Electronics Measurements
and Instrumentation
By
Dr. Amandeep Kaur
Contents
Electronic Instruments
Instruments
• An instrument is a device in which we can determine the magnitude or value of the quantity
to be measured. The measuring quantity can be voltage, current, power and energy etc.
Generally instruments are classified into two categories.
1) Absolute instrument
2) Secondary Instruments
• 1) Absolute Instruments:
• An absolute instrument determines the magnitude of the quantity to be measured in terms
of the instrument parameter. This instrument is really used, because each time the value of
the measuring quantities varies. So we have to calculate the magnitude of the measuring
quantity, analytically which is time consuming. These types of instruments are suitable for
laboratory use.
• 2) Secondary Instruments:
• This instrument determines the value of the quantity to be measured directly. Generally
these instruments are calibrated by comparing with another standard secondary instrument.
Examples of such instruments are voltmeter, ammeter and wattmeter etc. Practically
secondary instruments are suitable for measurement.
• Secondary Instruments Includes:
1.
Indicating Instruments: This instrument uses a dial and pointer to determine the value
of measuring quantity. The pointer indication gives the magnitude of measuring
quantity.
2.
Recordings: This type of instruments records the magnitude of the quantity to be
measured continuously over a specified period of time.
3.
Integrating: This type of instrument gives the total amount of the quantity to be
measured over a specified period of time.
4.
Electromechanically: For Electromechanical operation, certain forces are required.
(a) Deflecting force
(b) Controlling force
(c)Damping force
Deflecting force: When there is no input signal to the instrument, the pointer will be at its
zero position. To deflect the pointer from its zero position, a force is necessary which is
known as deflecting force.
• A system which produces the deflecting force is known as a deflecting system.
• Generally a deflecting system converts an electrical signal to a mechanical force.
Controlling force: To make the measurement indicated by the pointer definite (constant) a
force is necessary which will be acting in the opposite direction to the deflecting force. This
force is known as controlling force.
• A system which produces this force is known as a controlled system.
• When the external signal to be measured by the instrument is removed, the pointer
should return back to the zero position. This is possibly due to the controlling force and
the pointer will be indicating a steady value when the deflecting torque is equal to
controlling torque.
Damping force: The deflection torque and controlling torque produced by systems are
electro mechanical. Due to inertia produced by this system, the pointer oscillates about it
final steady position before coming to rest.
The time required to take the measurement is more. To damp out the oscillation is quickly,
a damping force is necessary.
Ballistic Galvanometer
• To measure a charge flow through the circuit for
very short duration of time.
Construction of Ballistic Galvanometer
• The ballistic galvanometer consists coil of copper
wire which is wound on the non-conducting frame
of the galvanometer. The phosphorous bronze strip
suspends the coil between the north and south
poles of a magnet.
• A mirror is also attached rigidly to phosphor
bronze wire.
• For increasing the magnetic flux the iron core
places within the coil. The lower portion of the coil
connects with the spring. This spring provides the
restoring torque to the coil.
• Here, Non-conducting frame is considered so that
induced current=0 and also the electromagnetic
damping is negligible.
Why we consider thin phosphorous bronze wire?
• It has small torsional constant.
• So we consider long and thin phosphorous bronze wire
Theory:
• Torque acting on a coil is
𝑛𝑖𝐵𝐴
• The current flows for small time
angular impulse is
𝑑𝑡 , then
the
𝜏 × 𝑑𝑡 = 𝑛𝑖𝐵𝐴 × 𝑑𝑡
When current flows for whole time, then the
angular impulse is calculated as
𝑡
𝑡
𝑛𝑖𝐵𝐴𝑑𝑡 = 𝑛𝑖𝐵𝐴
0
𝑑𝑡 = 𝑛𝐵𝐴 𝑖 × 𝑡 = 𝑛𝐵𝐴𝑞
0
𝐼
𝑇 = 2𝜋
𝐶
𝑇2𝐶
𝐼=
4𝜋
𝐼𝛼𝑇
Theory (Contd..)
• To calculate angular momentum
Let 𝜔 be the initial angular velocity of the coil and 𝐼 be the moment of inertia
around suspension
𝐼𝜔 = 𝑛𝐵𝐴𝑞
……………….(a)
The Kinetic Energy (K) deflects the coil through an angle θ, and this deflection is
restored through the spring.
