Emi Course File

EMI COURSE FILE T.V.D.R ECE DEPT 1

Padmasri Dr B V Raju Institute of Technology Vishnupur, NARSAPUR, Medak (dist)

Study material on

Electronic Measurements

& Instrumentation

(VI B.Tech ECE I Semester)

2011-12

Prepared by

T.VASUDEVAREDDY Sr Asst professor ECE department


ELECTRONIC MEASUREMENTS &

INSTRUMENTATION

SYLLABUS UNIT I Performance characteristics of instruments, Static characteristics, Accuracy, Resolution, Precision, Expected value, Error, Sensitivity, Errors in Measurement, Dynamic Characteristics, speed of response, Fidelity, Lag and Dynamic error. DC Voltmeters- Multirange, Range extension/Solid state and differential voltmeters, AC voltmeters- multi range, range extension, shunt. Thermocouple type RF ammeter, Ohmmeters series type, shunt type, Multimeter for Voltage, Current and resistance measurements. UNIT II Signal Generator- fixed and variable, AF oscillators, Standard and AF sine and square wave signal generators, Function Generators, Square pulse, Random noise, sweep, Arbitrary waveform. UNIT III Wave Analyzers, Haromonic Distortion Analyzers, Spectrum Analyzers, Digital Fourier Analyzers. UNIT IV Oscilloscopes CRT features, vertical amplifiers, horizontal deflection system, sweep, trigger pulse, delay line, sync selector circuits, simple CRO, triggered sweep CRO, Dual beam CRO, Measurement of amplitude and frequency. UNIT V Dual trace oscilloscope, sampling oscilloscope, storage oscilloscope, digital readout oscilloscope, digital storage oscilloscope, Lissajous method of frequency measurement, standard specifications of CRO, probes for CRO- Active & Passive, attenuator type, Frequency counter, Time and Period measurement. UNIT VI AC Bridges Measurement of inductance- Maxwell’s bridge, Anderson bridge. Measurement of capacitance, Schering Bridge. Wheat stone bridge. Wien Bridge, Errors and precautions in using bridges. Q-meter. UNIT VII Transducers- active & passive transducers : Resistance, Capacitance, inductance; Strain gauges, LVDT, Piezo Electric transducers, Resistance Thermometers, Thermocouples, Thermistors, Sensistors. UNIT VIII Measurement of physical parameters force, pressure, velocity, humidity, moisture, speed,


proximity and displacement, Data acquisition systems. TEXTBOOKS:

1. Electronic instrumentation, second edition - H.S.Kalsi, Tata McGraw Hill, 2004. 2. Modern Electronic Instrumentation and Measurement Techniques – A.D. Helfrick and W.D. Cooper, PHI, 5th Edition, 2002.

REFERENCES:

1. Electronic Instrumentation & Measurements - David A. Bell, PHI, 2nd Edition, 2003. 2. Electronic Test Instruments, Analog and Digital Measurements - Robert A.Witte, Pearson Education, 2nd Ed., 2004. 3. Electronic Measurements & Instrumentations by K. Lal Kishore, Pearson Education - 2005.


SUBJECT OBJECTIVE Measurement in one form or another is used regularly by all sorts of people in all

sorts of jobs. The primary emphasis of the subject is on the measurement of physical,

mechanical and electrical variables encountered in experimental investigators and industrial

processes. Engineers in various engineering branches who have to get themselves involved

with measurements of these electrical and non-electrical parameters by electrical

transudation methods will find this material immensely useful in their work. Similarly an

instrumentation engineer working in a continuous/batch process industry will be benefited

with these concepts as technology is shifting towards the implementation of electronic

instruments in place of pneumatic type which are being used earlier .the primary objective is

to bring these concepts within the reach of students (future engineers).


UNIT – I

OBJECTIVE:

The basic purpose of instrumentation in a process is to obtain requisite information

pertaining to the fruitful completion of process. The objective of fruitful completion in

industrial terminology is obtained when process efficiency is maximum with minimum cost

of production and desired level of product quality. Here measurement plays a vital role in

determining a quantity or a variable, which is done through a proper instrument. The purpose

this chapter is to study the basic characteristics of various kinds of instruments in conjunction

to their qualitative and quantitative performance.

Prerequisite:

1. Basic Electrical Technology 2. Basic engineering mathematics

Instrument: A device for determining the value or magnitude of a quantity or variable.

Functional elements of an Instrument: Any instrument or a measuring system can be described in general with the help of a block diagram.


Primary sensing element:

First receives energy from the measured medium and produces an output depending, in same way on, the measured quantify (measurand). An instrument always extracts some energy from the measured medium. Thus the measured quantify is always disturbed by the act of measurement, which makes a perfect measure theoretically impossible. Good instruments are designed to minimize this loading effect.

Variable-conversion element:

Convert the output signal of the primary sensing element to another more suitable variable, while preserving the information content of the original signal.

Variable-manipulation element: Change, in numerical value according to the some definite rule but a preservation of the physical nature of the variable, e.g. an electronic amplifier. Data-transmission element:

When the functional elements are separated, it becomes necessary to transmit the data from one to another.

Data-presentation element:

Presents the information about the measured quantity to one of the human senses. Data-storage/playback element:

Storage-pen/ink Storage/playback – Tape recorder/reproducer, memory Measurement system performance: The treatment of instrument and measurement system

characteristics can be divided into two distinct categories:

1. Static characteristics

2. Dynamic characteristics

1. Static characteristics: Measurement of the quantities that are either constant or Vary

slowly with time can be defined by a set of criteria that gives a meaningful description of

quality of measurement. These characteristics are Static characteristics. The main Static

characteristics discussed here are


1. Accuracy 2.Precision 3.Sensitivity 4.Reproducibility 5.Drift 5.Static error

Accuracy: Closeness with which an instrument reading approaches the true value of the

variable being measured.

Precision: A measure of the reproducibility of the measurements i.e., given a fixed value of a

variable, precision is a measure of the degree to which successive measurements differ from

one another.

Sensitivity: The ratio of output signal response of the instrument to a change of input or

measured variable.

Resolution: The smallest change in measured value to which instrument will respond.

Expected value: The most probable value that calculations indicate one should expect to

measure.

Error: Deviation from the true value of the measured variable.

Linearity: One of the best characteristics of an instrument is considered to be linearity.

Linearity defines in the terms of a linear relation between system’s input & output.

2. Dynamic characteristics:

Many are concerned with rapidly varying quantities and therefore for such cases we must

examine the dynamic relation which exists between input and output. This is normally done

with the help of differential equations. The performance criteria based upon dynamic

relations constitute the dynamic characteristics.

Dynamic characteristics of instruments

When dynamic or time-varying quantities are to measure, it is necessary to find the dynamic

response characteristics of the instrument being used for measurement. The dynamic inputs to

an instrument may be of following types:

1. Periodic inputs

2. Transient inputs

3. Random inputs

For studying the dynamic characteristics of an instrument it is necessary to represent each

instrument by its mathematical model, from which the governing relation between its input

and output is obtained.


Example: The relation between input and output signals of a temperature measuring element

like thermocouple is:

(1+ t D) Xo =KXi

Where Xi and Xo are input & output signals respectively and both being function of time,

while K and t are system constants. D is time derivative operator of first order and hence

system is called first order one.

The dynamic characteristics of an instrument can be determined experimentally with a known

dynamic input signals .For theoretical analysis for any dynamic input a solution of its

governing equations obtained from its mathematical model is desirable.

Example For second order system:

Taking second order instrument system, the governing equation is given by

a2 d2X0 + a1 dX0 + a0 X0 =b0Xi(t)

dt2 dt

The above equation can be written in dimensionless form as

( D2 + 2G D + 1) X0 =K Xi

W

(t) 2 W

Where K = b0/ a0 = static sensitivity

W= a0/ a2

G= a

= Un damped natural frequency

1

2 a

= damping ratio

0 * a2

D= d/dt

Higher Order Systems:

When the order of governing equation of an instrument or a combination instruments is high

it is convenient to plot a frequency response of the system by logarithmic plots known as

Bode Diagrams. The advantage of this method is the frequency response of a complex system

can be obtained by adding the response due to various first and second order system’s

responses


Errors in measurement:

Gross errors: Largely human errors, among them misreading of instruments, incorrect

adjustment and improper application of instruments.

Systematic errors: Short coming of instruments such as defective or worn parts, and effects

of environment on the equipment or the user.

Random errors: Those due to causes that can’t be directly established because of random

variation in the parameter or the system of measurement.

Errors are to be expected; they are intrinsic in the physical processes of measurement making. Categories of measurement errors and some subcategories, as follows.

1. Theoretical errors: The explicit or implicit model on which we base our interpretation of our measurements may be inapplicable or inaccurate.

(a) Range of Validity: A model is applicable only within a limited range of

conditions. Be- yond that, it will give inaccurate predictions. (b) Approximation: Models have finite precision even within their range of

validity. Don't quote ten significant figures when only two are trustworthy.

2. Static errors:

(a) Reading errors: Due to misreading, or a difficulty in accurately reading, the display of the instrument.

i. Parallax: Analog meters use a needle as a pointer to indicate the measured value.

Reading this at an oblique angle causes a misreading, known as a parallax reading error.

ii. Interpolation: The needle often rests between two calibrated marks. Guessing its position by interpolation is subject to an error that depends on the size of the scale, and on the visual acuity and experience of the person reading the meter.

iii. Last-digit bobble: Digital readouts re often observed to oscillate between two neighboring values, for example a digital voltmeter (DVM) may alternately show 3.455 and 3.456 volts. This occurs when the actual value is about midway between the two displayed values. Small variations in the system under test, or in the meter itself, are suffcient to change the reading when it is delicately poised between the two values.

(b) Environmental errors: Measurements can be affected by changes in

ambient factors.


i. Temperature. ii. Pressure.

iii.Electromagnetic (EM) fields: Static electric or magnetic f i elds, dynamic (changing) fields, and propagating fields (radiation) can interfere with measurements. A particularly common example is the mains electricity supply, which is ideally a sinusoid; in Australia this is specialized to have a frequency of 50 Hz. In reality, mains power is not a pure sinusoid, so it contributes interference at other frequencies also.

(c) Characteristic errors: Static errors intrinsic to the measuring instrument or process.

Physical limitations and manufacturing quality control are factors in several characteristic errors. Incorrect calibration can also contribute!

