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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…?