EE 5109
MODULE
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ELECTRONIC INSTRUMENTAT INSTRUMENTATION ION
1.0
I
Oscilloscopes: CRT, Construction, Basic CRO circuits, Block diagram of a modern oscilloscope, Y-amplifiers, X-amplifiers, Time base-single and mixed time base. Triggering, Oscilloscopic measurement (8) Module - I
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Draw the block diagram of a general purpose CRO and explain the function of the following controls intensity, focus, horizontal and vertical positioning, and synchronization. What are the major components of a cathode ray tube? How is the electron beam focused to a fine spot on the face of the cathode ray tube? Derive an expression for vertical deflection of an electron beam in a CRT. Explain the functioning of time base generator in a CRO. Why is a delay line used in the vertical section of the oscilloscope? Describe the different types of phosphor material used in a CRO and list their application. Explain the different types of graticules used in a CRO. Describe their advantages and disadvantages. Describe the different types of sweeps used in a CRO. Explain their spheres of application. Describe the phenomenon of synchronization of vertical input signal to its sweep generator. Explain the principle of sweeps synchronization. Explain the blanking circuit. How is magnitude and phase measured on a CRO for two different waves? What are the different types of probes used in oscilloscope explain in brief? What is oscilloscope probe compensation? How this is adjusted? What effects are noted when the compensation is not correctly adjusted? Why an attenuator probe is required? Explain the following: a) Why is a CRT considered as the heart of the oscilloscope? b)
For what a triggering circuit is provided in a CRO?
c)
Why is the grid in a CRT provided with a hole in it?
d)
Why is the focusing system of a CRO named as an electrostatic lens?
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e)
Why is the screen of a CRT coated with phosphor?
f)
Why are the inner wall of a CRT between neck and screen coated with conducting material?
How much voltage is required across two-deflection plate separated by 1 cm to deflect an electron beam 1 degree if the effective length of the deflection plates is 2 cm and the accelerating potential is 1 kV? What is the velocity of electron that has been accelerated through a potential of 2 kV? If an electro- statically deflected CRT has deflecting plate’s 2 cm long and 7.5 cm apart and distance of the screen from the centre of the deflecting plates of 50 cm. Find the velocity of the beam and deflecting sensitivity of the tube when the accelerating voltage is 2.5 kv. The deflection factor of an oscilloscope is 35 V/cm. If the distance from the deflection plates to the CRT screen is 16 cm the length of the deflection plates is 2.5 cm and the distance between the deflection plates is 1.2 cm. What is the accelerating anode voltage? [583 V] A CRO tube has two pairs of deflecting plates, which are at right angle to each other. The length of each plate is 2 cm and the distance between the two plates forming a pair is 1 cm, while the distance between the two pairs of plates is 3 cm. Each plate of the pair is connected to the plate of the other pair and the combination is connected to the 100 V rms supply. Determine the length of the waveform produced on the screen at a distance of 37 cm from the centre of the nearer plates. Assume accelerating potential of 1500V. [10.28] The horizontal and vertical deflecting plates of a cathode ray tube have their length of 2.2 cm and the distance between the two pairs of plates is 3.5 cm. Both the pairs of plates are at same potential difference of 100 V rms. Find the length of line produced on the screen at a distance of 35 cm from the centre of near plates. Let anode voltage be 1.5 kV. The distance between the two plates forming a pair is 1.2 cm. [Ans. 9 cm] Two sinusoidal waves have same frequency but different phases. The two voltage waves, with equal magnitude are applied respectively to x and y plates and the pattern obtained is an ellipse. The peak y deflection is 5 divisions and its intercept with positive y-axis is at 3 divisions. Find the relative angle when the major axis makes a positive slope with x-axis and when the major axis makes negative slope with x-axis. Sketch the pattern obtained on a CRO when the following voltages are impressed on X and Y plates i) X: A sin t, Y: A os t ii) X: A sin t, Y: A sin2 t If the vertical amplifier of an oscilloscope has a bandwidth of 15 M Hz. What is the fastest rise time that an input may have to be displayed without distortion? [23.33ns] An electro statically deflected cathode ray tube has plane parallel deflecting plates, which are 2.5 cm long and 0.5 cm apart and the distance from their centre to the screen is 20 cm. The electron beam is accelerated by a potential
difference of 2.5 kV and is projected centrally between the plates. Calculate the deflecting voltage required to cause the beam to strike a deflecting plate also find the corresponding deflection on the screen. [200 V, 4 cm]
MODULE
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II
Special Purpose CRO's: Dual trace, Dual beam, Sampling oscilloscope, Analog and digital storage CROs.