` i. Manufacturing Tolerances: Design and manufacturing processes are

frequently in- exact. For example, the calibrated marks on a ruler are not 1.0000 millimeters apart. Hopefully some will be slightly above and some slightly below, so that over a series of measurements these errors will be random and so balance out, but they might not the errors in the manufacturing process of one or more batches of rulers might be systematically biased.

ii. Zero Offset: a meter (for example) may read zero when the actual value is nonzero. This is a common form of calibration error.

iii. Gain error: amplifiers are widely used in instruments such as CRO probes, and we may trust that “times 10” means precisely what it says only when the amplification has been carefully calibrated.

iv. Processing error: modern instruments contain complex processing devices such as analog computers which can introduce errors into the process leading to the displayed value of a measurement. Digital devices have f i nite precision (see quantization errors, below) and are occasionally wrongly programmed: a small programming error often produces large errors in the results.

v. Repeatability error: instruments change over time, which is why they must be regularly calibrated, just as a car must be serviced. Instruments change, however slightly, even between consecutive measurements. The act of measurement itself may affect the instrument, for example spring scales lose some elasticity with every use.

vi. Nonlinearity: ideally, an instrument designed to be linear has an output which is proportional to its input, but this is only approximately true, and then only within a range of validity. Drive an amplifier to too high a gain and it will operate in its nonlinear regions, producing a severely distorted output signal.

vii. Hysteresis: Some measurement systems remember some of their past history, and produce different results if a different path is taken to the same nal set of external conditions.

viii. Resolution: devices can only resolve (that is, distinguish) values that are suffi ciently separated. For example, optical instruments cannot easily resolve objects less than one wavelength apart.


ix. Quantization: When analog values are recorded on a digital system (analog to digital conversion), the values are rounded to the nearest available step.

3. Dynamic errors: Due to the measured quantity changing, or the measured object

moving, during measurement. Carr mentions two kinds of dynamic errors.

(a) Mechanical: Such as the inertia of the needle of an analog meter. (b) Electronic: For example, a sample and hold circuit with a long time constant, used

in an attempt to record a high frequency sine wave. 4. Insertion errors: We wish to know the values that quantities have in a system when

the measuring instrument is absent, but of course we can only with the instrument present, and that constitutes a different system! The values can differ between the two systems, because the effect of the instrument is never zero, and may not be negligible!

(a) Classical insertion error: An example is the use of a voltmeter with a low

resistance compared to the component or subsystem across which it is connected. Another is an ammeter with a large resistance compared to the current loop in which it is placed.

(b) Quantum insertion error: The theory of quantum physics places restrictions on the precision with which certain quantities may simultaneously be measured (Heisenberg Uncertainty Principle). In optics this can be a significant concern.

Dealing with errors

To minimize measurement errors, one may improve the procedure, or one may use statistical averaging.

Procedural improvements may include using more accurate instruments, in particular meters that cause less disturbance to the system being measured, for example a voltmeter with an impedance that is much higher than that of the circuit under test.

Statistical averaging involves attempting the same measurement on different occasions, or by using different instruments — for example by measuring the current in a loop using several ammeters in series and reading them at the same time.

Example Problem1: In calculating voltage drop, a current of 3.18 A is recorded in a

resistance of 35.68 .Calculate the voltage drop across the resister to the appropriate number

of significant figures.

Sol:- E=IR=35.68 * 3.18 =113.4624=113V

Since there are three significant figures involved in the multiplication the answer can be

written only to a Maximum of three significant figures.


ExampleProblem2: Subtract 628 3 from 826 5 and express the range of doubt in the

answer as a percentage.

Sol: N1

N

=826 5(0.605%)

2

=6283(=0.477%)

Difference =1988(= 4.0.4%)

.

Suspension Galvanometer principle:

A coil of fine wire is suspended in a magnetic field produced by a permanent magnet.

According to fundamental law of electromagnetic force, the coil will rotate in the magnetic

field when it carries an electric current. The fine filament suspension of the coil serves to

carry current to and from it, and the elasticity of the filament sets up a moderate torque in

opposition to the rotation of the coil. The coil will continue to deflect until its electro

magnetic torque balances mechanical counter torque of the suspension. The coil deflection

therefore is a measure of the magnitude of the current carried by the coil.

The equation for the developed torque, is

T = B* A*I*N

Where T = torque

B = flux density

A = effective coil area

I = current in the coil

N = turns of wire in the coil.


Types of damping Mechanisms:

1. Mechanical damping: caused mainly by motion of the coil through the air

surrounding it.

2. Electromagnetic damping: caused by induced effects in the moving coil as it

rotates in the magnetic field.

PERMANENT-MAGNET MOVING –COIL MECHANISM:

The basic PMMC movement is often called the d’ Arsonval movement, after its inventor.

This design offers the largest magnet in a given space and is used when

maximum flux in the air gap is required.

The PMMC basic movement is inherently insensitive to temperature, but it may be

temperature – compensated by the appropriate use of series and shunt resistors of copper and

manganin.


DC Ammeters:

The basic movement of a dc ammeter is a PMMC galvanometer. Since coil winding of a

basic movement is small and light, it can carry only very small currents. When large currents

are to be measured, it is necessary to bypass the major part of the current through a

resistance, called a shunt. A universal or Ayrton shunt eliminates the possibility of having

the meter in the circuit without a shunt and this is gained at the price of a slightly overall

meter resistance.

The shunt resistance Rsh = Im Rm / I-Im

The addition of a series resistor converts the basic d’Arsonval movement into DC Voltmeter.

The multiplier limits the current movement so as not to exceed the value of full-scale

deflection. The voltmeter measures the voltage across the two points of a circuit or a voltage

across a circuit component.

DC Voltmeter:

The voltmeter must be connected across the two points or a component to measure the

potential difference with the proper polarity.

The multiplier resistance can be calculated as:

Let Rm

R

= Internal resistance of the coil i.e. meter

s

Rs

Rm

Im

+

- = Series multiplier resistance


Im

V = Full range deflection voltage to be measured

= Full scale deflection current

From figure,

V = Im (Rm + Rs

V = I

)

m Rs + Im Rm

R

s = V - Rm I

m

The multiplying factor for multiplier is the ratio of full range voltage to be measured and the

drop across the basic meter.

Rs m = 1 +

R m = Multiplying factor

m

Hence multiplier resistance can be expressed as Rs = (m – 1) Rm

Voltmeter Sensitivity:

The sensitivity S is reciprocal to full-scale deflection current of basic movement.

S = 1 Ω Ifsd

V

Voltmeter Loading Effects: When the voltmeter resistance is not high enough, connecting it across a circuit component can change circuit resistance, which changes circuit current and voltage. The measured voltage decreases compared to the voltage without the voltmeter. This effect is called voltmeter loading, because additional current is drawn by the voltmeter. An ideal voltmeter would have infinite resistance and no loading effects.

1. Always set the range to the highest voltage and reduce as needed

2. Observe polarity

3. The voltmeter must be connected across or in parallel with the component voltage to be measured.

4. Reading should be made as close to full scale as possible

5. Prevent parallax error

The following SAFETY PRECAUTIONS should be observed when using a voltmeter. • Always connect a voltmeter in parallel. • Always start with the highest range.


• De energizes and discharges the circuit before connecting or disconnecting the voltmeter.

• Never use a dc voltmeter to measure an ac voltage. • On a dc voltmeter, observe the proper polarity.

Extending the range of dc voltmeter. A dc voltmeter can be converted into a multirange voltmeter by connecting a number

of resistors (multipliers) along with a range switch to provide a greater number of workable ranges

Figure below shows a multirange voltmeter using four position switch and 4 multipliers R1, R2, R3, and R4 for voltage values V1, V2, V3 and V4

AC Ammeters and Voltmeter:

D’Arsonval movement responds to average or dc value of current through the moving coil. If

the movement carries an alternating current with positive and negative half-cycles, the

driving torque would be in one direction for positive alternation and the other direction for

negative alternation. To measure ac D’Arsonval movement some means must be devised to

obtain unidirectional torque that does not reverse each half-cycle.

There are two methods to achieve this

1. Using rectification methods

2. Using Heating effect of alternating current to produce indication of its magnitude

Extending the range of dc ammeter

To obtain a multi range ammeter, a number of shunts are connected across the

movement with a multi-position switch

Referring to the figure, the circuit has 4 shunts Ra, Rb, Rc and Rd which can be

placed in parallel with the movement to give four different current ranges


OHMMETERS

The basic meter movement can also be used to measure resistance. The resulting circuit is called an ohmmeter. In its basic form, the ohmmeter is nothing more than a meter movement, a battery, and a series resistance.

An ohmmeter forces current to flow through an unknown resistance and then measures the resulting current. For a given voltage, the current is determined by the unknown resistance.

When using an ohmmeter it is first necessary to zero it.

The ohmmeter scale is nonlinear. Voltmeter and ammeter scales are linear.

Because zero is on the right side of an ohmmeter it is referred to as a back-off scale.

1. Zero before using or changing ranges.

2. Ensure that power is removed from the circuit being measured

3. Connect the leads across the component and read the resistance multiplying by 1, 10, etc. depending on the range.

4. If the component under test has a parallel connection of another component, an invalid reading may be obtained due to the parallel combination of resistances. To overcome this isolates the component.

OHMMETER SAFETY PRECAUTIONS

The following safety precautions and operating procedures for ohmmeters are the MINIMUM necessary to prevent injury and damage.

• Be certain the circuit is de energized and discharged before connecting an ohmmeter.


• Do not apply power to a circuit while measuring resistance.

• When you are finished using an ohmmeter, switch it to the OFF position if one is provided and remove the leads from the meter.

• Always adjust the ohmmeter for 0 (or ∞ in shunt ohmmeter) after you change ranges before making the resistance measurement.

THE ANALOG MULTIMETER (VOM) The analog multimeter combines the ammeter, voltmeter, and ohmmeter into one unit. The

name multimeter comes from the term multiple meter. It is also commonly called a VOM

(volt-ohm-meter). The VOM is a DC ammeter, an AC ammeter, a DC voltmeter, an AC

voltmeter, and an ohmmeter all in one package.

Most multi meters use the D’Arsonval meter movement and have a built-in rectifier for AC measurement. Thermocouple meter The principle of operation of the thermocouple meter is shown in Figure. The measured a.c. voltage signal is applied to a small element. This heats up and the resulting temperature rise is measured by a thermocouple. The d.c. voltage generated in the thermocouple is applied to a moving-coil meter. The output meter reading is an R.M.S. quantity that varies in a non-linear fashion with the magnitude of the measured voltage. Very high-frequency voltage signals up to 50MHz can be measured by this method.

