Solution for Electronic Instruments and Measurements · Solution for Electronic Instruments and Measurements ... Explain alternate mode and chop mode in a dual trace ... mode of operation

Mumbai University EXTC (Semester -III)

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Solution for Electronic

Instruments and Measurements

May 2016

Index

Q.1)

a) …………………………………………………………………………. 2

b) …………………………………………………………………………. 3

c) …………………………………………………………………………. 4 -5

d) …………………………………………………………………………. 6

e) …………………………………………………………………………. 6

f) …………………………………………………………………………. 7

Q.2)

a) …………………………………………………………………………. 7-10

b) …………………………………………………………………………. 10-11

Q.3)

a) …………………………………………………………………………. 11-15

b) ………………………………………………………………………….16-18

Q.4)

a) ………………………………………………………………………….18-20

b) …………………………………………………………………………. 20-22

Q.5)

a) …………………………………………………………………………. N.A

b) …………………………………………………………………………. 23-24

Q.6)

a) …………………………………………………………………………. 24-25

b) …………………………………………………………………………. N.A

c) …………………………………………………………………………. 26-28

d) …………………………………………………………………………. 28-29



Q1) Answer any four.

a) Explain alternate mode and chop mode in a dual trace

oscilloscope.

Ans: Alternate Mode : A display mode of operation in which the oscilloscope

completes tracing one channel before beginning to trace another channel.

Chop Mode : A display mode of operation in which small time segments

of each channel are traced sequentially so that more than one waveform

can appear on the screen simultaneously.

Alternate and Chop Display

On analogue scopes, multiple channels are displayed using either an alternate or

chop mode. (Digital oscilloscopes do not normally use chop or alternate mode.)

Alternate mode draws each channel alternately - the oscilloscope completes one

sweep on channel 1, then one sweep on channel 2, a second sweep on channel 1,

and so on. Use this mode with medium- to high-speed signals, when the sec/div

scale is set to 0.5 ms or faster.

Chop mode causes the oscilloscope to draw small parts of each signal by

switching back and forth between them. The switching rate is too fast for you to

notice, so the waveform looks whole. You typically use this mode with slow

signals requiring sweep speeds of 1 ms per division or less.



b) What is cold junction compensation in thermocouples.

Ans:

Metal used for thermocouple

Copper-constantan alloy: 0-

200000 F

Iron-conatanatn alloy : 0-12000F

Platinum –Platinum/Rhodium

alloy: 0-30000 F

Chromel – Alumel alloy :0-9000 F



c) Write a note on piezoelectric transducer .

Ans:

Different modes of piezoelectric transducer

Different arrangements of using the piezoelectric transducer are shown as

in above diagram

(a) (b)Equivalent circuit of the piezoelectric





d) Which is fastest ADC and why?

Ans: Flash converters are extremely fast compared to many other types of

ADCs, which usually narrow in on the "correct" answer over a series of stages.

Compared to these, a flash converter is also quite simple and, apart from the

analogue comparators, only requires logic for the final conversion to binary.

For best accuracy, often a track-and-hold circuit is inserted in front of the ADC

input. This is needed for many ADC types (like successive approximation

ADC), but for flash ADCs there is no real need for this, because the

comparators are the sampling devices.

A flash converter requires a huge number of comparators compared to other

ADCs, especially as the precision increases. A flash converter

requires comparators for an n-bit conversion. The size, power consumption

and cost of all those comparators makes flash converters generally impractical

for precisions much greater than 8 bits (255 comparators). In place of these

comparators, most other ADCs substitute more complex logic and/or analogue

circuitry that can be scaled more easily for increased precision.

e) Define accuracy ,precision and sensitivity with suitable example.

Ans : a) Sensitivity: The sensitivity is defined as mV/mm . and it is typically 1

to 2 mV/0.01 mm for the LVDT Sensitivity should be as high as possible and

the typical value of LVDT sensitivity shows that it is a highly sensitive

transducer

b) Accuracy: It indicates the deviation of actual output from the theoretical

value. It depends on the accuracy of the resistors used in ladder and the

precision of the reference voltage used. It is also specified in terms of

percentage of full scale output e.g. If full scale output is 15V and accuracy is +

or – 0.1 % then the

maximum error =o.1x15/ 100 = 15mV

c) Precision: It is a measure of the responsibility of the measurement ,that is

given a fixed value of variable. Precision is a measure of the degree of which

successive measurements differ from each other.

https://en.wikipedia.org/wiki/Digital_electronics

https://en.wikipedia.org/wiki/Binary_numeral_system

https://en.wikipedia.org/wiki/Sample_and_hold

https://en.wikipedia.org/wiki/Successive_approximation_ADC

https://en.wikipedia.org/wiki/Successive_approximation_ADC

https://en.wikipedia.org/wiki/Comparator

https://en.wikipedia.org/wiki/Digital_circuit

https://en.wikipedia.org/wiki/Accuracy_and_precision



f) Compare Analogue instrument with digital instrument.

