at a constant time dependent

rate. So the x-axis on the screen can be as time which helps to display and analyze the time

varying signals.

Delay Line

In the vertical section to delay the signal for some time it is used. Delay line circuit is shown in

figure. There are two types of delay

lines used in CRO.

I) Lumped parameter delay line.

II) Distributed parameter delay line

Horizontal Amplifier

The saw tooth voltage produced by

the time base generator may not be sufficient strength. It is amplified using the horizontal

amplifier before giving it to the horizontal plates.

Power supply

To generate and accelerate an electron power supply provides the voltages. It provides the

voltage to other circuits of amplifier like horizontal amplifier and vertical amplifier.

There are two section of the power supply block. The high voltage and low voltage section. The

high voltage are required to CRT, of range 1000V to 1500V.The low voltage is for the heater of

the electron gun. This is positive voltage and few 100 volts.

Working

When voltage is applied to the input of CRO then vertical deflection of electron will be proportional to

the magnitude of the input voltage. It is then necessary to convert horizontal deflection into time axis.

The time base generator provides periodic saw tooth voltage (sweep).

This saw tooth voltage is applied to horizontal deflecting plates. During the trace Tr, Vx linearly

increasing w.r.t.So electron beam will move linearly in the horizontal direction. At the end of Tr beam

reaches at max poison at this instant the voltage suddenly drops to zero in a short time, known as fly

back period. After this beam sudden jumps back to the original position as extreme left in figure.


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For time Tr, spot moves horizontally across the face of

the screen along x-axis from left to right. Thus the

horizontal axis is converted in to time axis.

When input sinusoid applied voltage to CRO probe and

internally generated saw tooth voltage is applied tohorizontal plates then saw tooth voltages keeps on

shifting the spot horizontally while the applied voltage

shifts the spot vertically proportional to its magnitude.

Due to effect of both voltages a waveform can be seen

on the screen.

When signal and saw tooth voltage frequencies are equal a single cycle appears on screen. When the

sweep (saw tooth) is lower than signal several cycles appear on screen. When sweep is higher than

signal less than one cycle appear on the screen. We can say the numbers of cycles depend upon ration

of the two signal frequencies.

REFERENCES:

Electronic Measurements By V.U.Bakshi U.A.Bakshi

Electronic Instruments and Instrumentation Technology By M.M.S. Anand22

Electronics Instrumentation By H S Kalsi

Submitted To: Sir Amir Khalil

Submitted By: Abdul Shakoor

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