Thermocouple meter

THE DIGITAL MULTIMETER (DMM)

A digital multimeter displays a digital value of the measurement.

An analog to digital converter (A/D) is used to convert the analog values at the input into a digital (binary) form. An LCD or LED display shows the value.


The input resistance is typically 10 M ohm on all ranges. For AC measurements a rectifier is used.

The frequency range for AC measurements is limited to 45 to 1000 Hz.

In most DMMs the left or most significant digit is known as a half digit because it can display only a 0 or 1. a DMM with four digits is often called a 3 ½ digit DMM. A 3 ½ digit DMM can display 19.99 V but 29.99 V is displayed as 30.0 V.

A drawback is sample rate. 2.5 – 4 times per second. Some DMMs have a bar-graph that updates 25-40 times per second.

Many DMMs include additional types of measurements such as frequency, capacitance, inductance, and transistor testing. Many DMMs also have a diode test feature to measure the junction voltages.

Subjective questions:

1. What is the difference between accuracy and precision

2. List four sources of possible errors in instruments and Explain three general

classes of errors.

3. The resistance of an unknown resister is determined by wheat stone bridge

method. The solution for the unknown resistance is stated as R=R1*R2/R3 Where

R1=500 Ω ± 1%

R2=615 Ω ± 1%

R3=100 Ω ± 1%

Calculate (a) nominal value of unknown resistor

(b) Limiting error of in ohms

(c) Limiting error in percentage

4. A temperature measuring system with time constant of two seconds is used

To measure temperature of heating medium which changes sinusoidal between

350 and 300 degree C with periodic time of 20s . Find the maximum and

minimum of values of temp, as indicated by the measuring system and time lag

between output and input signals.

5. A second order instrument is subjected to a sine input. Un damped natural

frequency is 3 Hz and damping ratio is 0.5 .Calculate the amplitude ratio and

phase angle for an input frequency of 2HZ


Objective questions:

1. The basic movement of a DC ammeter or voltmeter is___________ galvanometer.

2. In order to convert PMMC into an ammeter ________ is employed and to convert

into voltmeter ______ is used.

3. In a PMMC excitation force is generated by _________, controlling force by_______

and damping force by___________.

4. The significant numbers of an instrument who’s output is the sum of two subsystems.

One having 3 significant figures and the other 4 significant figures is _______.

5. ________ is the smallest amount of input change that can be sensed by instrument.

6. _______ is the smallest amount of input that can be sensed by instrument.

7. Dead band is the total range of __________________.

8. In watt-hour meter the movable coil is _______ coil and fixed coil is _________ coil.


UNIT-II Signal generators A signal generator is very vital equipment in the test setups and in electronic developments and trouble shooting. Signal generators provide a variety of waveforms for testing electronic circuits, usually at low powers. A signal generator is an electronic device that generates repeating or non-repeating electronic signals (in either the analog or digital domains). They are generally used in designing, testing, troubleshooting, and repairing electronic or electro acoustic devices; though they often have artistic uses as well.

There are many different types of signal generators, with different purposes and applications (and at varying levels of expense); in general, no device is suitable for all possible applications.

Traditionally, signal generators have been embedded hardware units, but since the age of multimedia-PCs, flexible, programmable software tone generators have also been available. The term “Oscillator” used to describe an instrument that provides only sinusoidal output signal, and the term “Generator” to describe an instrument that provides several output waveforms, including sine wave, square wave, triangular wave and pulse trains, as well as an amplitude modulated waveform. Requirements of signal generators.

1. The output frequency and the amplitude of the signal should be very stable. 2. The amplitude of output signal should be controllable from very small to relatively

large values. 3. The signal should be distortion free.

The above mentioned requirements vary for special generators, such as function generators, pulse and sweep generators. The frequency band limits of signal generators are as follows.

Band

Approximate Range

Audio Frequency (AF)

20Hz-20 KHz

Radio Frequency (RF)

Above 30 KHz

Very Low 15-100 KHz


Frequency (VLF) Low Frequency (LF)

100-500 KHz

Broadcast 0.5-1.5 KHz Video dc-5 MHz High Frequency (HF)

1.5-30 MHz

Very High Frequency (VHF)

30-300 MHz

Ultra High Frequency (UHF)

300-3000 MHz

Microwave Beyond 3 GHz The Audio frequency oscillators are divided into two categories. 1. Fixed frequency AF oscillators 2. Variable frequency AF oscillators Fixed frequency AF oscillator: Many instrument circuits contain oscillator as one of its integral parts to provide output signal within the specified fixed audio frequency range. This specified audio frequency range can be 1 KHz signal or 400 Hz signal. The 1 KHz frequency signal is used to excite a bridge circuit and 400 Hz frequency signal is used for audio testing. A fixed frequency AF oscillator employs an iron core transformer. Due to this a positive feedback is obtained through the inductive coupling placed between the primary winding and secondary winding of the transformer and hence fixed frequency oscillations are generated. Variable frequency AF oscillator: It is a general purpose oscillator used in laboratory. It generates oscillations within the entire audio frequency range i.e. from 20 Hz to 20 KHz. This oscillator provides a pure, constant sine wave output throughout this AF range. The examples of variable AF oscillators are used in laboratory are RC feedback oscillator, beat frequency oscillator. Audio frequency Sine and Square wave generator: Wien bridge oscillator is the heart of an AF sine and square wave generator. Depending upon the position of the switch the generator gives the output as sine wave or square wave. The Wien bridge oscillator generates a sine wave. Depending upon the position of the switch, it is switched to either circuit. In the square wave generation section, the output of the Wien bridge oscillator is fed to square wave shaper circuit which uses Schmitt trigger circuit. The attenuators in both the sections are used to control output signal level. Before attenuation the


signal level is made very high using sine wave amplifier and square wave amplifier. Square wave and Pulse generator: The square wave generator and pulse generators are normally used as measuring devices in combination with the oscilloscope. The basic difference between the square wave generator and pulse generator is in the duty cycle. Duty cycle: The ratio of average value of a pulse over one cycle to the peak value. (OR) The ratio of the pulse width to the pulse period of one cycle. The square wave generator produces an output voltage with equal ON and OFF periods as duty cycle is 0.5 or 50% as the frequency of oscillation is varied. Then we can state that irrespective of the frequency of operation, the positive and negative half cycles extend over half of the total period. If you consider a pulse, the total period of the pulse is T. This pulse can be divide into two parts namely ON and OFF period. The ON and OFF period when combined together gives a period of one pulse. The pulse width is t.


Duty cycle for a pulse = Pulsewidth

Pulseperiod

= tT

According to other definition,

Duty cycle of a pulse = Averagevalue

Peakvalue =tT =

Thus depending on the ON period of the pulse the duty cycle may vary between 50% and 95%. Generally the pulse generator can supply more power than square wave generator during ON period of a pulse. The square wave generators are used when the system is analyzed low frequency characteristics, testing of audio system. Function generator: The function generator is an instrument which generates different types of wave forms with a wide range of frequencies. The most required common waveforms are sine wave, saw tooth wave, triangular wave and square wave. These different output waveforms of the function generator are available simultaneously.

An arbitrary waveform generator An arbitrary waveform generator (AWG) is an advanced signal generator that can generate a waveform of almost any shape. The generated waveform can then be inserted into the device you wish to test and then analyzed as it progresses through the device to confirm correct operation, or to highlight a fault. Arbitrary waveform generators are often expensive and so are usually only found in high-end test equipment, however, several Pico Scope PC Oscilloscopes include a built-in AWG.

The arbitrary waveform generator is programmed with a data file, supplied by the user, which defines the output voltage at a number of equally spaced points in time. The circuit uses this data to reconstruct the waveform with a specified amplitude and frequency


UNIT-III Harmonic Distortion Harmonic distortion is a measure of the amount of power contained in the harmonics of a fundamental signal. Harmonic distortion is inherent to devices and systems that possess nonlinear characteristics—the more nonlinear the device, the greater its harmonic distortion. Harmonic distortion can be expressed as a power ratio or as a percentage ratio. Use the following formula to express it as a power ratio:

where PHD is the power of the harmonic distortion in dBc, Pfund is the fundamental signal power in dB or dBm, and Pharm

Convert the powers to voltages to express harmonic distortion as a percentage ratio: is the power of the harmonic of interest in dB or dBm.

In some applications, the harmonic distortion is measured as a total percentage harmonic distortion (THD). This measurement involves the power summation of all the harmonics in the spectrum band, defined in the following equation:

Wave analyzer An apparatus that assesses a complex mixture of wave forms by separating out their component frequencies and displaying their distribution.

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Spectrum analyzer: The most natural way to look at waveforms is in the time domain - looking at how a signal varies in amplitude as time progresses, i.e. in the time domain. This is what an oscilloscope is used for, and it is quite natural to look at waveforms on an oscilloscope display. However this is not the only way in which signals can be displayed Spectrum analysis is nothing but viewing a signal in frequency domain for signal analysis. The instrument that is being used for this task is spectrum analyzer. Practical spectrum analyzers use the same principles as a super heterodyne receiver. The spectrum analyzer is similar to an up converting super hetero dyne receiver .The input of the spectrum analyzer is first converted to an IF higher than highest input frequency. The frequency of local first oscillator swept electronically usually using varactor. Spectrum analyzers are widely used within the electronics industry for analyzing the frequency spectrum of radio frequency, RF and audio signals. Looking at the spectrum of a signal they are able to reveal elements of the signal, and the performance of the circuit producing them that would not be possible using other means. Spectrum analyzers are able to make a large variety of measurements and this means that they are an invaluable tool for the RF design development and test laboratories, as well as having many applications for specialist field service.

In exact terms it is necessary that the signal must be evaluated over an infinite time for the transformation to hold exactly. However in reality it is sufficient to know that the waveform is continuous over a period of at least a few seconds, or understand the effects of changing the signal.

It is also worth noting that the mathematical Fourier transformation also accommodates the phase of the signal. However for many testing applications the phase information is not needed and considerably complicates the measurements and test equipment. Also the information is normally not needed, and only the amplitude is important.

By being able to look at signals in the time domain provides many advantages and in particular for RF applications, although audio spectrum analyzers are also widely used. Looking at signals in the frequency domain with a spectrum analyzer enables aspects such as the harmonic and spurious content of a signal to analyze. Also the width of signals when modulation has been applied is important. These aspects are of particular importance for developing RF signal sources, and especially any form of transmitter including those in cellular, Wi-Fi, and other radio or wireless applications. The radiation of unwanted signals will cause interference to other users of the radio spectrum, and it is therefore very important to ensure any unwanted signals are kept below an acceptable level, and this can be monitored with a spectrum analyzer.


