chapter 1 _ electronic instruments (part 1) _ electronics measurements and instrumentation

5/6/12 Chapter 1 : Electronic Instruments (Part 1) | ELECTRONICS MEASUREMENTS AND INSTRUMENTATION

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ELECTRONICS MEASUREMENTSAND INSTRUMENTATION

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Chapter 1 : Electronic

Instruments (Part 1)

Chapter 1 : Part 2

Chapter 2 : Instruments For

Generation of Waveforms

Chapter 3 : Instrument

Transformers

Chapter 4 : Tranducers (Part 1)

Chapter 4 : Part 2

Chapter 5 : Data Aquisition

System and Recorders

Chapter 6 : Display Devices

(Including Digital Instruments)

Chapter 7 : Telemetry

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Chapter 1 : Electronic Instruments (Part 1)

Remember These:

Electronic voltmeter, VTVM Transistor voltmeter,

Electronic Multimeter, CRO’s: study of various

stages in brief, measurement of voltage, current

phase and frequency, special purpose

oscilloscopes, measurement of resistance,

inductance, capacitance, using Kelvin’s Maxwell’s

and Schering bridges. Effective resistance at high

frequency, Q meter, LCR meter.

2. INSTRUMENTATION FOR GENERATION AND

ANALYSIS OF WAVEFORMS

Signal generator, function generator, wave

analyzer, harmonic distortion -“ analyzer,

spectrum analyzer.

I TRUMENT TRANSFORMER

Current and potential transformers,

constructional features, ratio and phase angle

error.

4 TRANSDUCERS

Principles of operation, qualitative treatment of

strain gauge, L’’D1 thermoquple,

piezoelecte79rital and-photo eIectric transducers.

5. DATA ACQUISTION SYSTEMSAND RECORDERS

Necessity of recorders, recording requirements,



graphic recorders, strip chart recorders, magnetic

tape recorders, digital tape recorders.

6. DISPLAY DE VICES

Electronic indicating instruments, seven segment

display, fourteen segmental display, Nixie tube.

7. TELEMETRY

Introduction, method of data transmission, types

of telemetry systems and applications.

Q. 1. Write a short history of “units and

measurements”.

Ans. The concept of measurement is introduced since

olden days. History goes, that once upon a time, a

King decided that in his kingdom, the length would be

measured. with the length of his “Foot”, The unit of

length was thus equal to the length of “foot of the ruler

i.e. King” (Fig. 1).

Later on, for day to day use at different places

wooden pieces were made, to be used as “foot

ruler”. Needless to say that length of each piece

was equal to the length of “one foot” of the king.

With time, one foot was supposed to be equal to

12 inches (Fig. 2).



It is believed that in Egypt, “Cubit” was

introduced as unit of length.

The unit was used during construction of

“pyramids” and every worker was given a stick

measuring ‘one cubit’. All the workers were

ordered to get the stick compared with the

“Royal Cabit’ possessed by the King on every full

moon day, failing which, he was punished to hang

till death.

The researchers have found that the unit of

length called Angulam mentioned in Katilya’s

treatise, “Arthshastra” dated 300 BC was used

without a break by Indian builders for 3900

years, till the British imposed their units in 18th

century.

The ancient “Angulam” has been found to be

equal to 1.763 modern centimetres.

Researchers have also found that “Angulam” and

its multiples “Vitasti” (equal to 12 Angulam) and

“Dhanus” (equal to 9 vitasties), have been used

as unit of measurement right from the “Harappan

period” till the pre modern era.

“Dhanus” have been used for designing “Iron

Pillar” and “Qutub minar” built in Delhi, 1600 Yrs

ago.

The total height of the “iron piller” has been

measured exactly as 4 Dhanus.

The study has established that design and

architecture of “Taj Mahal” (Agra) is also based

on these old units and there is nothing “foreign”

in its design. The grids used in the Taj has sides

of 60 x 90 Vitasti.

In fact, the unit of Angulam (= 1.763 cm)

matches very well with the dimensions of

historical monuments establishes continuity of

India’s engineering traditions though the ages for

as long as 3900 years.



It is believed that, the Angulam tradition

remained unbroken for to long, because the

workers from Harappan days handed over this to

next generations.

Q. 2. Define measurement.

Ans. It is an act or the result of a quantitative

comparison between a given quantity and a quantity of

same kind selected as a unit.

Q. 3. Define measuring instrument.

Ans. It is a device used for comparing the unknown

quantity with the unit or standard.