1
Kinetic Energy (K) = 𝐼𝜔2
2
Restoring energy =
1 2
𝐶𝜃
2
The resorting torque of the coil is equal to their deflection. Thus,
1 2 1 2
𝐶𝜃 = 𝐼𝜔
2
2
• Time period of oscillation
𝐼
𝑇 = 2𝜋
𝐶
𝑇2𝐶
𝐼=
4𝜋
𝐼𝛼𝑇
• On multiplying equation
𝐼 2 𝜔2
𝑇2𝐶 2𝜃2
=
4𝜋 2
𝐼𝜔 =
• On comparing (a) and (b)
𝑇𝐶𝜃
2𝜋
𝑇𝐶𝜃
𝑛𝐵𝐴𝑞 =
2𝜋
𝑇𝐶𝜃
𝑞=
𝑛𝐵𝐴2𝜋
𝑞 = 𝐾𝜃
………………(b)
Permanent Magnet Moving Coil (PMMC)instrument
• A Permanent Magnet Moving Coil (PMMC) meter – also known as a D’Arsonval meter or galvanometer –
is an instrument that allows you to measure the current through a coil by observing the coil’s angular
deflection in a uniform magnetic field.
• A PMMC meter places a coil of wire (i.e. a conductor) in between two permanent magnets in order to
create stationary magnetic field. According to Faraday’s Laws of electromagnetic induction, a current
carrying conductor placed in a magnetic field will experience a force in the direction determined by
Fleming’s left hand rule.
• The magnitude (strength) of this force will be proportional to the amount of current through the wire. A
pointer is attached to the end of the wire and it is put along a scale.
• When the torques are balanced the moving coil will stop, and its angular deflection can be measured by
the scale. If the permanent magnet field is uniform and the spring linear, then the pointer deflection is
also linear. Hence we can use this linear relationship to determine the amount of electrical current passing
through the wire.
• PMMC instruments (i.e. D’ Arsonval meters) are only used for measuring the Direct Current (DC) current. If
we were to use Alternating Current (AC) current, the direction of current will be reversed during the
negative half cycle, and hence the direction of torque will also be reversed. This results in an average value
of zero torque – hence no net movement against the scale.
PMMC Construction
A PMMC meter (or D’Arsonval meters) is constructed of 5 main components:
1.
Stationary Part or Magnet System
2.
Moving Coil
3.
Control System
4.
Damping System
5.
Meter
1) Stationary Part or Magnet System
• In the present time we use magnets of high field intensities, high coercive force U shaped
permanent magnet having soft iron pole pieces. The magnets which we are using now-adays are made up of materials like alcomax and alnico which provide high field strength.
2) Moving Coil:
• The moving coil can freely moves between the two permanent magnets as shown in the
figure given below. The coil is wound with many turns of copper wire and is placed on
rectangular aluminium which is pivoted on jeweled bearings.
3) Control System
• The spring generally acts as control system for PMMC instruments. The spring also serves another
important function by providing the path to lead current in and out of the coil.
4) Damping System
• The damping force hence torque is provided by movement of aluminum former in the magnetic
field created by the permanent magnets.
5) Meter
• Meter of these instruments consists of light weight pointer to have free movement and scale which
is linear or uniform and varies with angle.
• PMMC Torque Equation
The deflecting torque is given by the expression
Td = NBAi
(1)
• N is number of turns,
• B is magnetic flux density in air gap,
• l is the length of moving coil,
• d is the width of the moving coil,
• i is the electric current.
• For a moving coil instrument the deflecting torque should be proportional to current,
Td = Gi
(2)
• On Comparing (1) and (2)
G = NBA
• At steady state, when both the controlling and deflecting torques are equal. Tc is
controlling torque, on equating controlling torque with deflection torque we have GI =
K.x where x is deflection thus current is given by
• Since the deflection is directly proportional to the current therefore we need a uniform
scale on the meter for measurement of current.
Errors in Permanent Magnet Moving Coil Instruments
1. Errors due to permanent magnets: Due to temperature effects and aging of the
magnets, the magnet may lose their magnetism to some extent. The magnets
are generally aged by the heat and vibration treatment.
2. Error may appear in PMMC Instrument due to the aging of the spring. However
the error caused by the aging of the spring and the errors caused due to
permanent magnet are opposite to each other, hence both the errors are
compensated with each other.
3. Change in the resistance of the moving coil with the temperature: Generally
the temperature coefficients of the value of coefficient of copper wire in
moving coil is 0.04 per degree celsius rise in temperature. Due to lower value of
temperature coefficient the temperature rises at faster rate and hence
the resistance increases. Due to this significant amount of error is caused.