Ans: An analogue instrument gives an output which varies continuously as the

quantity being measured changes. The output can have an infinite number of

values within the range that the instrument is designed to measure. The

deflection type of pressure gauge a good example of an analogue instrument. As

the input value changes, the pointer moves with a smooth continuous motion.

Whilst the pointer can therefore be in an infinite number of positions within its

range of movement, the number of different positions which the eye can

discriminate between is strictly limited, this discrimination being dependent

upon how large the scale is and how finely it is divided.

A digital instrument has an output which varies in discrete steps and so can only

have a finite number of values. A cam is attached to the revolving body whose

motion is being measured, and on each revolution the cam opens and closes a

switch. The switching operations are counted by an electronic counter. This

system can only count whole revolutions and cannot discriminate any motion

which is less than a full revolution.

Q2)

a) Explain the principle, working and construction of LVDT. What

is meant by residual voltage.:

Ans: It is a variable inductance displacement transducer .The construction is

shown in the figure below

Construction of LVDT



(a) Construction of LVDT winding (b)transfer characteristic of LVDT

Operation of LVDT :



Advantages



Disadvanatges:

b) Draw neat block diagram of Dual beam oscilloscope. Give the

comparison between Dual trace and trace beam oscilloscope.

Ans: Dual Beam Oscillator.

The block diagram of dual beam CRO is shown in the figure below.

It consist of two separate electron beam .Each electron beam has its own

vertical deflection plates and a common set of horizontal deflection plates

.This means time base is common for both the channels.



comparison between Dual trace and trace beam oscilloscope.

Q3)

a) What are the various D/A converting techniques? Explain any

one technique.

Ans:

Classification of data converters



Digital to Analog Converters.

The basic block diagram of D to A conversion is shown in the figure/

It consists of a resistive network, digitally controlled electronic switches,

voltage reference and current to voltage converter.

A digital input is applied to the resistive network via digitally controlled

switches.

The digitally controlled switches are turned on or off by the digital input

bits.

Block diagram of DAC

The supply to resistive network is given by reference voltage. The output

of resistive network is in the form of current .It can be converted to

proportional voltage by I-V converter. Thus we obtain analogy output

voltage proportional to the digital input code.

Depending on the type of resistive network used ,there are mainly two

types of D to A converter circuits .

1) Binary weighted resistor DAC

2) R-2R ladder type DAC

1) Binary Weighted Register DAC

The circuit of binary weighted resistor type DAC is shown in the fig.

The circuit uses a network of binary weighted resistors and a summing

amplifier in inverting mode of operation. The resistors 21 R ,22R

,…….2nR form the network of binary weighted resistors.

There are ‘number of electronic switches used, one switch per digital bit.

They are single pole double throw type switches. Every switch is

controlled by the digital input bit.

In this circuit op-amp operates as a voltage to current converter .It is

possible to connect the op-amp in the non-inverting summing amplifier

configuration.



The polarity of reference voltage depend on the type of switches used.

For example if we use the TTL switches then the reference voltage VR

=+5V and the output voltage of DAC will be negative. For example, if digital input bit d1 =1,then switch will connect resistor

21R to a negative reference voltage (-VR) and when digital input bit d1

=0,then switch will connect resistor 21 R to ground.

Thus an n-bit digital input will decide the position of all the switches and

connect their corresponding resistor to either –VR or ground.

The current will flow through binary resistor if switch connect resistor to

reference voltage .the current will not flow through binary weighted

resistor if switch connects resistor to ground.

Circuit diagram of binary weighted resistor DAC

Applying KCL at node 2 ,we get

I1+ I2+…………+In = Iin + If

Current going into op-amp terminal Iin is zero as op-amp has very high

input resistance.

I1 = -VR – V2 / 21 R , I2 = -VR – V2 / 2

2 R ,

In = -VR – V2 / 2n R ,

If = -VR – V0 / Rf



From virtual ground concept ,as node 1is at ground potential so node 2 is

also at ground potential ,therefore V2= 0.

Therefore ,

By substituting the values of d1,d2 ……dn we can obtain corresponding

analogy output voltage V0.

Consider 3 bit digital input d1,d2 d3.The table below shows the analogy

output voltage for all possible combination of 3 bit digital input.

