Download Chapter 5- Oscillloscope

PRINCIPLES OF
MEASUREMENT AND
INSTRUMENTATION
EKT 112
Oscilloscope
Contents
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The Basic Oscilloscope
Beam Deflection
Vertical Amplifier
Horizontal Amplifier
Sweep Generators
Introduction
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The oscilloscope is basically a graph-displaying device
– it draws a graph of an electrical signal.
The graph shows how signals change over time:
oY - axis: represents voltage
oX - axis: represents time
oZ - axis: represents intensity or brightness
(see Fig. 6.1)
Introduction…contd
Figure 6.1: X, Y, and Z components of a displayed waveform
Introduction…contd
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This simple graph can tell you many things about a
signal, such as:
o The time and voltage values of a signal.
o The frequency and phase.
o DC and AC components.
o Spectral analysis.
o Mathematical analysis.
o Rise and fall time.
o How much of the signal is noise and whether the
noise is changing with time.
Contents
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The Basic Oscilloscope
Beam Deflection
Vertical Amplifier
Horizontal Amplifier
Sweep Generators
Oscilloscope
Block Diagram of Oscilloscope
Function of the various blocks
Cathode ray tube (CRT) - generates
the electron beam, accelerates the beam
to a high velocity, deflects the beam to
create the image, and contains a phosphor
screen where the electron beam
eventually becomes visible.
 Vertical Amplifier – wide band
amplifier used to amplify signals in the
vertical section.
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Function of the various blocks…cont’d
Delay Line – used to delay the signal for
some time in the vertical sections.
 Time Base – used to generate the
sawtooth voltage required to deflect the
beam in the horizontal section.
 Horizontal Amplifier – used to amplify
the sawtooth voltage before it is applied
to horizontal deflection plates.
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Function of the various blocks…cont’d
Trigger circuit – used to convert the incoming
signal into trigger pulses so that the input signal
and the sweep frequency can be synchronized.
 Power supply - Low voltage supply is required
for the heater of the electron gun for generation
of electron beam and high voltage, of the order
of few thousand volts, is required for cathode ray
tube to accelerate the beam. Normal voltage
supply, say a few hundred volts, is required for
other control circuits of the oscilloscope.
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The Basic Oscilloscope
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A basic Oscilloscope is as shown below.
Figure 6.2: Basic Oscilloscope
The Basic Oscilloscope
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The basic controls are:
◦ Brightness – to adjust the intensity of display.
◦ Focus – To adjust the focusing of display.
◦ Trigger – To select a trigger source.
◦ Trace – To select which trace is to be displayed.
◦ Timebase (sec/div)– To select the speed which the
trace moves across the tube face.
◦ Input Level- To adjust the input level.
◦ Pos (Position) – To set the position of the trace on
the display.
Contents
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The Basic Oscilloscope
Beam Deflection
Vertical Amplifier
Horizontal Amplifier
Sweep Generators
Beam Deflection
Figure 6.3: Basic construction of a CRT
Beam Deflection
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Cathode-ray tubes (CRT) used in oscilloscopes consist
of an electron gun, a deflection system, and a
fluorescent screen.
The electron gun generates electrons and focuses
them into a narrow beam.
The deflection system moves the beam horizontally
and vertically across the screen.
The screen is coated with a phosphorous material
that glows when struck by the electrons.
Beam Deflection
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The electron beam is developed, focused, and
accelerated by the electron gun.
The beam appears on the screen of the CRT as a
small, bright dot.
The beam of electrons passes through an electrostatic
field between two plates.
Electrons are negatively charged and that they will be
deflected in the direction of the electric force (from
negative to positive).
This deflection causes the electrons to follow a
curved path while in the electrostatic field.
Beam Deflection
When the electrons leave the electrostatic field, they
will take a straight path to the screen at the angle at
which they left the field.
 The beam can be positioned anywhere on the screen by
adjusting the controls marked horizontal position and
vertical position.
 When the horizontal and vertical position controls are
set to their midpoint position, the deflection voltages
divide equally across both halves of the potentiometers.
 There is therefore no deflection of the beam; it simply
travels along the axis of the CRT and strikes the centre
of the screen.
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Beam Deflection
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Adjusting the horizontal and vertical position control
deflect the beam to any desired position on the
screen.
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Factors influencing deflection angle:
o Length of the deflection field.
o Spacing between the deflection plates.
o The difference of potential between the plates.
o The accelerating voltage on the second anode.