Q. 4. What are analog instrument?

Ans. The analog measuring instrument has a

calibrated scale and pointer. The pointer deflects on

the scale and gives value of the quantity (Fig. 3).

Q. 5. What are digital instruments?

Ans. These have neither a scale nor a pointer. They

have an electronic circuit within. The reading is

obtained directly in digits on a monitor (Fig. 4).



Q. 6. Define resolution of an instrument.

Ans. The least interval between two adjacent discrete

details, which can be distinguished one from other is

called “Resolution” of the instrument.

Q. 7. Define sensitivity of an instrument.

Ans. Sensitivity : The ratio of change in the output to

the change in the input, which causes after the steady

state has reached, is known as sensitivity of an

mstrument

Q. 8. What is difference between resolution and

sensitivity of an instrument?

Ans. Almost the two terms are same with little

difference.

Resolution is the smallest change in the input signal

(quantity under measurement), which can be detected

by the instrument

The sensitivity is the ratio of output signal (or response

of the instrument) to a change of input signal (or the

quantity under measurement.)

Q. 9. Define accuracy and precision related to

measuring instruments.

Ans. Accuracy : This is the closeness, with which an

instrument reads the true value.

Precision : This means when a quantity is measured

repeatedly ; the instrument should give the same

value i.e. precision is the measure of reproducibility of

the- measurements.

Q. 10. What is the principle of an electronic

voltmeter ? What are its types?

Ans. In all electronic voltammeters, the principle

involved is that of a permanent



magnet moving coil (PMMC) instrument; popularly

known as a D’ Arsonval instrument. An indication in

the voltmeter is obtained by means of amplification

through one or more stages and this is proportional to

the input voltage.

The electronic voltmeters are of the following types

1. Which use vacuum tubes, popularly known as

“Vacuum Tube Voltmeter (VTVM)”.

2. Which use solid state devices (i.e. bipolar

transistors, FET etc.) popularly known as solid state or

“Transistorised Voltmeter” (TVM) ; or solid state

multimeter. They are also sometimes referred as

“Electronic Multimeter.”

Q. 11. Enlist advantages of electronic voltmeter

over ordinary voltmeters.

Ans. 1. The electronic voltmeter draws very low

power from the test circuit as it has very high input

impedance. This feature is useful for voltage

measurement in very high impedance circuit, such as

in communication circuits of the order of microvolts.

2. The electronic voltmeters have a wide frequency

range from 10 Hz to 100 MHz or even higher.

3. The Transistorized voltmeters (TVM) can measure

very high voltages upto thousand of volts.

4. Owing to amplifying properties, electronic

voltmeters possess very high sensitivity as a result,

their input impedance is increased. It reduces” loading

effect”, while carrying measurements.

5. These can give flexibility and a fast response.

Q. 12. What is solid state voltmeter?

Ans. It uses bi-junction transistors or field effect

transistors. It is called Transistorised Voltmeter (TVM).

Q. 13. Enlist TVMs (Transistorised Voltmeters).



Ans. (a) TVM with cascaded transistors.

(b) TVM with FET (Field effect transistor)

(c) Balanced bridge transistor voltmeter.

(d) Chopper amplifier voltmeter.

Q. 14. What is a differential

voltmeter/potentiometer voltmeter?

Ans. In this, unknown voltage is compared with

known voltage. It is therefore also called as

Potentiometer voltmeter.

Q. 15. What is a RMS reading voltmeter?

Ans. It has a permanent magnet moving coil (PMMC)

meter, which is calibrated to read the root mean

square (rms) value of the input voltage.

Q. 16. What is sampling voltmeter?

Ans. This voltmeter uses sampling technique to

measure voltages at frequencies as high as 1

Gigahertz (1 GHz = IO Hz)

Q. 17. What are the two sampling techniques

used in sampling voltmeter?

Ans. (a) Coherent sampling.

(b) Inherent sampling.

Q. 18. What are the factors which are to be kept

in mind while selection of electronic voltmeters ?

Ans.

(a) Input impedance

(b) Voltage range

(c) Decibel

(d) Sensitivity

(e) Battery operation.



Q. 19. What is the advantage of an A.C.

voltmeter?

Ans. It uses negative feedback, hence greatly reduces

the response time.

Q. 20. What the disadvantage of AC voltmeter?

Ans. By negative feedback, sensitivity of the

voltmeter is reduced.

Q. 21. What are advantages of ‘TVM’ over

‘VTVM’?

Ans. Few advantages of transistorized voltmeter

(TVM) over vactium tubes voltmeter (VTVM) are given

below

1. The TVM does not need any” Worm up time” but a

VTVM needs warm-up time due to presence of a

heating element.

