D.S.Vidyasagar

Understanding

INSTRUMENTS

Contents

This chapter covers three most

important laboratory

instruments.

• Cathode Ray Oscilloscope

• Function Generator

• Digital Multimeter

Cathode Ray Oscilloscope

A cathode ray oscilloscope

is abbreviated as CRO. It

is a versatile lab

instrument. It can measure

ac/dc voltage, ac/dc

current, resistance,

frequency, wavelength,

phase etc.

A prototype of CRO

What is inside a CRO?

The CRO consists

of three main

sections:

• Cathode ray tube

• Controlling circuit

• Power supply

What is CRT?

• The CRT is heart of CRO.

• It is used to display different types of

waveforms or patterns on its screen.

• The waveforms or patterns are drawn with

the help of high speed electron beam on the

electron sensitive fluorescent screen by

deflecting the beam horizontally and vertically.

Inside the CRT

Schematic of CRT

accelerating anodes

focusing anode

control grid

cathod

filament

sealing base

co

nne

cting

pin

s

e-beam

vertical plates

hor. def. plates

aquadag coating

phosphor coating

vacuum lighted spotglass tube

scre

en

Details of CRT

• The filament heats up the cathode.

• Cathode emits large number of electrons.

• Control grid covers cathode. It has a tiny

hole on its surface.

• So all the electrons come out of this hole

and produce diverged beam due to mutual

repulsion.

Electron Gun Assembly

Control grid Cathode

Details of CRT…

• The 3 accelerating anodes are connected

to high-to-low positive voltage in reducing

order from base to screen.

• Due to gaps in between the anodes,

electrostatic lens system is produced within

accelerating anodes.

Details of CRT…

Scre

en

control grid

cathode

divergedelectron bean High +ve

voltage

High +ve voltage Focus

adjustment

Pre-accelerating anode

Second accelerating anodeFocusing anode

Deflection Systems in CRT

The electron beam

is deflected using

electric field applied

across the two pairs

of deflection plates.

The electron beam

is deflected using

magnetic field which

is created using

yoke system.

Electrostatic Deflection

SystemMagnetic Deflection System

Electrostatic Deflection System

Two pairs of deflection plates used in this system. They are:

Vertical deflection plates

Horizontal deflection plates

distance (d)

screen

parabolic path

straight path

verticaldeflection plates

D

P

P(+)

(-) electric field (flux)

Magnetic Deflection System

Two pairs of deflection coils are used in this system.

They are fitted on the neck of CRT from outside.

It is called as YOKE.

deflection coilselectron gun

assembly

picture tube

screen

deflectionof beam

Applications

Used in CRO

Used in ECG

Used in earthquake

monitoring system

The electron beam

is deflected using

magnetic field which

is created using

yoke system.

Electrostatic Deflection

SystemMagnetic Deflection System

Why it is called yoke…?

Block

Diagram of

CRO

Applications of CRO

AC/DC voltage measurement

AC/DC current measurement (indirect

method)

Frequency and Phase measurement

using Lissajou’s patterns

DC Voltage Measurement

AC Voltage Measurement

Frequency Measurement (Direct Method)

Lissjou’s Patterns

Frequency measurement

More Lissjou’s Patterns

Frequency measurement

More Lissajou’s Patterns

Such pattern

may obtain

during relative

frequency

measurement

only.

Frequency Measurement (Relative Method)

Lissjou’s Patterns

Phase measurement

ba

Phase difference 0 or 360

0 0Phase difference 90 or 270

0 0Phase difference 180

0

(a) (b) (c)

Special cases of phase difference between the waveforms

Function Generator

It generates different types

of waveforms electrically. It

uses oscillator circuits to

generate sine wave, square

wave and triangular wave,

etc.

How it works…?

Digital Multimeter

It generates different types of

waveforms electrically. It uses

oscillator circuits to generate

sine wave, square wave and

triangular wave, etc.

How it works…?

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Designed by: D.S.Vidyasagar