Advantages of Permanent Magnet Moving Coil Instruments
1. The scale is uniformly divided as the current is directly proportional to deflection
of the pointer. Hence it is very easy to measure quantities from these
instruments.
2. Power consumption is also very low in these types of instruments.
3. A high torque to weight ratio.
4. These are having multiple advantages, a single instrument can be used for
measuring various quantities by using different values of shunts and multipliers.
• Disadvantages of Permanent Magnet Moving Coil Instruments
1. These instruments cannot measure AC quantities.
2. The cost of these instruments is high as compared to moving iron instruments.
Cathode Ray Oscilloscope
• A cathode ray oscilloscope (C.R.O.) is an instrument that converts electrical
signals to a visual display.
• The main structure of the C.R.O. is a highly evacuated cathode ray tube
(C.R.T.) which emits an electron beam known as cathode ray beam.
• The cathode ray tube consists of three main components:
(a) The electron gun
(b) The deflection system
(c) The fluorescent screen
Main Structure of CRO
Construction of CRO
Electron Gun
1) The electron gun is used to produce a narrow beam of electrons. It consists of:
(a) A filament enclosed in a cylindrical cathode
(b) A ring-shaped electrode called the control grid
(c) Two cylindrical anode electrodes known as focusing anode and accelerating
anode
2) The filament is heated when current flows through it. It is used to heat up the
cathode.
3) The cathode of a C.R.O. consists of a small diameter nickel cap. The closed end of
the cap is coated with emitting material as shown in Figure. This type of cathode
can produce a highly concentrated electron beam.
4) The control grid is placed between the cathode and anodes. It is made negative
with respect to the cathode.
5) This is to control the number of electrons in the beam. When the grid is adjusted
more negative, the number of electrons emitted from the electron gun decreases
and the spot on the screen is less bright. Thus, the negative voltage of the grid is
used as the brightness control.
6) These electrons are accelerated towards the anodes by the electric field set up
between the cathode and anodes.
7) The focusing anode serves a dual purpose; to attract electrons from the area of
the control grid and to focus the electrons into a beam.
8) The accelerating anode is used to accelerate the electrons in the beam towards
the front of the tube or screen.
• Deflection System
1) The deflection system allows the electron beam to be deflected from its straightline path when it leaves the electron gun.
2) The deflection system consists of two sets of parallel plates. One set which is
arranged vertically is known as X-plates and the other set which is arranged
horizontally is known as Y-plates, as shown in Figure.
3) If no input voltage or potential difference is applied between the X-plates and Yplates, the electron beam does not experience any force. No deflection occurs and
the bright spot is at the centre.
5) When a positive voltage or potential difference is applied to the Y-plates, the
electrons in the beam will experience a force acting upwards causing the electron
beam to deflect upwards. The bright spot moves to the top of the screen.
6) When a negative voltage is applied to the Y-plates, the electron beam deflects
downwards. The bright spot moves to the bottom of the screen
7) When an A.C. voltage is applied to the Y-plates, the electron beam deflects up
and down. The bright spot moves up and down rapidly to form a bright vertical
trace on the screen.
8) Thus we can conclude that the function of the Y-plates is to move the electron
beam up and down the screen when an input voltage is applied across it.
9) The function of the X-plates is to sweep the electron beam across the screen
horizontally from left to right at a steady speed. The X-plates are usually connected
to a time-base circuit that generates a time-varying voltage as shown in Figure.
10) Therefore, by applying appropriate voltages to the deflection plates, the
position of the bright spot on the screen can be controlled.
Fluorescent Screen
• The fluorescent screen is coated on the inside surface with some fluorescent
material such as phosphor or zinc sulphide.
• When electrons in the beam strike the screen, the material fluoresces and
becomes luminous or glows. This enables a bright spot to appear wherever an
electron beam strikes the screen.
• Electrons are particles and they have mass. Since they move with high speed,
they have kinetic energy.
• When these high-energy electrons strike the screen, the fluorescent coating on
the screen converts the kinetic energy of the electrons into light energy.
Block Diagram of CRO
• 1)CRT
• 2)Vertical Amplifier
• 3)Delay line
• 4)Horizontal amplifier
• 5)Time base generator
• 6)Trigger circuit
• 7)Power Supply.