2. The TVM is light in weight and can easily by

transported.

3. The TVM can be operated on batteries and can be

used anywhere, but the VTVM needs a power supply.

4. The TVM can measure a current directly, whereas a

VTVM cannot.

5. The TVM consumes very less power as compared to

the VTVM.

Q. 22. Give types of electronic voltmeters.

Ans. These may be

1. l)C voltmeters

2. AC voltmeters.

Q. 23. Draw block diagram of electronic

voltmeter.



Ans. The Fig. 5 shows the block diagram.

Q. 24. What are types of DC voltmeters?

Ans. The DC voltmeter can be of two types

1. Directly coupled amplifier DC voltmeter.

2. Chopper type DC voltmeter.

Q. 25. What is a multimeter? Give types and

uses.

Ans. (a) A multimeter is the basic instrument of an

engineer. The instrument can

measure currents (amperes), voltage (volts) and

resistance (ohms) and is therefore also called an AVO

(amperes-volts-ohms) meter. Thus it can measure

more than one quality hence the name ‘multimeter’

(multi means ‘many’).

These are two major types

1. Analog MM

2. Digital MM.

(c) The multimeter helps in ‘trouble shooting’ of

radio/T.V. and other electronic systems, as it can

locate an ‘open’ circuit, short circuit or earth fault.

Needless to mention that a multimeter can also be

used to check ‘continuity’ of a circuit. At that time, it is

used as an ohmmeter and the equipment under

checking is disconnected from the supply mains.

The multimeter should not be connected across an

energized circuit.

Q. 26. What do you means by sensitivity of a MM

7 Explain.

Ans.



The resistance offered by a multimeter per

volt of full scale deflection is called its

sensitivity. If the fs.d. current of a multimeter

is I,,,, the meter sensitivity would be

It is clear that the sensitivity indicates internal

resistance (impedance) of a meter. If a meter

has high sensitivity, it has high internal resistance

and when connected in a circuit it will draw

negligible current and measurement will be

accurate.

On the other hand, a low sensitivity meter (i.e.

low internal resistance) will draw a larger current

and due to loading effect it will not give accurate

measurements.

During measurements the internal impedance of

the meter comes in parallel with the load. This is

called ‘loading effect’.

Note that internal resistance of the meter is not

same in all ranges. Higher the range selected,

greater is its internal resistance. Theoretically, a

meter should have infinite internal resistance for

giving accurate reading.

We can say that higher the sensitivity (Q/V), the

lesser is the loading effect. The maximum

sensitivity available is 20 kQ/V.

Q. 27. A multimeter has a f.s.d. current of 2 mA.

Determine its sensitivity.

Ans. Here 1,,, = 2 mA = 2 c i- A

Meter sensitivity, I/I,,, = 1/(2 x 10-s) 500 Q per volt.

Q. 28. What are merits of a MM.

Ans. 1. It is of small size and is a portable instrument.



2. It is a single instrument which can perform number

of functions.

3. Due to loading effect, its measurements are not

very accurate.

Q. 29. Write down MM operating instructions.

Ans. The proper use of MM improves its efficiency and

accuracy. The following guide lines should be kept in

mind:

1. To prevent overloading and possible damage, when

checking voltage or current, start with the highest

range and move down successively.

2. For accuracy and minimum loading, choose a

voltmeter range such that voltmeter resistance (ohm

per volt x full scale voltage) is at least 100 times the

resistance of the circuit under test.

3. As for as possible, make all resistance reading in the

uncrowded portion of the meter scale.

4. Verify the circuit polarity before making a test,

particularly when measuring direct voltage and

currents.

5. When checking resistance in the circuit, see that

power to the circuit is switched ff, otherwise the

voltage across the resistance may damage the meter.

6. Renew the MM batteries time to time to ensure

accuracy of resistance measurement.

7. Recalibrate the instrument frequently.

8. Protect the instrument against dust, dirt, moisture,

heat etc..

Q. 30. What are the applications of CRT?

Ans. A cathode ray tube (CRT) is widely used for the

study of waveform of voltage and current, and for the

analysis of electrical signals.

Q. 31. What is Cathode Ray Oscilloscope (CR0)?



Ans. The CR0 is an electronic device with a CRT as its

“heart” and other associated circuits consisting of a

power supply unit, a saw-tooth wave generator,

horizontal and vertical amplifier. It is used to display

waveforms and other applications.

Q. 32. How is a CR0 superior to ordinary

measuring instruments?