The graph of digital input Vs analogy output voltage is plotted in fig.



Advantages:

1) Circuit is simple

2) Easy calculation

Disadvantages:

1) The accuracy and stability of this type of DAC depends on the accuracy

of the resistor used.

2) This type of DAC requires wide range of resistor values. If number of

digit per binary word is 12 then the smallest resistor is 2R and largest is

212R =4096 thus the largest resistor is 2048 times the smallest one.

3) The finite resistance of the switches will disturb the currents particularly

in MSB position when the current setting are small in value.



b) What is the basic principle of wave analyser? Explain

heterodyne type wave analyser with application.

Ans: WAVE ANALYSER:

PRINCIPLE:

The intermediate stage is a full wave rectifier .It is used to obtain the

average value of the input signal, on the indicating peak. The indicating

meter is a dc voltmeter calibrated to read the peak value of the input

voltage .

There are two types of wave analysers depending on the frequency range



HETRODYNE WAVE ANALYZER:



Application:

Q4)

a) Explain Kelvin’s double bridge and its application in very low

resistance measurement?

Ans: * This method is one of the best available method for the precise

measurement of low resistance .

It is a development of the Wheatstone bridge by which the errors due to

contact and lead resistance are eliminated.

The schematic diagram of kelvin’s double is shown in the fig.

The known resistance R3 is adjusted such that galvanometer indicates

zero

Kelvin’s double bridge





Advantages :

1) In atypical Kelvin’s double bridge the range of resistance covered is 1Ω

to 10 µΩ with an accuracy of + or – 0.05 % to + or – 0.2%

2) The resistance of the connecting lead Ry has no effect on measurement .

b) Draw and explain Hay bridge and its application for measurement

of inductance.

Ans: Hay’s bridge:

This method of measurement is particularly suited for the measurement of

inductance having high Q value.

The schematic of Hay’s Bridge is shown below:



Hay’s Bridge differs from Maxwell’s bridge by having a resistance R1 in

series with a capacitor C1 instead of being parallel.

For large phase angles,R1 needs to be low, therefore this bridge is more

convenient for measuring high Q coils .For Q=10 ,the error is+ or – and

for Q = 30 ,The error is + or – 0.1%





Q5)

a) Explain the principle and working of operation of dual slope

DVM.

Ans: N.A

b) Define Q factor and explain working of a Q meter for Q factor

measurement.

Ans:



Q6)

a) Draw block diagram for generalized measurement system and

explain its components.

Ans:



b) List various sensors for pressure and temperature along with their

ranges.

Ans N.A



c) Brief out classification of error in measurement.

Ans: 1)Gross Error:

2) Systematic error:



(a) Instrumental errors:

(b) Environmental errors:

3) Schematic error



4) Format error:

d) Explain Electrodynamometer type wall meter.

Ans:

CONSTRUCTION



1) The Fixed Coil.

The fixed coils carry current of the circuit,

They are divided into two halves.

The fixed coils are easily chosen to be current coils because they can be

made more massive and can be easily constructed to carry considerable

current.

They are air cored and wound with heavy wire. This wire is stranded or

laminated especially when carrying losses in conductor.

The fixed coils of earlier watt meters were designed to carry a current of

100 A but modern design usually limit the maximum current to 20 A.

In the case of precision watt meters, the two halves of the fixed coil,

which are connected in series for a basic measuring range, can be

connected in parallel to increase the wattmeter current range to twice its

original value. Shunt are not used for extension of current range since

they are subject to temperature errors.

2) Moving coil:

The moving coil is mounted on a pivoted spindle and is entirely embraced

by the fixed current coils. It is air cored.

The spring control is used for the movement of coil.

Since the current of the moving coil is carried by the instrument springs,

it is limited to the values which can be carried safely by springs without

appreciable heating .A series resistor is used to limit the current.

The voltage rating of the wattmeter is limited to about 600 V by the

power requirements of the voltage circuit since most of the power is

absorbed by the resistance in series with the moving coil.

3) Damping element:

The light aluminium vane carried by moving system acts as damping

element .It moves in a sector shaped box and provides air friction

damping. Electromagnetic or eddy current damping is not used as

introduction of permanent magnet will gently distort the weak operating

magnetic field.

4) Control element:

Spring control is used for this instruction.

5) Scales and pointers:

They are equipped with minor type scales and knife edge pointers to

remove reading errors due to parallax.



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Solution for Electronic Instruments and Measurements · Solution for Electronic Instruments and Measurements ... Explain alternate mode and chop mode in a dual trace ... mode of operation