Vertical Amplifier
• The first element of the pre-amplifier is the input stage,
often consisting of a FET source follower whose high input
impedance isolates from the attenuator.
• This FET input stage is followed by a BJT emitter followers to
match the medium impedance of FET output with the low
impedance input of the phase inverter.
• The phase inverter provides two anti-phase output signals
which are required to operate the push pull output amplifier.
• The push pull output stage delivers equal signal voltages of
opposite polarity to the vertical plates of the CRT.
• The vertical amplifier consists of several stages, with fixed
overall sensitivity or gain expressed in v/divisions. The
advantage of fixed gain is that the amplifier can be more
easily designed to meet the requirements of stability and
between the vertical amplifier is kept within its signal
handling capability by proper selection of the input
attenuator switch.
Delay Line
• It is observed that the deflection signal is initiated or triggered, by a portion of the output
signal applied to the vertical CRT plates.
• Signal processing in the horizontal channel consists of generating and shaping a trigger
pulse that starts the sweep generator, whose output is fed to the horizontal deflection
plates. This whole process takes time on the order of 80 ns.
• To allow the operator to observe the leading edge of the signal waveform, the signal drive
for the vertical CRT plates must therefore be delayed by atleast the same amount of time.
This is the function of time delay line.
Horizontal Amplifier
• Time base generator:
It is used to generate the saw tooth voltage required to
deflect the beam in the horizontal section.
•
Trigger circuit: This is used to convert the incoming signal into trigger pulses that the
input signal and the sweep frequency can be synchronized.
• Power supply: There are two power supplies, a negative high voltage (HV) supply and
a positive low voltage (LV) supply. Two voltages are generated in the CRO ranges from 100V to +1500V. This voltage is passed through a bleeder resistor at a few mA. The
intermediate voltages are obtained from the bleeder resistor for intensity, focus and
positioning controls.
TYPES OF OSCILLOSCOPES
• a)Dual beam oscilloscope
b)Dual trace oscilloscope
c) Sampling oscilloscopes:
The oscilloscopes presently can be used for continuous
display for frequencies in the 50-300 MHz range depending upon the design of the
oscilloscopes.
The display may have upto 1000 dots of luminescence. The vertical deflection for each dot is
obtained from progressively later points in each successive cycle of input waveform as shown
• The sampling oscilloscope is able to respond
and store rapid bits of information and present
them in a continuous display.
• The sampling techniques immediately the input
signals into lower frequency domain, where
conventional low frequency circuitry is then
capable of producing a highly effective display.
• The input signal is fed into the sampling gate. When the
sampling pulse is provided to the sampling gate, it gets open in
order to sample the input waveform. It is noteworthy that
sampling is to be done in synchronization with the frequency of
the applied input signal.
• The vertical amplifier employed in the circuit delays the input
signal and after amplification, the signal is given to the vertical
plates.
• When the sampling cycle begins, the oscillator gets activated by
the trigger pulses. Due to which, linear ramp output voltage is
produced. The signal generated from the ramp generator is then
fed to the voltage comparator unit.
• Here, the ramp signal gets compared with the staircase signal,
generated by the staircase generator. During comparison when
the amplitude of the two signals is equal, it advances the
staircase by one step. Thus generating a sampling pulse. This
again opens the sampling gate and the cycle is repeated in a
similar manner.
• The size of the steps generated by the staircase generator
determines the resolution of the image at the output. When the
size of the steps is smaller, the number of samples will be larger.
Thus, the image resolution will be higher.
d) Storage type Oscilloscope:
• Usually in conventional CRTs, the persistence of phosphor ranges from
microseconds to seconds. In applications where the persistence of the
screen is smaller than the rate at which the signal sweeps across the
screen, the start of screen will have disappeared before the end of the
display is written.
• In storage oscilloscopes, the persistence times are much greater than
a few seconds or even hours are available, making it possible to store
events on the CRT screen.
• The special CRT of storage oscilloscope contains electron gun,
deflection plates, phosphor bronze screen but also it holds many
number of special electrodes. The CRT used here is called as storage
tube.
• The storage mesh or the storage target is mounted just behind the
phosphor screen is a conductive mesh covered with a highly resistive
coating of magnesium fluoride.
• The write gun is a high-energy electron gun, similar to the
conventional gun giving a narrow focused beam which can be
deflected and used to write the information to be stored.
• Because of the excellent insulating properties of the magnesium
fluoride coating, the positively charged pattern remains exactly in the
same position on the storage target which it was first deposited.