Ans. The C.R.0 is an equipment that give graphical

representation of alternating quantities under

examination. In addition it can measure voltage,

frequency and phase.

Q. 33. What are the merits of C.R.O.?

Ans. The C.R.0. gives very accurate measurements,

as it is free from the errors introduced by moving

parts. It is also free from damping mechanism and

other inertia containing parts.

Q. 34. Is there any difference between a CRT

(Cathode Ray Tube) and C.R.0. Cathode Ray

Oscilloscope).? State if any.

Ans. The CRT is a part of CR0.

Q. 35. Enlist the basic constructional features of

C.R.T.

Ans. It consists of

1. An evacuated or vacuum glass-tube.

2. A cathode for emitting electrons.

3. Anodes

4. Fluorescent screen.

Q. 36. What is deflection sensitivity ?

Ans. This is the deflection of the spot on the screen of



a CR0 per unit deflection voltage.

Q. 37. Enlist accessories used with CR0?

Ans. (a) Calibrators

(b) Electronic switch

(c) Camera

(d) Probes.

Q. 38. What is a CR0 probe?

Ans. It connects the circuit with CR0 without changing

“loading” of the circuit.

Q. 39. What are various focussing techniques?

Ans. Two most commonly employed focussing

techniques are:

1. Electrostatic focussing

2. Electromagnetic focussing.

Q. 40. Give two examples where electrostatic

focussing is used ?

Ans. In general purpose C.R,0. and in Television

camera tubes.

Q. 41. How do you achieve X-shift and Y-shift.

Ans. There are two commonly employed techniques

for obtaining X-shift and

Y-shift. They are

1. Electrostatic deflection

2. Electromagnetic deflection.

Q. 42. Why do we use horizontal and vertical

deflection amplifiers ?

Ans. Vertical deflection amplifiers increase the

amplitude of the signal so that its waveform can be



studied very precisely. If the waveform does not fill

the trace (from left to right), the horizontal deflection

amplifier can be used to achieve this.

Q. 43. What is a Time-Base-Signal ?

Ans. A time-base-signal is one, that has a linear

variation with time.

Q. 44. Where do we apply a time basic signal in

C.R.0?

Ans. It is applied to the X-plates.

Q. 45. What is the purpose of using a time basic

signal in C.R.0?

Ans. It makes the spot move frQm left to right & back

and so on.

Q. 46. How does Sync. control help in obtaining

a stationary pattern?

Ans. The stationary pattern is obtained only when

time period of the time base

signal applied is equal to time occupied by whole

number of cycles of the applied input.

The sync. control varies the frequency of the time base

signal till the above stated

condition are achieved.

Q. 47. Write short notes on ECG (Electro cardio

graph).

Ans. The function of an electro cardio graph usually is

to draw an amplitude Vs tin gap of the waveform on a

ial gray j:dper. Th.; .trucardio graphs are generated

by the cells of the heart.



Q. 48. Give two examples of special

oscilloscopes?

Ans. 1. Storage oscilloscopes.

2 oscilloscopes.

Q. 49. Enlist applications of CR0.

Ans.

1. Measurement of voltages and currents.

2. Measurement of phase.

3. Measurement of frequency.

4. Comparison of two frequencies.

5. CR0 is a versatile instrument which can be used for

display, measurement, waveform analysis and other

phenomenon in electrical and electronic circuits.

Q. 50. Enlist special scopes.

Ans.

(a) Sampling scope

(b) Multiple beam scope

(c) Double beam scope

(d) Multiple trace scope

(e) Impulse waveform scope

(f) Scanning scope

(g) Storage type scope.

Q. 51. What is Deflection factor ?

Ans. This is reverse of deflection sensitivity.

Q. 52. What is a double-beam C.R.0?

Ans. A double beam C.R.0. has got electron beams

that are controlled independently. The constructional

features are almost same as in the case of a single-

beam CR0.



Q. 53. What do mean by fluorescence?

Ans. it is the property by which fluorescent materials

emit light when electrons are bombarded on them.

Q. 54. What are the commonly used fluorescent

materials?

Ans. They are

1. zinc sulphate + Manganese

2. Zinc sulphate + Silver

3. Zinc sulphide + Copper.

Q. 55. What is afterglow?

Ans. When electrons are bombarded on a particular

spot of a fluorescent material, it emits light. If the light

doesn’t die out immediately on removal of electron

beam, the phenomenon is termed as afterglow.

Q. 56. What is an oscilloscope probe ? What for it

is used?

Ans. Oscilloscope probe is basically a shielded cable

used to connect the signal to be displayed from the

source to the vertical input of the oscilloscope. A

shielded cable is used to prevent the pick-up of

interfering signals or hum due to hand capacitance.