Beam Deflection
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Length of the deflection field
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The longer deflection plates can bend the beam to a
greater deflection angle.
Beam Deflection
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Spacing between the deflection plates
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The closer together the plates, the more effect the
electric force has on the deflection angle of the
electron beam.
Beam Deflection
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The difference of potential between the plates
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The greater the potential, the wider the deflection
angle.
Beam Deflection
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The accelerating voltage on the second anode
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The faster the electrons are moving, the smaller their
deflection angle will be.
Contents
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The Basic Oscilloscope
Beam Deflection
Vertical Amplifier
Horizontal Amplifier
Sweep Generators
Vertical Amplifiers
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The vertical amplifier is the principal factor in
determining the sensitivity and bandwidth of an
oscilloscope.
The gain of the vertical amplifier determines the
smallest signal that the oscilloscope can be
satisfactory reproduce on the CRT screen.
The sensitivity of an oscilloscope is directly
proportional to gain of the vertical amplifier; that is, as
gain increases sensitivity increases, which allows us to
observe smaller-amplitude signals.
Vertical Amplifiers
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The vertical sensitivity is a measure of how much the
electron beam will be deflected for a specified input
signal.
Bandwidth of an oscilloscope determines the range of
frequencies that can be accurately reproduced on the
CRT screen.
The greater the bandwidth, the wider the range of
frequencies that can be observed with the instrument
Contents
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The Basic Oscilloscope
Beam Deflection
Vertical Amplifier
Horizontal Amplifier
Sweep Generators
Horizontal Amplifiers
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The horizontal amplifier basically serves two purposes:
1.
When the oscilloscope is being used in the ordinary
mode of operation to display a signal applied to the
vertical input, the horizontal amplifier will amplify
the sweep generator output.
2.
When the oscilloscope is being used in the X-Y
mode, the signal applied to the horizontal input
terminal will be amplified by the horizontal amplifier.
Contents
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The Basic Oscilloscope
Beam Deflection
Vertical Amplifier
Horizontal Amplifier
Sweep Generators
Sweep Generators
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Oscilloscopes are used to display a waveform that
varies as a function of time.
If the waveform is to be accurately reproduced, the
beam must have a constant velocity.
Since the beam velocity is a function of the deflecting,
voltage must increase linearity with time.
A voltage with this characteristic is called a ramp
voltage.
If the voltage decreases rapidly to zero with waveform
repeatedly reproduced, as shown in Figure 6.4, the
pattern is generally called a sawtooth waveform.
Sweep Generators
Figure 6.4: Typical sawtooth waveform applied to the horizontal
deflection plates.
Sweep Generators
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During the sweep time, Ts the beam moves from left
to right across the CRT screen.
The beam is deflected to the right by the increasing
amplitude of the ramp voltage and the fact that the
positive voltage attracts the negative electrons.
During retrace time, Tr the beam returns quickly to
the left side of the screen.
The control grid is generally “gated off”, which blanks
out the beam during retrace and prevents an
undesirable retrace pattern from appearing on the
screen.
Applications of Oscilloscope
1. Voltage Measurement
2. Period and Frequency
measurement
Volts/ div =
 Time/div =
 X10 probe =
 Coupling =
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Voltage =
 Period =
 Frequency=
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CONTROL
Volts/ div =
 Time/div =
 X10 probe =
 Coupling =
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Voltage =
 Period =
 Frequency=
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CONTROL
Volts/ div =
 Time/div =
 X10 probe =
 Coupling =
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Voltage =
 Period =
 Frequency=
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CONTROL
Volts/ div =
 Time/div =
 X10 probe =
 Coupling =
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Voltage =
Period =
Frequency=
CONTROL
Volts/ div =
 Time/div =
 X10 probe =
 Coupling =
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Voltage =
Period =
Frequency=
Exercises
1. If the horizontal sweep rate control is set to
0.1msec/Div, and a complete cycle of the
sinewave occupies 7 divisions, what is the
signal’s period?
 2. Suppose the vertical switch is set to
0.5V/div and a 10X scope probe is used. If the
signal occupies 5 divisions vertically.
 What is the peak to peak voltage?
 If the time base is set to 2 msec/division and the
period of signal is 6 msec. Sketch 3 cycles of
sinewave signal.
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CONTROL
Volts/ div =
 Time/div =
 X10 probe =
 Coupling =
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Voltage =
 Period =
 Frequency=
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