• The stored pattern may be made available for viewing at a later time by the use of two
special electron guns called flood guns. The flood guns are placed inside the CRT in a
position between the deflection plates and the storage target and they emit low-velocity
electrons over a large area towards the entire screen.
• When the flood guns are switched for viewing mode low energy electrons are sprayed
towards the screen. The electron trajectories are adjusted by the collimating electrodes
which constitute a low-voltage electrostatic lens system , so that the flood electrons cover
the entire screen area.
• To erase the pattern which is etched on the storage mesh, a negative voltage is applied to
the storage target, neutralizing the stored positive charge.
• To get variable persistence, the erase voltage is applied in the form of pulses instead of a
steady dc voltage. By varying the width of these pulses, the rate of erase is verified.
MEASUREMENT OF PHASE AND FREQUENCY USING
LISSAJOUS PATTERNS IN AN OSCILLOSCOPE
• The patterns that appear on the CRT when sinusoidal voltages are simultaneously applied
horizontal and vertical plates. These patterns are called as Lissajous patterns.
• When two sinusoidal voltages of equal frequency which are in phase with each other are
applied to the horizontal and vertical deflection plates, the pattern appearing on the
screen is a straight line as is clear from the figure below:
• (i) A straight line results when the two voltages are equal and are in either in phase with
each other or 180 out of phase with each other. The angle formed with the horizontal is
45 when the magnitudes of voltages are equal. An increase in the vertical deflection
voltage causes the line to have an angle greater than 45 with the horizontal.
• (ii) Two sinusoidal waveforms of the same frequency produce a lissajous pattern, which
may be a straight line, a circle or an ellipse depending upon the phase and magnitude of
the voltages.
• A circle can be formed only when the magnitude of the two signals are equal and the
phase difference between them is either 90 or 270 . However if the two voltages are not
equal and/or out of phase an ellipse is formed.
• If the 'Y' voltage is larger, an ellipse with vertical major axis is formed while if the X-plate
voltage has a greater magnitude, the major axis of the ellipse lies along horizontal axis.
To determine the phase difference ø between two signals applied to the horizontal
and vertical plates
• Case – I: When, 0 < ø < 90o or 270o < ø < 360o
• The phase difference will be,
• Another possibility of phase difference,
• From Above given Lissajous pattern
• Another Possibility of Phase Difference,
• Case – II: When 90o < ø < 180o or 180o < ø < 270o
In this condition the phase difference will be,
Another possibility of phase difference,
From Above given Lissajous pattern
Another Possibility of Phase Difference,
Frequency Measurement
• Lissajous patterns are used for accurate measurement of frequency. The signal whose
frequency is to be measured is applied to the 'Y' plates. An accurately calibrated standard
variable frequency source is used to supply voltage to the 'X' plates, with the internal
sweep generator switched off. The standard frequency is adjusted till the unit pattern
appears as a circle or an ellipse, indicating that both signals are of same frequency.
Examples
INDUSTRIAL APPLICATIONS OF CRO
• Because the oscilloscope is an extremely flexible and versatile instrument, it can be used to
measure a number of parameters associated with DC and AC signals. Using a single channel
oscilloscope, it is capable of making measurements of voltage current, time, frequency and
rise/fall time. If a dual trace oscilloscope is used the phase shift between two synchronous
signals can be measured. Other major applications of CRO are listed below:
• (A) In Radio Work
• 1. To trace and measure a signal throughout the RF, IF and AF channels of radio and television
receivers.
• 2. It provides the only effective way of adjusting FM receivers, broadband high frequency RF
amplifiers and automatic frequency control circuits;
• 3. To test AF circuits for different types of distortions and other spurious oscillations;
• 4. To give visual display of wave-shapes such as sine waves, square waves and their many
different combinations;
• 5. To trace transistor curves
• 6. To visually show the composite synchronized TV signal
• 7. To display the response of tuned circuits etc.
• (B) Scientific and Engineering applications:
• 1.Measurement of ac/dc voltages,
• 2. Finding B/H curves for hysteresis loop,
• 3. for engine pressure analysis,
• 4. for study of stress, strain, torque, acceleration etc.
• 5. Frequency and phase determination by using Lissajous figures,
• 6. Radiation patterns of antenna,
• 7. Amplifier gain,
• 8. Modulation percentage,
• 9. Complex waveform as a short-cut for Fourier analysis,
• 10. Standing waves in transmission lines etc