Spectrum analyzer basics There are many different types of RF test equipment that can be used for measuring a variety of different aspects of an RF signal. It is therefore essential to choose the right type of RF test equipment to meet the measurement requirements for the particular job in hand.

Test Instrument Type

Frequency measurement

Intensity (I) amplitude measurement

Application

Power meter N Y Use for accurate total power measurements

Frequency counter Y N

Used to provide very accurate measurements of the dominant frequency within a signal

Spectrum analyzer Y Y

Used primarily to display the spectrum of a radio frequency signal. Can also be used to make power and frequency measurements, although not as accurately as dedicated instruments

RF network analyzer Y Y Used to measure the properties of RF

devices Properties of RF measuring instruments in common use The spectrum analyzer is able to offer a different measurement capability to other instruments. Its key factor is that it is able to look at signals in the frequency domain, i.e. showing the spectrum, it is possible to see many new aspects of the signal.

A spectrum analyzer display, like that of an oscilloscope has two axes. For the spectrum analyzer the vertical axis displays level or amplitude, whereas the horizontal axis displays frequency. Therefore as the scan moves along the horizontal axis, the display shows the level of any signals at that particular frequency.

This means that the spectrum analyzer, as the name indicates analyses the spectrum of a signal. It shows the relative levels of signals on different frequencies within the range of the particular sweep or scan. General format of the display on a spectrum analyzer In view of the very large variations in signal level that are experienced, the vertical or amplitude axis is normally on a logarithmic scale and is calibrated in dB in line with many other measurements that are made for signal amplitudes. The horizontal scale conversely is normally linear. This can be adjusted to cover the required range. The term span is used to give the complete calibrated range across the screen. Terms like scan width per division may also be used and refer to the coverage between the two major divisions on the screen. Types of spectrum analyzer Just as in the case of other instruments, there are a number of types of spectrum analyzer that


can be seen in the manufacturers catalogues. These two types are:

• Swept or super heterodyne spectrum analyzers: The operation of the swept frequency spectrum analyzer is based on the use of the super heterodyne principle, sweeping the frequency that is analyzed across the required band to produce a view of the signals with their relative strengths. This may be considered as the more traditional form of spectrum analyzer, and it is the type that is most widely used.

• Fast Fourier Transform, FFT analyzers: These spectrum analyzers use a form of Fourier transform known as a Fast Fourier Transform, FFT, converting the signals into a digital format for analysis digitally. These analyzers are obviously more expensive and often more specialized.

• Audio spectrum analyzer: Although not using any different basic technology, audio spectrum analyzers are often grouped differently to RF spectrum analyzers. Audio spectrum analyzers are focused, as the name indicates, on audio frequencies, and this means that low frequency techniques can be adopted. This makes them much cheaper. It is even possible to run them on PCs with a relatively small amount of hardware - sometimes even a sound card may suffice for some less exacting applications.

Both swept / super heterodyne and FFT spectrum analyzer technologies have their own advantages. The more commonly used technology is the swept spectrum analyzer as it the type used in general-purpose analyzers enabling these analyzers to operate up to frequencies of many GHz. However a swept frequency analyzer is only capable of detecting continuous signals, i.e. CW as time is required to capture a given sweep, and they are not able to capture any phase information.

FFT analyzer technology is able to capture a sample very quickly and then analyze it. As a result an FFT analyzer is able to capture short lived, or one-shot phenomena. They are also able to capture phase information. However the disadvantage of the FFT analyzer is that its frequency range is limited by the sampling rate of the analogue to digital converter, ADC. While ADC technology has improved considerably, this places a major limitation on the bandwidths available using these analyzers.

In view of the fact that both FFT and super heterodyne analyzer technologies have their own advantages, many modern analyzers utilize both technologies, the internal software within the unit determining the best combinations for making particular measurements. The super heterodyne circuitry enabling basic measurements and allowing the high frequency capabilities, whereas the FFT capabilities are introduced for narrower band measurements and those where fast capture is needed. An analyzer will often determine the best method dependent upon factors including the filter settling time and sweep speed. If the spectrum analyzer determines it can show the spectrum faster by sampling the required bandwidth, processing the FFT and then displaying the result, it will opt for an FFT approach, otherwise it will use the more traditional fully super heterodyne / sweep approach. The difference between the two measurement techniques as seen by the user is that using a traditional sweep approach, the result will seen as sweep progresses, when an FFT measurement is made, the result cannot be displayed until the FFT processing is complete.


Advantages and disadvantages of a swept or sweep spectrum analyzer The sweep or swept spectrum analyzer has a number of advantages and disadvantages when compared to the main other type of analyzer known as the FFT spectrum analyzer. When choosing which type will be suitable, it is necessary to understand the differences between them and their relative merits.

Advantages of the super heterodyne spectrum analyzer technology

• Able to operate over wide frequency range: Using the super heterodyne principle, this type of spectrum analyzer is able to operate up to very high frequencies - many extend their coverage to many GHz.

• Wide bandwidth: Again as a result of the super heterodyne principle this type of spectrum analyzer is able to have very wide scan spans. These may extend to several GHz in one scan.

• Not as expensive as other spectrum analyzer technologies: Although spectrum analyzers of all types are expensive, the FFT style ones are more expensive for a similar level of performance as a result of the high performance ADCs in the front end. This means that for the same level of base performance, the super heterodyne or sweep spectrum analyzer is less expensive.

Disadvantages of the super heterodyne spectrum analyzer technology

• Cannot measure phase: The super heterodyne or sweep spectrum analyzer is a scalar instrument and unable to measure phase - it can only measure the amplitude of signals on given frequencies.

• Cannot measure transient events: FFT analyzer technology is able to sample over a short time and then process this to give the required display. In this way it is able to capture transient events. As the super heterodyne analyzer sweeps the bandwidth required, this takes longer and as a result it is unable to capture transient events effectively.

Balancing the advantages and disadvantages of the swept or super heterodyne spectrum analyzer, it offers excellent performance for the majority of RF test equipment applications. Combining the two technologies in one item of test equipment can enable the advantages of both technologies to be utilized. Swept or sweep spectrum analyzer basics The swept spectrum analyzer uses the same super heterodyne principle used in many radio receivers as the underlying principle on which its operation depends. The super heterodyne principle uses a mixer and a second locally generated local oscillator signal to translate the frequency.

The mixing principle used in the spectrum analyzer operates in exactly the same manner as it does for a super heterodyne radio. The signal entering the front end is translated to another frequency, typically lower in frequency. Using a fixed frequency filter in the intermediate frequency section of the equipment enables high performance filters to be used, and the analyzer or receiver input frequency can be changed by altering the frequency of the local


oscillator signal entering the mixer.

Although the basic concept of the spectrum analyzer is exactly the same as the super heterodyne radio, the particular implementation differs slightly to enable it to perform is function as a spectrum analyzer.

Super heterodyne or swept frequency spectrum analyzer block diagram The frequency of the local oscillator governs the frequency of the signal that will pass through the intermediate frequency filter. This is swept in frequency so that it covers the required band. The sweep voltage used to control the frequency of the local oscillator also controls the sweep of the scan on the display. In this way the position of the scanned point on the screen relates to the position or frequency of the local oscillator and hence the frequency of the incoming signal. Also any signals passing through the filter are further amplified, detected and often compressed to create an output on a logarithmic scale and then passed to the display Y axis. Elements of a super heterodyne spectrum analyzer Although the basic concept of the sweep spectrum analyzer is fairly straightforward a few of the circuit blocks may need a little further explanation.

1. RF attenuator: The first element a signal reaches on entering the spectrum analyzer is an RF attenuator. Its purpose is to adjust the level of the signal entering the mixer to its optimum level. If the signal level is too high, not only may the reading fall outside the display, but also the mixer performance may not be optimum. It is possible that the mixer may run outside is specified operating region and additional mix products may be visible and false signals may be seen on the display. In fact when false signals are suspected, the input attenuator can be adjusted to give additional attenuation, e.g. +10 dB. If the signal level falls by more than this amount then it is likely to be an unwanted mix product and insufficient RF attenuation was included for the input signal level. The input RF attenuator also serves to provide some protection to very large signals. It is quite possible for very large signals to damage the mixer. As these mixers are very high performance components, they are not cheap to replace. A further element of protection is added. Often the input RF attenuator includes a capacitor and this protects the mixer from any DC that may be present on the line being measured.


2. Low pass filter and pre-selector: This circuit follows the attenuator and is included

to remove out of band signals which it prevents from mixing with the local oscillator and generating unwanted responses at the IF. These would appear as signals on the display and as such must be removed. Microwave spectrum analyzers often replace the low pass filter with a more comprehensive pre-selector. This allows through a band of frequencies, and its response is obviously tailored to the band of interest

3. Mixer: The mixer is naturally key to the success of the analyzer. As such the mixers are high performance items and are generally very expensive. They must be able to operate over a very wide range of signals and offer very low levels of spurious responses. Any spurious signals that are generated may mix with incoming signals and result in spurious signals being seen on the display. Great care must be taken when using a super heterodyne spectrum analyzer not to feed excessive power directly into the mixer otherwise damage can easily occur. This can happen when testing radio transmitters where power levels can be high and accidentally turning the attenuator to a low value setting so that higher power levels reach the mixer. As a result it is often good practice to use an external fixed attenuator that is capable of handling the power. If damage occurs to the mixer it will disable the spectrum analyzer and repairs can be costly in view of the high performance levels of the mixer.

4. IF amplifier: Despite the fact that attenuation is provided at the RF stage, there is also a necessity to be able to alter the gain at the intermediate frequency stages. This is often used and ensures that the IF stages provide the required level of gain. It ahs to be used in conjunction with the RF gain control. Too high a level of IF gain will increase the front end noise level which may result in low level signals being masked. Accordingly the RF gain control should generally be kept as high as possible without overloading the mixer. In this way the noise performance of the overall unit is optimized.

5. IF filter: The IF filters restrict the bandwidth that is viewed, effectively increasing the frequency resolution. However this is at the cost of a slower scan rate. Narrowing the IF bandwidth reduces the noise floor and enables lower level spurious signals to be viewed.

6. Local oscillator: The local oscillator within the spectrum analyzer is naturally a key element in the whole operation of the unit. Its performance governs many of the overall performance parameters of the whole analyzer. It must be capable of being turned over a very wide range of frequencies to enable the analyzer to scan over the required range. It must also have a very good phase noise performance. If the oscillator has a poor phase noise performance then it will not only result in the unit


not being able to make narrow band measurements as the close in phase noise on the local oscillator will translate onto the measurements of the signal under test, but it will also prevent it making any meaningful measurements of phase noise itself - a measurement being made increasingly these days.