Q. 57. With reference to oscilloscope, what are

(a) Alternate display mode

(b) Chopped display mode?

Ans. The above mentioned modes are used whenever

it is desired to display two signals simultaneously with

a single beam.

The ALTERNATE mode is useful for higher frequency

signals, Cl-TOPPED mode is preferred for low

frequency signals.



Q. 58. Where are storage oscilloscopes?

Ans. Storage oscilloscopes are used to view transient

or one shot events. They are also used to get a useful

display in case of very low frequency signals.

Q. 59. What is a SAMPLING OSCILLOSCOPE?

Ans. Conventional oscilloscopes are limited in

bandwidth to several hundred MHz. Sampling

oscilloscopes have bandwidths upto typically 10 GHz.

This permits the sampling oscilloscope to view signals

of extremely high frequencies.

Q. 60. How do you achieve focussing?

Ans. Focussing is achieved by various sets of plates

forming a sort of electronic lens. In general, there are

three sets of plates : Voltages at anodes 1 and 2 are

kept fixed while it is variable on the third anode. By

varying the voltage on the third anode, the spot may

be focussed.

Q. 61. What is the principle of operation of a

digital voltmeter.

Ans. A digital voltmeter comprises of a clock

generator, a circuit that converts the voltage to he

measured into a pulse and a digital frequency counter.

Q. 62. What are the advantages of digital meters

over analog types?

Ans. Numerical reading reduces human reading

errors, eliminates parallax error, increases reading

speed and provides outputs in digital form suitable for

further processing or recording.

Q. 63. What are the typical operating and



performance characteristics of digital voltmeters

?

Ans.

1. Absolute accuracy as high ± 0.005 percent of the

reading.

2. Resolution : The V can be read on the 1 V input

range.

Q. 64. Name the different types of digital

voltmeter (DVMs).

Ans. Digital voltmeters can be classified as

1. Ramp-type DVM

2. Integrating DVM

3. Continuous balance DVM.

4. Successive approximation DVM.

Q. 65. What is the use of Electronic Counter?

Ans. it can measure Time interval, Time period,

Frequency Ratio of two frequencies etc.

Q. 66. What is a logic probe?

Ans. A logic probe can test a logic circuit for ‘High’ and

‘Low’ states.

Q. 67. What are the components used for display

in digital instruments?

Ans. (a) Light emmitting diodes

(b) Liquid crystal displays.

Q. 68. What are display devices used in digital

instruments?

Ans.(a) Nixte tube

(h) Lucit sheet



(c) Grid illuminated dots

(SI) l3eam switching tube.

Q. 69. What is a digital instrument?

Ans. A digital instrument indicates the measured value

in the form of decimal digits.

Q. 70. What is the important equipment in digital

instruments?

Ans. The important equipment are “Analog to Digital”

converter at the input and

“Digital to Analog” converter at the output.

Q. 71. What are the advantages of digital

instruments over analog instruments?

Ans. The digital instruments are

(a) more accurate

(b) error free

(c) have more resolution

(d) these draw negligible power.

Q. 72. What is resolution of a digital instrument?

Ans. This is the number of digits used in the digital

instrument, if no. of digits = U, the resolution is =

1/10”.

Q. 73. Define sensitivity of a digital meter.

Ans. Sensitivity S = R x V

R = Resolution in decimal

V = Lowest full scale value of the meter.

Q. 74. Define sensitivity, range and error in

instruments.



Ans. 1. Sensitivity: This is the ratio of output signal

to the change of input signal (or the quantity under

measurement).

2. Range: The region between the limits, within which

a quantity is measured, received, transmitted or

expressed by stating the lower and upper limits is

called Range”. Such as 0-50 Amp., 50 to — 500 volt

etc.

3. Error : The difference between the “measured

value” and the “true value” of an unknown quantity is

called the “error”.

Q. 75. Define accuracy, precision and resolution.

Ans. 1. Accuracy : This is the closeness, with which

the instrument reading approaches the true value of

the variable under measurement.

2. Precision : This is the measure of the

reproducibility of the measurements, i.e. Precision is a

measure of the degree to which successive

measurements differ from each other.

3. Resolution : The resolution of an instrument is the

smallest change in the quantity under measurement,

which can be detected by the instrument.

Q. 76. Differentiate between Ammeter and

Voltmeter.

Ans. Table 1.