(5) Module - II
1. Describe the following types of oscilloscope dual beam, dual trace and sampling oscilloscope. What are their advantages and disadvantages? 2. Describe in detail the construction and working of an analog and digital storage oscilloscope. What are the advantages of each? 3. What precautions must be taken when using a sampling oscilloscope? 4. How does the sampling oscilloscope increase the apparent frequency response of an oscilloscope? 5. What is the function of the electronic switch? 6. A sampling oscilloscope is being used to observe a 400 MHz sine wave. A sampling pulse occurs every 3 ns. Draw five cycles of the signal and place a dot at the sampled point on each of the five cycles.
MODULE
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III
Electronic Instruments: Electronic voltmeter, Digital voltmeter, vector voltmeter, Vector Impedance meter and Q-meter.
(6)
Module - III
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A coil with a resistance of 3 ohm is connected to the terminal of the Q-meter. Resonance occurs at an oscillator frequency of 5 MHz and resonating capacitance of 100 pF. Calculate the percentage error introduced by the insertion resistance, R sh = 0.1 ohm. Explain with the help of a block diagram the various parts of an electronic multimeter. Explain the ohmmeter part of an electronic multimeter. Describe how Rx 1. Rx 10 and Rx 100 ranges are obtained in this instrument. Describe the operation of a vector voltmeter with the help of a block diagram. List its applications. Describe the operation of a vector impedance meter with help of a block diagram. Describe the circuit and working of a Q meter. Describe its applications. Describe how corrections for shunt resistance and distributed capacitance are applied when measuring Q factor of a coil with a Q meter. Describe the measurement of following using a Q meter i)
Q factor
ii)
Inductance
iii)
Effective resistance
iv)
Self capacitance
v)
Bandwidth
vi)
Capacitance
To check the distributed capacitance of a coil, the coil is resonated at 10 MHz with 120 pF and then is resonated at 15 MHz with 40 pF. What is the inductance of the coil and what is the equivalent distributed capacitance? A coil with a resistance of 3 ohm is connected to the terminals of the basic Qmeter circuit. Resonance occurs at an oscillator frequency of 5 MHz and resonating capacitance of 100 pF. Calculate the percentage of error introduce by the insertion resistance Rsh = 0.1 ohm. With a suitable standard inductor connected to a Q meter resonance is obtained with an angular frequency with a capacitor setting C1 the indicated Q factor is Q1. The unknown impedance is connected in series with the standard inductor and resonance is re-established by re-setting the capacitor to C2 the corresponding Q factor being Q2. Calculate the resistive and reactive component of the unknown impedance C1 = 208 pF, C2 = 185.5 pF, Q1 = 170, Q2 = 52. The frequency is 159 KHz. [35 ohm, 584 ohm] A Q meter gave the following reading during a certain test on a coil: Q = 250, C = 150 pF and f = 750 kHz. A VTVM was then connected across the standard capacitor and resonance re-established. The corresponding readings now
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become Q = 150, C = 150 pF and f = 750 kHz. Calculate the input resistance and capacitance of the VTVM. [496 ohm, 10 pF] A coil was tested using a Q meter and following information was obtained. At a frequency of 3 MHz the value of capacitance to give maximum voltage across the capacitor was 251 pF while at 6 MHz frequency it was 50 pF. Calculate the selfcapacitance of the coil. What are the different types of the digital voltmeter? What are the advantages of the digital voltmeter over analog meters?
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Explain the working of ramp type, integrating type, successive – approximation type and continuous balance type digital voltmeter .
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Describe electronic voltmeters. What are the advantages of TVM over VTVM?
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Describe direct-coupled amplifier dc voltmeter and chopper type dc voltmeter.
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What are the different types of electronic AC voltmeters?