7. Ramp generator: The ramp generator drives the sweep of the local oscillator and also the display. In this way the horizontal axis of the display is directly linked to the frequency.

8. Level detector: The level detector converts the signal from the IF filter into a signal voltage that can be passed to the display. Normally a logarithmic output is required for the display, although occasionally linear displays may be required. Any conditioning and switching for this will be contained within the level detector and associated display circuitry.

9. Display: In many respects the display is the heart of the test instrument as this is where the spectra are viewed. Originally cathode ray tubes were used, but a variety of more modern types of display are used these days. Additionally significant amounts of signal processing are used in spectrum analyzers these days, and this enables far higher degrees of functionality to be introduced into these test instruments.

FFT analyzer

The FFT analyzer technology is the less commonly used on its own, but it is able to offer some distinct advantages over the more common swept frequency analyzer. By combining the two technologies the advantages of each can be utilized to offer extremely high performance items of test equipment.

In general, spectrum analyzers are used to provide a view of radio frequency, or in some case audio frequency waveforms in the time domain. With other instruments able to provide views of other aspects of signals, the spectrum analyzer is uniquely placed to offer views of the spectrum of a signal, revealing aspects that other instruments are unable to do. With the FFT analyzer able to provide facilities that cannot be provided by swept frequency analyzers, enabling fast capture and forms of analysis that are not possible with sweep / super heterodyne techniques alone. Advantages and disadvantages of FFT analyzer technology FFT spectrum analyzer technology has a number of advantages and disadvantages when compared to the more familiar super heterodyne or swept frequency analyzer. When choosing which technology will be suitable, it is necessary to understand the differences between them and their relative merits.

• Analogue front end attenuators / gain: The FFT analyzer requires attenuators of gain stages to ensure that the signal is at the right level for the analogue to digital conversion. If the signal level is too high, then clipping and distortion will occur, too


low and the resolution of the ADC and noise become a problems. Matching the signal level to the ADC range ensures the optimum performance and maximizes the resolution of the ADC.

• Analogue low pass anti-aliasing filter: The signal is passed through an anti-aliasing filter. This is required because the rate at which points are taken by the sampling system within the FFT spectrum analyzer is particularly important. The waveform must be sampled at a sufficiently high rate. According to the Nyquist theorem a signal must be sampled at a rate equal to twice that of the highest frequency, and also any component whose frequency is higher than the Nyquist rate will appear in the measurement as a lower frequency component - a factor known as "aliasing". This results from the where the actual values of the higher rate fall when the samples are taken. To avoid aliasing a low pass filter is placed ahead of the sampler to remove any unwanted high frequency elements. This filter must have a cut-off frequency which is less than half the sampling rate, although typically to provide some margin, the low pass filter cut-off frequency is at highest 2.5 times less than the sampling rate of the FFT spectrum analyzer. In turn this determines the maximum frequency of operation of the FFT spectrum analyzer.

• Sampling and analogue to digital conversion: In order to perform the analogue to digital conversion, two elements are required. The first is a sampler which takes samples at discrete time intervals - the sampling rate. The importance of this rate has been discussed above. The samples are then passed to an analogue to digital converter which produces the digital format for the samples that is required for the FFT analysis.

• FFT analyzer: With the data from the sampler, which is in the time domain, this is then converted into the frequency domain by the FFT analyzer. This is then able to further process the data using digital signal processing techniques to analyze the data in the format required.

• Display: With the power of processing it is possible to present the information for display in a variety of ways. Today's displays are very flexible and enable the information to be presented in formats that are easy to comprehend and reveal a variety of facets of the signal. The display elements of the FFT spectrum analyzer are therefore very important so that the information captured and processed can be suitably presented for the user.

Advantages of FFT spectrum analyzer technology

• Fast capture of waveform: In view of the fact that the waveform is analyzed digitally, the waveform can be captured in a relatively short time, and then the subsequently analyzed. This short capture time can have many advantages.

• Able to capture non-repetitive events: The short capture time means that the FFT analyzer can capture non-repetitive waveforms, giving them a capability not possible with other spectrum analyzers.

• Able to analyze signal phase: As part of the signal capture process, data is gained


which can be processed to reveal the phase of signals.

Disadvantages of the FFT spectrum analyzer technology

• Frequency limitations: The main limit of the frequency and bandwidth of FFT spectrum analyzers is the analogue to digital converter, ADC that is used to convert the analogue signal into a digital format. While technology is improving this component still places a major limitation on the upper frequency limits or the bandwidth if a down-conversion stage is used.

• Cost: The high level of performance required by the ADC means that this item is a very high cost item. In addition to all the other processing and display circuitry required, this results in the costs rising for these items.

Logic analyzers Logic analyzers are widely used for testing digital or logic circuits. They appeared shortly after the first microprocessors were used because to fault find these circuits required the instrument to have access to a large number of lines, more than could be seen using a conventional oscilloscope. Since then the need for logic analyzers has grown, especially as the complexity of circuits has continued to grow.

Although oscilloscopes can perform many of the functions of a logic analyzer, the analyzer is more suited to operating in a digital environment because it is able to display relative timing of a large number of signals. Essentially a logic analyzer enables traces of logic signals to be seen in such a way that the operation of several lines in a digital circuit can be monitored and investigated.

Logic analyzers come in a variety of formats. One of the most popular is a typical test instrument case. However it is also possible to utilize the processing power of a computer and PC based logic analyzers are available. The actual choice of logic analyzer will depend upon the cost budget and the actual requirements. TH PC logic analyzers are a particularly cost effective method of creating an analyzer. However the main drawback of the PC logic analyzers is that their functionality is not as great as the dedicated logic analyzers, which is only to be expected in view of the cost differential. What makes a logic analyzer Logic analyzers are designed to monitor a large number of digital lines. They possess a horizontal time axis and a vertical axis to indicate a logic high or low states. As logic analyzers are optimized for monitoring a large number of digital circuits, typically they may have anywhere between about 32 and 132 channels they can monitor, each channel monitoring one digital line. However some specialized logic analyzers are suitably scaled to be able to handle many more lines, and in this way enable tracking and fault finding on much more complex systems.

One of the main points to note about a logic analyzer is that it does not give a full analogue display of the waveform. Although it shows the logical high and low states as a waveform on the display, it only looks for whether the state of a line is high, i.e. above a certain trigger voltage, or low, i.e. below the trigger voltage. Having decided whether a line is high or low,


the logic analyzer then displays the relevant level. This means that it is not possible to see small amplitude variations such as ringing on the signal, however the state of the lines and their timings are displayed.

To achieve this logic analyzer will sample the waveforms states and store the level as either high or low at each sample time. The displayed waveform will then look like a timing diagram from a simulator or data-book. It will display the state of the lines, and the timings of any transitions.

In this way it is possible to analyze the waveforms produced by the circuit and ensure they match those that are expected. Any differences can then be used to trace any problems in the circuit design Logic analyzer probes With the large number of signals required to be monitored, often from a small area on a board and possibly even from one integrated circuit, the design of the probes can be a critical issue.

The logic analyzer probes contain an internal comparator where the voltage of the waveform on the board is compared against the threshold voltage. This can be set using the main instrument to a variety of levels so that signals from a variety of logic families can be monitored.

Logic analyzer probes can take a variety of physical forms, but generally fall into one of three categories:

• Multichannel probes that use a dedicated connector on the circuit board. These probes enable a large number of points to be accessed using a high density connector. This facility has to be designed on to the board and may only be retained for the development phase where access for a logic analyzer is required.

• High density compression probes. These probes use a compression contact that does not have a dedicated connector. Contacts on the board are required for this type of connection.

• Flying lead probes. As the name implies, these probes are on a flying lead that is connected to a small electronic unit that contains the electronics for detecting the high and low levels. However these flying lead probes are used to monitor points that may not be included on any of the other access points.

Triggering One of the key features of a logic analyzer is its triggering capability. When investigating and debugging complex software driven digital circuits it is necessary to be able to see the response of the system after a particular occurrence. As this may involve a number of lines to be in a given state, it is necessary for the logic analyzer to be able to trigger after this combination occurs. This facility is one of the key advantages of logic analyzers and enables them to be used to quickly home in on problems that may only occur under a particular set of circumstances.


Logic analyzer operation Although the operation of a logic analyzer may appear to be fairly complicated at first sight, a methodical approach to the use of one enables it to be set up correctly and to be used effectively. Once the probes are connected, the logic analyzer is programmed with the names of each signal. The analyzer can also associate several signals into groups so that they can be manipulated more easily.

With the basic set-up of the logic analyzer complete the capture mode for the data needs to be chosen. This can be set to one of two modes:

• Timing mode Using this mode signals are sampled at regular intervals based on an internal or external clock.

• State mode Here one or more of the signals are defined as clocks, and data is sampled on the edges of these clocks.

Once the logic analyzer mode is chosen then the trigger condition can be set. The analyzer trigger condition may vary from a very simple signal edge to a set of conditions that must be met across a variety of lines. The complex trigger conditions aid in locating problems that occur when a particular set of conditions occur.

With the trigger condition set, the logic analyzer can be set to run, triggering once only, or repeatedly. The data that is captured can then be displayed and analyzed.

UNIT-IV

OSCILLOSCOPES


OVERVIEW AND IT’S OBJECTIVE The cathode ray oscilloscope is probably the most versatile tool for the development of electronic circuits and systems, and has been one of the more important tools in the development of modern electronics. The cathode ray oscilloscope is a device that allows the amplitude of electrical signals, whether they be voltage, current, power, etc., to be displayed primarily as a function of time. The oscilloscope depends on the movement of an electron beam, which is then made visible by allowing the beam to impinge on a phosphor surface, which produces a visible spot. If the electron beam is deflected in either of two orthogonal axes, such as the familiar X and Y axes used in conventional graph construction, the luminous spot can be used to create two-dimensional displays. The overall objective of this chapter is to study the principle of working of a CRO and its applications. Prerequisites: Electrical sciences Basic Mathematics Important points: -

The heart of the oscilloscope is the cathode ray tube, which generates the electron beam, accelerates the beam to a high velocity, deflects the beam to create the image, and contains the phosphor screen where the electron beam eventually becomes visible. To accomplish these tasks, various electrical signals and voltages are required, Relatively high voltages are required by cathode ray tubes, in the order of a few thousand volts for acceleration's as well as a low voltage for the heater of the electron gun, which emits the electrons.