Ammeter Voltmeter

It is used to measure

current

It is used to measure

voltage

It has low resistance It has high resistance

It is connected in series

in the circuit

It is connected in parallel

in circuit

It carries full load current It carries a current



proportional to the circuit

voltage.

Q 77. How the resistances are classified ?

Ans. Resistances are classified into the following

groups

1. Low Resistance: Resistance below I ohm.

2. Medium Resistance: Resistance between 1 ohm and

100 k ohm.

3. High Resistance: Resistance above 100 kQ.

Q 78 What are the methods of measuring

resistance?

Ans. Following are the different methods of measuring

resistance:

1. Low Resistance: 1. Ammeter-Voltmeter method

2. Potentiometer

3. Kelvin’s Double bridge

4. Ohm Meter.

1. Ammeter-voltmeter method

2. Substitution method.

3. Wheatstone bridge.

1. Price guard wire method

2. Meggar.

Q. 79. Which is the most accurate and commonly

used method for measurement of medium

resistances?

Ans. Wheatstone bridge.

Q. 80. What is bridge sensitivity?

Ans. It is the minimum change or alteration required

under balance conditions to give full deflection in

galvanometer. Smaller the change required, greater is

the sensitivity.



Q. 81. On what factors does sensitivity of

Wheatstone bridge depend?

Ans. It depends upon the sensitivity of detector used.

More sensitive is the detector, more is the bridge

sensitivity.

Q. 82. What is the use of Bridges?

Ans. The bridges are used for measurement of

resistance, inductance, capacitance etc.

This is done by balancing the bridge.

Q. 83. What is a detector ? Name the devices

which can be used as detectors.

Ans. Detector is an instrument used in a.c. bridge to

obtain null deflection under balanced condition. The

common detectors used in a.c. bridges are

1. Head phones

2. Vibration galvanometer -

3. Tuneable amplifier.

Q. 84. What are the desirable characteristics of a

detector?

Ans. 1. Detector should be sensitive.

2. Detector should not damage due to over loading.

3. Detector should have short response time.

Q. 85. What are the sources of errors in Bridge

measurements?

Ans. Following are the sources of errors in Bridge

measurements

2. Medium Resistance

3. High Resistance.:

1. Errors due to stray magnetic field.



2. Lkage errors.

3. Eddy current errors.

4. Residual errors.

5. Frequency and waveform errors.

Q. 86. What is the most commonly used type of

method for measurement of inductances and

capacitances?

Ans. AC bridges.

Q. 87. What is the difference between an DC

bridge and AC bridge?

Ans. The DC bridge employs real impedances

(resistance) in its armsand the source of current is DC.

In AC bridges, the bridge arms have resistances,

capacitances, inductances etc. The supply used is AC.

Q. 88. Name the detector used in AC and DC

bridges.

Ans. The detector used in AC bridges is either a

vibration galvanometer or head phone whereas in DC

bridges, a galvanometer (D’ Arsonaval Type) can be

used.

Q. 89. What is the principle of operation of an AC

bridge?

Ans. It operates on principles of Wheatstone bridge.

There is one additional factor:

Since AC has both magnitude and phase, it is essential

to have balance in magnitude as well as in phase to

obtain null reading in the detector.

Q. 90. Name the different methods for

inductance measurement.



Ans. Following are the methods for measuring

inductance:

1. Three voltmeter method.

2. Three ammeter method.

3. Voltmeter and ammeter method.

4. Maxwell’s Bridge.

5. Anderson Bridge.

6. Owin’s Bridge.

7. Hay’s Bridge.

Q. 91. Name the different methods of

capacitance measurement.

Ans. Capacitance can be measured by the following

methods:

1. Do Sauty’s Bridge

2. Schering Bridge

3. Wien Bridge.

Q. 92. Explain the limitations of Maxwell’s

Bridge.

Ans. 1. The Maxwell bridge is limited to the

measurement of medium-Q coils

(1 <Q < 10)

2. The Maxwell bridge is also unsuited for the

measurement of coils with a very low value.

Q. 93. What is RLC Bridge?

Ans. This single bridge is a combination of Wheatstone

Bridge, Maxwell Bridge and De Sauty Bridge. The RLC

bridge can measure resistance, inductance and

capacitance.

Q. 94. What is RLC meter?

Ans. This single instrument can measure resistance,



inductance and capacitance.

Q. 95. What is Q-factor?

Ans. Q-factor stands for quality factor. It is equal to

wL/ R for a coil. Higher the Q-factor, more accurate

the performance of a coil.

Q. 96. What is a Q-meter?

Ans. The Q-meter is an equipment used to measure

the Q-factor of coils and resonant circuits.

