PRINCIPLES OF MEASUREMENT AND INSTRUMENTATION

EKT 112

Oscilloscope

ContentsThe Basic OscilloscopeBeam DeflectionVertical AmplifierHorizontal AmplifierSweep Generators

IntroductionThe 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 timeoZ - axis: represents intensity or brightness

(see Fig. 6.1)

Introduction…contd

Figure 6.1: X, Y, and Z components of a displayed waveform

Introduction…contdThis simple graph can tell you many things

about a signal, such as:oThe time and voltage values of a signal.oThe frequency and phase.oDC and AC components.oSpectral analysis.oMathematical analysis.oRise and fall time.oHow much of the signal is noise and

whether the noise is changing with time.

ContentsThe Basic OscilloscopeBeam DeflectionVertical AmplifierHorizontal AmplifierSweep 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.

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.

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.

Function of the various blocks…cont’d

The Basic OscilloscopeA basic Oscilloscope is as shown below.

Figure 6.2: Basic Oscilloscope

The Basic OscilloscopeThe 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.

ContentsThe Basic OscilloscopeBeam DeflectionVertical AmplifierHorizontal AmplifierSweep Generators

Beam Deflection

Figure 6.3: Basic construction of a CRT

Beam DeflectionCathode-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 DeflectionThe 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 DeflectionWhen 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.

Beam DeflectionAdjusting the horizontal and vertical

position control deflect the beam to any desired position on the screen.

Factors influencing deflection angle:oLength of the deflection field.oSpacing between the deflection plates.oThe difference of potential between the

plates. oThe accelerating voltage on the second

anode.

Beam DeflectionLength of the deflection field

The longer deflection plates can bend the beam to a greater deflection angle.

Beam DeflectionSpacing between the deflection plates

The closer together the plates, the more effect the electric force has on the deflection angle of the electron beam.

Beam DeflectionThe difference of potential between the

plates

The greater the potential, the wider the deflection angle.

Beam DeflectionThe accelerating voltage on the second

anode

The faster the electrons are moving, the smaller their deflection angle will be.

ContentsThe Basic OscilloscopeBeam DeflectionVertical AmplifierHorizontal AmplifierSweep Generators

Vertical AmplifiersThe 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 AmplifiersThe 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

ContentsThe Basic OscilloscopeBeam DeflectionVertical AmplifierHorizontal AmplifierSweep Generators

Horizontal AmplifiersThe 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.

ContentsThe Basic OscilloscopeBeam DeflectionVertical AmplifierHorizontal AmplifierSweep Generators

Sweep GeneratorsOscilloscopes 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 GeneratorsDuring 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 =

Voltage =Period =Frequency=

CONTROL

Volts/ div =Time/div =X10 probe =Coupling =

Voltage =Period =Frequency=

CONTROL

Volts/ div =Time/div =X10 probe =Coupling =

Voltage =Period =Frequency=

CONTROL

Volts/ div =Time/div =X10 probe =Coupling =

Voltage =Period =Frequency=

CONTROL

Volts/ div =Time/div =X10 probe =Coupling =

Voltage =Period =Frequency=

Exercises1. 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.

CONTROL

Volts/ div =Time/div =X10 probe =Coupling =

Voltage =Period =Frequency=