The strength of the electric field is categorized by the amount of force a charged particle would experienced in the field and is described by e = f/q.

The deflection sensitivity S of a CRT is defined as the deflection on the screen per volt of deflection voltage. S=D/E

The deflection sensitivity and deflection factor indicate that the sensitivity of a CRT is independent of the deflection voltage but varies linearly with the accelerating potential


Screens for CRTs: The front of the CRT is called the face plate which converts electrical signal in to light in accordance with the variation of vertical input signal. It is also feasible to have a small fiber optic region which is direct contact exposure of photographic film.

The inside surface of the face plate is coated with phosper. The effects of the characteristics of phosper are its luminance efficiency, spectral emission and persistence. Many types of phosper are used for CRT like P3,P2,P1,P11,P31…….A phosper must be refreshed by electrons before end of its decay time inorder to give a flicker free display. Short persistence phosper require more frequent refresher, where as long persistence phosper may result in characters feeding slowly. Long persistence prospers more prone to permanent discoloration, and loss of luminance efficiency. Long persistence is even needed storage type oscilloscopes.

The luminance of phosper is measure of its brightness. It is determined by the luminance efficiency of phosper.

1. The metal layer acts as a heat sink and reduces the danger of phosper burn 2. The light scatter from the phosper is reduced and the aluminuim reflects light

going back in the tube towards the viewer, So it increases the brightness 3. Electron are strike the phosper the screen, causes released the secondary

electrons. These electrons are collected by Aquadag coating. Graticule: The graticule is a grid of lines that serves as a scale when making the time and amplitude measurements. There are three types of graticule

1. External graticule 2. Internal graticule 3. Projected graticule

External graticule: The graticule can be easily changed to make different types of measurements and its


position can be adjusted to align it when the trace on the CRT. But it suffers from parallax errors. Internal graticule:

This is deposited on the internal surface of the CRT face plate and is there fore on the same surface is phosper.it require some method of electrical alignment. This is also difficult to illuminate for photography unless special illumination is provided. Projected graticule:

It is available with some camera’s and provides flexibility .The face plate is some times tinted neutral grey to reduce ambient light interference or external filters. Aquadag: The bombarding electrons, striking the screen release secondary emission electron. These secondary are collected by a solution of graphite called Aquadag. Time base generators: Oscilloscopes are generally used to display a wave form that varies as a function of time. If the wave form is to be accurately reproduced the beam must be constant horizontal velocity with respect to the wave form. A voltage with this characteristic is called ramp voltage. If the voltage decreases rapidly the input amplitude will be decreases repeatedly reproduced. During the sweep time the beam moves left to right with respect to the input amplitude. During the retrace time the beam returns quickly to the left of the screen. We can change the sweep rate in steps by switching different capacitors into the circuit. This entire operation done by active devices like transistors and UJT, resistors and capacitors timing circuits. The timing decided by the values of resistor and capacitors. Vertical Amplifier: The vertical is principle factor is determining sensitivity and bandwidth of the oscilloscope. Its totally depends up on the bandwidth of the vertical amplifier. It consists of pre-amplifier and main vertical amplifier. The preamplifier is generally field effect transistor (FET). It offer high input impedance and low output impedance. i) It provides excellent impedance matching purpose. ii) It provides large bandwidth and high gain. Voltage gain may be scarified in favour of greater bandwidth, or vice versa with out significantly effecting cost of the amplifier. However if the gain and bandwidth product increases the cost of the amplifiers is increases. The gain of the amplifier increases a sensitivity increase which allows observing small amplitude levels. Horizontal Amplifier:


The horizontal amplifier basically serves two purposes.

1. When the oscilloscope is being used in ordinary mode of operation to display a signal applied to vertical input. It amplifies the sweep generator output.

2. When the oscilloscope in the X Y mode, the signal applied to the Horizontal is amplified by the horizontal amplifier.

The amplifier stage preceded by the attenuator network and followed by the push pull amplifier. The sweep generator output is amplified by the horizontal amplifier and is fed to the push pull amplifier, which increases the power level of the signal and is sufficient to drive the Horizontal deflection system.

Delay Line: The oscilloscope having attenuators, pulse shapers and amplifiers, causes certain amount time delay in the transmission signal voltage to the deflection system. Especially in the horizontal generators, to generate ramp signal and is fed to the deflecting systems to move the electron beam so it takes the some nano seconds time so we need delay line. Delay lines are two lines

1. Lumped parameter delay line 2. Distributed parameter delay line

Lumped parameter delay lines are simply constructed a cascaded T-sections

terminated by the characteristic impedance Z0

. and the pass band upper limit is given by

Fc1/

lc.

The distributed parameter consists of specially manufactured co-axial cable with a high value of inductance for unit length causes to reduce eddy currents losses. The inductance can be o increased helical inner conductor on a ferromagnetic core and causes to increases time delay terminated by characteristic impedance.

Dual Beam CRO


• A Dual Beam CRO consists of two complete separately electron guns, two sets of VDPs and a single set of HDPs. • Since a single set of HDPs is used, only one beam can be synchronized at a time. • The Sweep produced by the Time Base Generator is common for both channels; hence the signals must have the same frequency or must be related harmonically in order to obtain both beams locked on the CRT screen.


UNIT-V Special Oscilloscopes Dual Trace Oscilloscope A single beam CRO gives a single trace on the screen. • It is often required to display and compare two signals at the same time. Hence two traces are required one the screen of the CRO. The Single Beam Dual Trace CRO consists of one electron gun, an electronic switch which switches the two signals to a single vertical amplifier and a single pair of horizontal deflection plates, as compared to a Dual beam CRO. Two channels A and B are used. Signals from A and B are applied through a pre-Amplifier and attenuator, which bring the signal within an acceptable level for amplification. The output from both channels A and B are applied to an electronic switch, which operates in two modes, alternate and chop mode

Alternate mode: • When the switch is in the ALTERNATE position, electronic switch feeds each signal alternately to the Vertical Amplifier. • ALTERNATE mode, the switching frequency is as sawtooth (time base) signal frequency. • The electronic switch alternately connects the main Vertical Amplifier to channels A and B and at same time adds a different DC offset component to each signal. • This DC component directs the beam alternately to the upper or lower half of the screen, i.e. the input to channel A is traced in the upper half of the screen then the input to channel B is traced in the lower half. • The repetition frequency is usually so high that the waveforms appear to be displayed simultaneously.


• The alternate mode is preferred for high frequency inputs.

CHOP MODE: When the switch is In the CHOP mode position, the duration of sawtooth (time base) signal is divided into small segments and the electronic switch moves between the two input with the frequency of these segments (i.e a very high rate). So the switch successively connects small segments of A and B waveforms to the main Vertical Amplifier at a relatively fast chopping rate. • If the chopping rate is slow, the continuity of the display will be lost. So, • The chop mode is preferred for low frequency inputs. Storage Oscilloscope:

In the conventional CRT the persistence of phosphorus ranges from a few

milliseconds to several seconds so that event that occurs only disappears from the screen

after a relatively short period of time. So target is appearing for longer time to go for storage

oscilloscope.


Storage CRTs can be classified as bistable tube and half tone tube. The bistable can

store an event and produces only one level of image of brightness, but in case of tone tube

image varying lengths of time and that different levels of image bright\ness, both use

Phenomena of secondary emission to build up store electro static charges. So we increase the

ratio of the secondary machine the storage time will be increased.

Sampling Oscilloscope: When the frequency of the vertical deflection signal increases, the writing speed of

electron beam increase causing reduction in image intensity on the CRT screen. In order to

obtain sufficient image brilliance the electron beam must be accelerated with higher velocity.

So we need sampling oscilloscope. The sampling oscilloscope basically pickup samples from

the sampling pulses in accordance with the input applied signal. The reconstructed from

many samples taken during recurrent cycles of the input wave form. The horizontal

displacement of the beam is synchronized with trigger pulses which also determine the

moment of sampling. The resolution of final image on the CRT screen is determined by the

size of steps of stair case generator

FREQUENCY AND TIME MEASUREMENT

Frequency and time are reciprocal to one another is their corresponding domains. Electronic

systems have developed to estimate their time intervals of signals and frequency counting. In

most communication systems an absolute precise estimation of frequency is required with

small distortion with the advent of digital logic. The frequency and phase measurement has

become much easier compared to early frequency comparator and interpolation oscillators.

In order to understand the basic signal characteristics (Frequency Phase), we need to study

their basic features and as well as their measurement techniques.

FREQUENCY MEASUREMENT Frequency measurement has not always been easy measurement task. Frequency measurement usually requires high precision standard, Frequency comparators and a lot of operator skill. This came to an end with the development of frequency counter. FREQUENCY SYNTHESIZER


One method of frequency synthesizers is called the indirect method or phase locked loop. It

has five main components.

1. Voltage Controlled Oscillator (VCO) : It is the source of the output frequency and

has the ability to be tuned electronically usually by applying variably voltage 2. Programmable dividers: It divides the frequency of VCO by an integer that can be

entered via programming switches, microprocessors.

3. Phase Detector: It provides an analog output i.e., function of the phase angle

between the two inputs. 4. Reference Source: It is very accurate and stable frequency source which is typically

a quartz crystal. 5. The loop Filter: It is an analog filter and is required to assure stable and noise free

operation of synthesizer.

FREQUENCY COUNTER The frequency counter operates on the principle of gating the input frequency into the counter

for a predetermined time.

• The unknown input is accumulated into the counter by using AND gates or an OR

gate • If an unknown frequency were gated into the counter for exact 1 second, the number

of counts allowed into the counter would be precisely the frequency of input.

Digital Counter

• The counting circuit is constructed by using integrated circuit counters

• The heart of frequency counter is the decade counter, which is constructed by using four flip flops and AND gate.

• But decade counter involves propagation delay

• A superior method of constructing a counter is to use a synchronous counter.

• In synchronous counter all the flip flops to be clocked are connected together which

greatly reduces propagations delay and allows higher counting speeds.


TIME MEASUREMENT If two input signals were substituted for the open and close gate signal and one of the internal clock signals, the time interval between the two input signals could be measured. The input signal must be proceed in the same fashion as the count input signal. Another time measurement can be made using a single input. This would be useful for determining the period of pulses and other signals. In this mode of operation, the gating signal in the input and interval frequency clocks are used as the timing sources. To measure the period of pulse wave form it is necessary to open the count gate at the rising edge of the pulse. And to close the gate at the falling edge of the pulse in case of negative going pulse, their procedure is reversed One very important period measurement is the period measurement to determine the frequency. In this case, the gate is to be opened at a point of the input wave form and closed at precisely the same point in next cycle. In a period measurement, the number of pulses counted equals Np = Fc/fWhere, F

x

c f

= crystal frequency of instrument x

= frequency of unknown input signal.