Q. 97. What is the principle of operation of a Q-

meter?

Ans. If applied voltage is maintained at a constant and

known level, a voltmeter connected across the

capacitor can be calibrated directly in terms of the

circuit Q.

Note : More short answer questions are given at the

end of the Chapter

Q. 98. What is impedence of a circuit?

Ans. The total opposition offered by a circuit is called

impedence. It is Eombined effect of resistance,

inductance

capacitance. It is represented by Z.

The impedence is given by



XL = Inductive reactance of the circuit = wL, where L

is the value of inductance X = Capacitive reactance of

the circuit = , where C is the capacitance

Q. 99. What are impedence Bridges?

Ans. These are bridges used to measure impedence

(resistance, inductance, capacitance of a circuit).

Q. 100. Give value of Q factor for coil and

capacitor.

Ans. By Q-factor we mean quality factor of a coil or a

capacitor. A coil or capacitor contains certain amount

of resistance which is not desirable as it causes power

loss.

The ratio of inductive reactance of a coil (or capacitive

reactance of a capacitor) to the resistance contained is

called its Q-factor.

A high value of Q is always desirable as it shows less

power loss.

Recall that in capacitor the power loss is called as

“Dielectric loss”.

Q. 101. What ‘is a Q-meter write in one line?

Ans. This is an instrument which measures Q-factor.

Q. 102. Write a list of 44sources and detectors

for bridges”.

Ans. (a) For measurement at low frequencies (50 Hz),



power line may act as source of supply for the bridges.

For higher frequencies, electronic oscillators should be

used An oscillator gives constant frequency (50 Hz —

120 KHz) and power (7W), frequency can be adjusted.

(b) The detectors used for bridges are

(i) Head Phones : These may be used for frequencies

250 Hz to 5 kHz. They are most reliable for this

frequency range. At balance condition, head phone is

completely silent.

(ii) Vibration Galvanometers: These may be used

from 5 Hz to 200 Hz frequencies. For low frequencies

they are more reliable than head phones. Sometime

micrometer or CR0 is also used for this frequency

range.

(iii) Transistor Tuned Amplifiers : These may be

used for narrow band of frequencies of 10 Hz to 100

kHz.

(iv) Telephone Receiver : This is the simplest

detector used in bridges.

(v) Galvanometer : For d.c. bridges a simple

galvanometer is used, which shows zero (null)

deflection in balance condition.

Q. 103. Which device is widely used as detector

in Bridges?

Or

Write a note on vibration galvanometer?

Ans. It was found, that sensitivity of the detector used

in a.c. bridges can be increased by employing

“principle of resonance”. For this, frequency of the

moving system of the galvanometer is mechanically

tuned, so that its natural frequency is same as that of

the if electromagnetic force” causing the deflection.



The vibration galvanometer is used as detector in

a.c. bridges below 300 Hz. In construction, it is

almost similar to any moving coil galvanometer.

The moving coil consists of a single turn of bronze

or of platinum silver. This coil passes over a small

pulley and is pulled tight by a spring. The coil is

stretched between two bridge pieces. The moving

system carries a mirror on which a light beam is

thrown.

The moving system (i.e. coil) vibrates, when an

a.c. flows through it, and beam of light reflected

from mirror throws a band of light which reduces

to a “spot on the scale” when the bridge is

balanced”.

The damping torque is very small and can be

increased by shunting the instrument.

Tuning is done by varying the distance between

the bridge pieces.

This changes the length of the moving coil, which

is free to vibrate and this varies the natural’

frequency of the moving system. The fine tuning

is done by changing the tension of the spring.

Q. 104. Describe components of A.C. impedance

Bridges.



Ans. The basic components of impedance bridges are:

1. Four arms containing impedances including the one

unknown impedance.

2. The, detectors commonly used in a.c. bridges are:

(a) Head phones/telephones—They are used for

frequencies from 250 Hz upto 5 kHz.

(b) Vibration galvanometer—They are used for low

frequencies below 200 Hz.

3. The supply sources commonly used in a.c. bridges

are:

(a) A.C. supply mains employing a step down

transformer

(h) A motor driven alternator

(c) An audio/radio frequency oscillator.

Q. 105. Enlist the various applications of

Impedance Bridges.

Ans. The following are the applications of Impedance

Bridges:

2. Measurement of resistances, inductances and

capacitances,

2. Measurement of loss angle, loss factor, dielectric

loss in capacitors,

3. Measurement• of impedances,

4. Measurement of audio frequencies,

5. In communication systems,

6. For phase shifting (phase shift oscillators).

7. For providing feedback in oscillator (Wein Bridge

oscillator).

