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Chapter 1 : Electronic
Instruments (Part 1)
Chapter 1 : Part 2
Chapter 2 : Instruments For
Generation of Waveforms
Chapter 3 : Instrument
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Chapter 4 : Tranducers (Part 1)
Chapter 4 : Part 2
Chapter 5 : Data Aquisition
System and Recorders
Chapter 6 : Display Devices
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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,
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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).
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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.
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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).
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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
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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).
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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.
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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.
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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.
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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.
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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)?
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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
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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
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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.
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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.
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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.
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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
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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
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(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.
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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
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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.
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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.
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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.
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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,
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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
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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),
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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.
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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.
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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
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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
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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
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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.
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(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.
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Thus a potential difference is established between
A and B, a current flows thought the meter, which
gives direct value of the input voltage.