For frequency measurement the No. of pulses counted Nf = Fx

GATING ERROR

Gating error occurs when frequency and time period measurement are made. In measuring

low frequencies the gating error may have appreciable effect on the result. TIME BASE ERROR Time base error consists of calibration error, Short term crystal stability errors and long term

crystal stability error

1. Calibration Error : One of the simplest calibration techniques is to zero – Beat the

crystal oscillator against the standard frequency transmitter. 2. Short term stability error: It is caused by voltage transients short frequency 3. Long term stability error: It is because of aging and deterioration of crystal


OBJECTIVE QUESTIONS:

1. What is counter 2. Contrast frequency and time 3. What is full form BCD word 4. What is gating error 5. What is time base error 6. Define cross over frequency 7. Define gate time

1. In CRO, the deflection sensitivity & accelerating voltage exhibit--------relationship 2. The purpose of Aquadag in CRO is to ----------------------------- 3. --------------is the phenomenon in which the energy level of phosphor crystal increases when electron beam strikes phosphor. Emission of light during phosphor excitation is called ------------------- when the electron beam is switched off, phosphor crystals emit light by returning to their initial states which is called as-----------------. 4. Focusing lenses in CRO consists of -------------- , ---------- and -------------- 5. The phase measurements are made by use of ------------- in CRO 6. In order to examine periodic and very fast signals ----------- oscilloscopes are used. 7. --------------Oscilloscopes are used to study the transient responses. 8. Non circular apertures are used in CROs to reduce ------------- Essay Type Questions:

1. Why is an attenuator probe used?

2. What are major blocks of oscilloscope and what does each do

3. What are major components of CRT

4. What is delay sweep and when it is used and write short notes on delay lines


5. Explain storage oscilloscope

6. Explain sampling oscilloscope

7. What are the applications of oscilloscope? Explain.

8. Write short notes on I). Graticules II) CRT Screens.


UNIT -VI

AC & DC Bridge measurements Overview and Objective:

Bridge measurements play a vital role in instrumentation .The simplest form of bridge is for the purpose of measuring resistance is called Wheat stone bridge. there is an entire group of ac bridges for measuring very high and low resistances. General-purpose bridges are hardly used anymore. There is an entire group of ac bridges for measuring inductance, capacitance, admittance, conductance etc. These are fully automatic bridges that electronically null a bridge to make precision component measurements. For this reason, we study this chapter which is devoted to bridge circuits. The concept of guarded measurements and three –terminal resistance measurement is covered.

Prerequisites: Basic Electrical technology Elementary mathematics. At the end of the course:

The reader should be familiarized with different bridge circuit topologies and their operations. He should also be familiar to various measurement errors and guard circuits.

Important points: Wheat stone bridge:

This bridge has four arms (resistive), together with source of emf(a battery) and a null detector, usually a galvanometer or the current meter. Condition for bridge balance is

R1*R4 = R2*R3.

If three of the resistances are known values, the fourth may be determined from equation.

Measurement errors:

The main source of measurement error is found in limiting errors of the three un known resistors and other errors as follows:

1. Insufficient sensitivity of null detector. 2. Change of resistance due to heating effects. 3. Thermal emf in bridge circuit. 4. errors due to the resistance leads.

To calculate the sensitivity of galvanometer ,it is required to change the given circuit into its Thevenin’s equivalent. The wheat stone bridge is limited to the measurement of resistances ranging from a few


ohms to several mega ohms. The upper limit is set by the reduction in sensitivity to unbalance. The lower limit is set by the resistance of the connecting leads and the contact resistance at the binding posts. Kelvin Bridge: Kelvin bridge is a modification of the Wheatstone bridge and provides greatly increased accuracy in the measurement of low –value resistances, generally below one ohm Kelvin double bridge:

The term double bridge is used because the circuit contains a second set of ratio arms. This second set of arms connects the galvanometer and it eliminates the effect of the yoke resistance. Commercial Kelvin bridge is capable of measuring resistances from 10 ohm to 0.00001 ohm. Measurement of high resistance: Difficulties in measurement of high resistances: High accuracy is rarely required in such measurements, hence simple circuits are used. Here small currents are encountered in measurement circuits. Due to electrostatic effects, stray changes occur in the measuring circuit causing errors. Here, much sensitive galvanometers are required in operations. Guarded wheat stone bridge: Guard circuits: these are the special circuits used for eliminating errors caused by leakage currents over insulation. Methods of Measurements of high resistance:

1. Direct deflection method 2. Loss of charge method 3. Megohm method.


AC BRIDGES AND THEIR APPLICATIONS

AC bridge is natural outgrowth of DC bridge and its basic form consists of four bridge arms a source of excitation and null detector. The power source supply an AC voltage to the bridge at the desired frequency for measurements at low frequencies. The power line may serve as a source of excitation and at higher frequencies an oscillator generally supplies the excitation. The balancing equation is given by Z1Z4=Z2Z3

• The products of magnitudes of the opposite must be equal • The sum of the phase angles of the opposite arms must be equal.

MAXWELL Bridge: The Maxwell bridge measures an unknown inductance in terms of known capacitance. One of the ratio arms has a resistance and capacitance in parallel and it may now prove somewhat easier to write the balance equations. Zx=Z2Z3Y

1

The Maxwell bridge is limited to the measurement of medium Q-coils. Maxwell bridge is unsuited for the measurement of coil with very low Q value because of convergence problems. Hay Bridge: Hay bridge differs from Maxwell bridge by having a resister R1 in series standard capacitor C1 instead of parallel. The Hay circuit is therefore more convenient for measuring High Q-coils. Schering Bridge: The schering bridge is used and extensively for the measurement of capacitance. It is particular useful for measuring insulation properties. The standard capacitor is usually a high quality Mica capacitor for general measurement work or an air capacitor for insulation measurements. Power Factor:

This is defined for RC combination as cosine of Phase angle of the circuits Dissipation factor: It is defined for an RC combination as Cotangent of phase angle. Wien Bridge: Wien bridge is used to measure frequency and this is used in harmonic distortion analyzer where it is used as notch filter. The Wien Bridge has a series RC


combination in one arm and parallel RC combination in the adjoining arm. Equation for frequency (F) is given by F=1/2C1C3R1R3. Wagner ground connection: In practice however stray capacitances exist between various bridge elements and ground, and also between the bridge arms themselves. These stray capacitances shunt the bridge arms and cause measurement error particularly at high frequencies. One way to control stray capacitances is by shielding the arms and connecting the shields to the ground. This does not eliminate completely but makes them a constant in value and they can there fore be compensated. One of the most widely used method of eliminating the stray capacitance is the Wagner ground connection Objective Questions:

1. Why a Wheatstone bridge cannot be used for precision measurement 2. The value of resistance of an earthing electrode depends on ------- 3. Why in a Kelvin ‘s double bridge to sets of readings are taken when

measuring low resistances 4. Why low resistances are provided with four terminals 5. The frequency can be measured by using ------------- 6. The equation under balance conditions for bridge is ------------ 7. The Maxwell bridge is used for measurement of inductance of-----coils 8. Wagner’s earth devices are used in AC bridge circuits for ---------

Essay Type questions:

1. Explain Maxwell bridge and its application 2. Explain Wheatstone bridge and guard circuits 3. Write about Kelvin double bridge 4. Write the importance of three terminal resistances 5. Write about hay bridge 6. Describe about capacitance measurement using appropriate bridge 7. What is stray capacitance and explain about wagner ground connection


UNIT -VII

TRANSDUCERS

Overview and Objective: An instrumentation system generally consists of three major elements: An

input device receives the quantity under measurement and delivers a proportional electrical signal to the signal-conditioning device. Here the signal is amplified, filtered, or otherwise modified to a format acceptable to the output device. The output device may be a simple indicating meter, an oscilloscope or chart recorder for visual display.

The input quantity for most instrumentation systems is non electrical. In order to use electrical methods and techniques for measurement manipulation or control the non electrical quantity is converted into an electrical signal by a device called Transducer. One definition states ” A transducer is a device which ,when actuated by energy in one transmission system, supplies energy in the same form or in another form to a second transmission system. The transmission may be electrical, mechanical, chemical, optical(radiant), or thermal. In this chapter ,we are going to study different types of transducers and their principles of operation, specifications and related concepts. Prerequisite

1. Basic physical sciences. 2. Basic mathematics 3. Solid state physics

At the end of the course student should understand the following

1. Various process measuring Device’s operations and 2. Instrumental sizing and selection

Important points: Transducer is confined to device that covers the entire detector Transducer stage wherein the Transducer converts a non electrical quantity into an electrical signal. Transducer consists of two parts.


1. Sensing element: A detector or a sensing element is a part of Transducer which responds to physical

phenomenon the response of sensing element must be closely related to a physical phenomenon.

2. Transduction Element: Transduction element transforms the output of sensing element to an electrical output. Classification of Transducers:

1. On basis of Transduction form used 2. Primary and secondary Transducers 3. Passive and active Transducers 4. Analog and digital Transducers 5. Transducer and inverse Transducers

Characteristics and choice of Transducer: When choosing a Transducer for any application the input, transfer and output

characteristics has to be taken into account.

Summary of factors influencing the choice of Transducer: Operating Principle: The operating principles used may be resistive, inductive, capacitive

etc Sensitivity: Transducer must be sensitive enough to produce detectable output Operating range: Transducer should maintain the requirements Accuracy:

High degree of accuracy and repeatability are required for some critical applications Errors: Transducer should maintain input and output relationship Loading effects: Transducer should have high input impedance and low output impedance Likewise environmental computability and stability are required Strain gauges:

If a metal conductor is stretched or compressed its résistance changes on account of the fact both length and diameter of conductor change. Also there is change in the value of resistance. This is property is called Piezoresistive effect.