8. Filtering out undesirable signals.

Q. 106. What is loading effect in instruments?

Ans. When an instrument is used for measurement, its

impedence comes into the circuit under test, this

changes the real value of the quantity under

measurement. This is called Loading Effect. This takes



place in all stages of measurement.

Q. 107. Write specifications of Electronic

Voltmeter.

Ans.

Table 2. Specifications of Electronic Voltmeter

AC/DC voltage range 0 to 500 V

Input Impedance 10 M for DC

1.5 M for AC

Accuracy 1.3% for DC

1.5% for AC

Resistance range Up to 1000 M

DC current range From zero to 200 mA.

Q. 108. What are probes in C.R.O?

Ans. C.R.O. Probes: The CR0 probes are the means

that connect a test circuit to the C.R.0., without

altering loading or disturbing the circuit. These are of

the following types

1 Direct Probe : It is the simplest probe and uses a

shielded co-axial cable. It i used for low frequency

circuit.

2. Isolation Probe : This probe is made by placing a

carbon resistor in series with the test load. The probe

removes the undesired effects of the direct probe.

3. Current Probe : This probe can be clamped around

a wire carrying current without any physical contact to

the probe, allowing the C.R.0. to be used upto 50 MHz.

Q. 109. Name the instruments used for testing

electronic devices.

Ans. The instruments used for testing electronic

devices are:

1. CR0



2. Multimeter

3. Signal Generator

4. V.T.V.M.

5. Function Generator

6. Power supply

7. LCR meter

8. Distortion meter

9. Frequency counter

10. Q-meter.

Q. 110. Write about different torques required

for indicating instruments.

Ans. For operations of indicating instruments, three

torques are required.

(i) Deflecting torque—This deflects the moving system

from its rest position.

(ii) Controlling torque—It controls the movements of

the moving system. The pointer stops when deflecting

and controlling torques are equal.

(iii) Damping torque—This torque does not allow

unnecessary oscillations of the pointer before coming

to rest.

Q. 111. Enlist, various types of ac voltmeters.

Ans. Types of AC voltmeters. We have the following

types of voltmeters:

1. Average reading voltmeter.

2. Peak reading voltmeter.

3. True rms reading voltmeter.

4. Sampling voltmeter.

Q. 112. Write about RF probe.

Ans. RF Probe : The type of measurement made by

RF milli-voitmeter depends upon the type of probe

used. Voltage measurements are made with a probe



shown in (Fig. 8). The measurement is made with a

relatively high impedance.

Q. 113. Explain a Balanced Bridge V.T.V.M.

(Electronic Voltmeter).

Ans. (a) It is also called a vacuum tube multimeter. It

is multimeter using vacuum tubes and can measure

voltage, current and resistance, but popularly it is used

to measure voltage (D.C.).

The V.T.V.M. has internal impedance in megaohms (as

it uses vacuum triodes) and does not load the circuit

under measurement, but it needs a supply voltage for

its operation. Moreover, it is costlier and also takes

“warm up” time to start.

(b) Construction (Fig. 9)

The V.T.V.M. is basically a moving coil meter, plus two

vacuum triodes (exactly identical) and an external

power supply. The moving coil voltmeter (V) is

connected across the anodes. The voltage to be

measured is fed to the grid of either of the triodes. A

variable resistance r is used for zero adjustment.



(c) Operation:

When the instrument is switched on to the

supply, it start warming up i.e., the cathodes

heat up to the temperature at which they start

emitting electrons and then the instrument starts

operation.

This is an important precaution while using a

V.T.V.M. that it should be switched ON well in

advance.

When no voltage is applied at the grid, both the

triodes act as diodes and as both are similar,

anode current is same and potential difference

between A and B is zero. Therefore, no current

flows through meter and the meter reads zero.

Ideally, at this position the meter should read

ZERO, however, as the diodes cannot be exactly

similar (Two things of even the same kind in this

universe cannot be exactly similar.), and due to

some difference in the characteristics of the two

triodes some current may flow through the

meter. If this happens, the pointer of V.T.V.M.

may be brought to Zero by Zero Adjuster.

Now the voltage to be measured is connected at

the grid of either of the triodes (say T1) across

CD. Now the grid of T1 will be more negative,

thus the plate current of T1 will change and also

the potential at point A.



Thus a potential difference is established between

A and B, a current flows thought the meter, which

gives direct value of the input voltage.

chapter 1 _ electronic instruments (part 1) _ electronics measurements and instrumentation

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