Theory of Strain Gauge: The change in the value of résistance by straining the gauge material, corresponds to the dimensional change in the length and the area of cross section. This property is exploited in designing strain gauges for strain measurements. Types of Strain gauges:

1. Unbonded metal strain gauges 2. Bonded metal wire strain gauges 3. Bonded metal foil strain gauges 4. Vaccum deposited thin metal film strain gauges 5. Sputter deposited thin metal strain gauges 6. Bonded semiconductor strain gauges 7. Diffused metal strain gauges

The strain gauge has been in use for many years and is the fundamental sensing element for many types of sensors, including pressure sensors, load cells, torque sensors, position sensors, etc. The majority of strain gauges are foil types, available in a wide choice of shapes and sizes to suit a variety of applications. They consist of a pattern of resistive foil which is mounted on a backing material. They operate on the principle that as the foil is subjected to stress, the resistance of the foil changes in a defined way. The strain gauge is connected into a Wheatstone Bridge circuit with a combination of four active gauges (full bridge), two gauges (half bridge), or, less commonly, a single gauge (quarter bridge). In the half and quarter circuits, the bridge is completed with precision resistors.

Resistance Thermometers:


The resistance of a conductor changes when temperature is changed. This property is utilized for the measurement of temperature. Gold and silver rarely used for construction of RTDs. Thermistors: Thermistor is contraction of a term “thermal resister”. This exhibits negative temperature coefficient. This is also used as a temperature Transducer, measurement of power at high frequency measurement of composition gases and vaccum measurements. Thermistors are composed of sintered mixture of metallic oxide such as manganese, Nicole, Cobalt, Copper, Iron and uranium. Thermocouples: In thermocouple, EMF depends up on the difference between hard junction and cold junction temperatures. The temperature of the latter should remain constant in order that calibration holds good and there are no errors on account of change in ambient temperature. Thermocouples are cheaper compared to RTDs. Synchros: A synchro is an electro magnetic Transducer which is commonly used to convert the position of shaft into an electrical signal. There are two types of Synchros

1. Control or error detection type 2. Torque transmission type

Resolvers: Resolvers are used for conversion of angular position of shaft into Cartesian coordinates.

Piezo Electric Transducer: The piezo electric material is one in which an electric potential appear across certain surfaces of crystal if dimensions of the crystal are changed by mechanical force. This effect known as piezo electric effect. The common piezo electric materials include Rochelle salts, ammonium die hydrogen phosphate Quartz and ceramics. Photo Voltaic cell: The photo voltaic cell or solar cell producers an electrical current when connected to load and here the light is transferred into an electrical energy. Pressure Transducer: Bourdon tube and pressure gauges are used for pressure measurements Hotwire anemometer: This Transducer is used for gas flow and pressure measurements based on heat exchange. OBJECTIVE QUESTIONS

1. A Transducer converts-------------- 2. An inverse Transducer converts----------------


3. ---------------- acts as an inverse Transducer 4. ---------------acts as an ACTIVE Transducer 5. A strip chart recorder is------------ 6. ------------------ is considered While selecting Transducer for a particular

application 7. A renold’s number of 1000 indicates------------- 8. A resistance potentiometer is a ---------------------- 9. The gauge factor is defined as---------------- 10. what is the principle of strain gauge? 11. what is the principle of RTD? 12. why RTDs use platinum so commonly? 13. what is the property of thermister? 14. the thermocouples are_____________________. 15. what are piezo-electric transducers? 16. what is hall effect? 17. what is photodiode? 18. name a gas flow transducer. 19. what is synchros?

ESSAY TYPE QUESTIONS

a. Describe with neat sketch the following types of primary detecting elements

i.Bourdon Tubes ii.Bellow and iii.Diaphragms b. Explain the following types of errors for a Transducer

i.Scale errors ii.Dynamic errors iii.Noise and drift errors c. Explain the operation of Thermistors for temperature measurment d. Describe the functionality of hot wire anemometer e. Describe the functionality of thermocouples f. Write about various transduction methods g. Write short note on piezo electric and photo electric Transducers


UNIT VIII In many industrial applications it is required to measure large number of mechanical

quantities like temperature, pressure, flow, velocity, acceleration and so on. These quantities are measured with high degree of accuracy by using primary measuring system which is made up of primary sensing element or sensor and supporting elements. To measure the above mentioned physical quantities the mechanical transducers are used .These are primary sensing elements which are in direct contact with the system and follow the changes in system during measurement. Measurement of displacement In general displacement can be classified as a) Translational displacement: The motion of a body in a straight line between two points. b) Rotational displacement: The motion of a body of angular type, about some rotation axis.

In practice different variety of translational displacement transducers and rotational displacement transducers are available. The translational displacement transducers are not only useful to translational motion but can be used as secondary transducers in measurement systems which are used to measure various physical quantities like pressure, force, acceleration and temperature. One of the important translational displacement transducer is LVDT.

Linear variable differential transformer (LVDT): The most widely used inductive Transducer translate the linear motion into electrical signals is linear variable differential transformer. Here the change in the displacement corresponds to change in inductance, which gives differential voltage output.


(a) A sectional view of a linear variable differential transformer (LVDT). (b) Circuit diagram for an LVDT. An LVDT consists of three symmetrically spaced coils wound onto an insulated bobbin. The magnetic core can move within the bobbin and provides a path for the magnetic flux linkage between the primary and the secondary coils. The motion (from a mechanical input) changes the characteristics of the flux path and the changes can be detected in the circuit. The primary coil is excited by an a.c

. signal and voltages are induced in the two secondary coils. The induced voltages depend on the position of the core inside the bobbin. The circuit is shown in part (b) of the figure. The secondary coils are wired in a series-opposing circuit so that when the core is centered between them the voltages induced are opposite but equal. When the core is centered between the two secondary coils, the induced voltages v 1 and v 2 are equal but out of phase by 180 degrees, and they cancel to give a zero output voltage. When the core moves from the center position, an output voltage v 0 =v 1-v 2 is developed, and as long as the displacement is within the working range of the LVDT, the voltage and displacement will be linearly related.


QUESTION BANK 1. a) Define terms

i) Accuracy ii) Measurement iii) Precision iv) Error v) Resolution vi) Sensitivity

b) Distinguish between accuracy and precision and explain about them in detail with the help of example

2. a) What is meant by an electronic volt meter, explain with respect ordinary (D’Arsonval) voltmeter

b) What is the type of voltmeter that uses thermocouples? Explain the same with the help of neat block diagram.

3. a) Describe the time base unit in a sophisticated universal frequency Counter timer. Draw block diagram and state various measures taken to maintain accuracy.

b) How the above instrument is organized to measure (i) Frequency (ii) Period (iii) Time delay. Draw various block diagrams connected with the above.

4. a) How do you understand the Distortion of a given wave; Explain the difference between super heterodyne wave analyzer and Distortion factor meter, with the help of neat sketches. b) Draw the block factor meter and explain the procedure to measure distortion factor, suggest suitable indicating element (Meter). 5. a) Explain the difference between AC bridges and D.C. bridges with help of diagrams and expressions. b) Draw the circuit diagram of a Maxwell’s bridge. Derive expression for the unknown element for which it is meant. Explain procedure to obtain the bridge balance. 6. a)Draw the Trigger circuit of an oscilloscope and wave forms with common time scale. Explain the use of trigger circuit in getting wave forms displayed on the oscilloscope. b) Show the arrangement of spectrum analysis along with a high frequency oscilloscope as an attachment. Explain spectrum analyzer block diagrams and principle of operation. 7. a) What are the applications of a piezo electric transducers, and Hot wire ammeter in Engineering measurements.

b) Draw the diagram showing constructional details of LVDT. Explain principle of operation with the help of neat wave form and characteristics.

8. Write short notes on the following :


i) Resistance thermometer Delay lines ii) Q meter

9. a) Define the following : i) Accuracy

ii) Error iii) Linearity iv) Precision b) Discuss main differences between accuracy and precision.

10 a) Draw the circuit arrangement to convert a.c. voltage to d.c. voltage in case of general purpose electronic voltmeter.

b) Explain terms i) input impedance i i) Sensitivity, iii) resolution and iv) accuracy with reference to a DVM.

11. What method can be used to increase the frequency range of a frequency counter? How can this be achieved without degrading the accuracy of the counter?

12. a) Derive an expression for balance in a Wheatstone bridge, and discuss the various sources of measurement errors.

b) Discuss the use of guard circuits in measuring very high resistances using wheat stone bridge.

13. Draw the neat sketch of the CRT and explain the main components of it.

b) Explain the electrostatic focusing used in all CROs.

14. a) Explain the operation of photodiode. b) Explain the use of photoconductive cell in controlling a relay.

15. a) Draw the rotational type potentiometer to measure angular position, b) What are linearity and sensitivity of resistance transducers?

16. a) Classify errors and explain them. b) Suggests methods to minimize and eliminate errors.

17. a) In case of Digital Volt Meter (DVM) discuss importance of number of digits. Does the accuracy depend upon number of digits of display. Explain with sufficient reasoning.

b) What is the importance of control logic block in case of Dual slope integration type of DVM.

18. Explain the block diagram of frequency counter with waveforms associated with the gating function of the frequency counter.

19. a) Discuss the working of Analog Phase meter and Digital Phase meter, b) Explain the basic principle of digital frequency measurements.

20. a) Derive an expression for balance in an Anderson's bridge.

21. Draw the phasor diagram under balance conditions, b) List the advantages and


disadvantages of Anderson's bridge.

22. a) Derive the relation for deflection sensitivity "S" of a CRT. b) What are the applications of CRO?

23. a) Explain the operation of fiber optic transducer for measuring temperature, b) Explain the operation of drum type movie fringe optical encoder.

24. a) What is heliport? Where it is used? b) Explain the principle of operation of resistive transducers. 24.. a) Explain terms : i) Significant figures ii) Conformity.

b) How are random errors are analyzed ? Explain with examples. 25. a) Explain in case of DVM i) 3 1 / 2 digits, i i) 5 1/2 digits. What is the significance of 1/2 digit.

b) Explain about the conversion logic used in case of simultaneous type of A/D DVM.

26. Briefly explain the logic diagram of a binary synchronous counter and also

explain the principle and operation of a cascading synchronous counter.

27. a) What is delay and briefly explain how do you measure the delay practically, b) Distinguish between period and frequency measurements.

28. a) Draw the circuit of a Schering bridge and discuss its principle with the help of suitable derivations and phasor diagram at balance, b) Write a note on the dissipation factor of a capacitor.

29. Write short notes on the following: a) Electrostatic Deflection

b) Screens for CRTs

30. a) Explain with neat diagram the operation of ultrasonic method for

measuring flow in a pipe,

b) What are the advantages & limitations of ultrasonic method of measuring flow.

31. a) Draw the optical spectrum. Classify it in terms of wavelength in nanometer, b) Draw vacuum type photo emissive cell and explain its operation.

32. a) Explain briefly about Wave meter. b) Discuss the working of fundamental suppression distortion meter.

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