MODULE
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IV
Transducers: Classification, Inductive, Resistive and Capacitive transducers, Analog and Digital Transducers with applications. Hall effect, Piezo Electric, Photovoltaic transducer, Measurement of temperature and pressure
(9) Module
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IV
What is a sensor and how does it differ from a transducer? What is an inverse transducer? How do you classify transducers? Why do we go for electrical transducers? List out the parameters that are needed for selecting a transducer. Distinguish between active and passive transducers. Explain the operation of potentiometric transducers. List the advantages and disadvantages. What is meant by gauge factor of a strain gauge? Explain different types of strain gauge. Differentiate between a Resistance Wire Strain gage and a Foil type Strain gage and list the advantages of Foil type over the other. Differentiate between a bonded and unbounded Strain gage.
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Describe the effect of temperature on a strain gage explain the temperature compensation method used in a strain gage. Explain the working principle and operation of a semiconductor strain gage. Analyze the behavior of Quarter-bridge, Half-bridge and Full-bridge circuit for strain gage transducers. Describe the working principle of Inductive transducers and capacitive transducers. Describe the operation of the change in self-inductance due to a change in (i) the number of turns (ii) permeability and (iii) reluctance. Describe construction, operation, advantages and disadvantages of a LVDT. Describe different types of variable reluctance transducers. What do you mean by push-pull arrangement used in capacitive transducers? Describe DC and AC Deflection Bridge as signal conditioning element. Describe different transducers used for measuring temperature. Describe the characteristics and operating principle of each and compare them. What are the advantages and limitations of resistance thermometers? State the different elements used as a sensor in RTD. Describe the different techniques used to eliminate the lead wire effect of a RTD connection in a bridge circuit Compare a RTD with a thermistor. What is meant by NTC and PTC? Mention their applications. Differentiate a thermistor with a Thermocouple. What is a thermopile? Describe Hall sensors. Get the equivalent circuit. Describe the frequency response. What are the applications of a Hall sensor? Describe piezo-electric transducers. Explain Photovoltaic transducers. A platinum resistance sensor is to be used to measure temperatures between 0 0 and 2000 C. Given that the resistance R T Ω at T0 C is given by R T =R0(1+αT+βT2) and R0=100Ω, R100=138.50, R200=175.83Ω. Calculate: (a) The value of α and β. (b) The non-linearity at 100 0 C as a percentage of full-scale deflection.
28. Four strain gauges are bonded onto a cantilever. Given that the gauges are placed halfway along the cantilever and the cantilever is subject to a downward force of 0.5 N, use the data given below to calculate the resistance of each strain gauge. Cantilever data
Strain gauge data
Length
l = 25cm
Gauge factor
Width
w = 6cm
Thickness
t = 3mm
G = 2.1
Unstrained resistance Ro = 120 Ω
Young’s modulus E = 70 × 109 Pa
29. A variable dielectric capacitive displacement sensor consists of two square metal plates, side 5 cm, separated by a gap of 1 mm. A sheet of dielectric material 1 mm thick and the same area as the plates can be slid between them as shown in Fig. 8.5. Given that the dielectric constant of air is 1 and that of the dielectric material 4, calculate the capacitance of the sensor when the input displacement x
0.0, 2.5, 5.0 cm.
(88.5, 55.3, 22.1 pF)
30. A variable reluctance sensor consists of a core, variable air gap and an armature. The core is a steel rod of diameter 1 cm, relative permeability 100, bent to from a semi-circle of diameter 4 cm. A coil of 500 turns is wound onto the core. The armature is a steel plate of thickness 0.5 cm and relative permeability 100. Assuming the relative permeability of air = 1.0 and the permeability of free space = 4π × 10 -7 H m-1, calculate the inductance of the sensor for air gaps of 1 mm and 3 mm. (7.6,3.4mH)
MODULE
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VI
Signal Generation: Sine wave generator, Triangular wave and square wave generator. Signal Analysis: Wave analyzer, Frequency selective and heterodyne wave analyzer. Distortion measurement, Harmonic analyzer
(7)
Module - VI
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Describe the circuit and working of wave analyzers used for audio frequency and megahertz ranges. Describe the engineering applications of wave analyzers. Describe the functioning of a total harmonic distortion meter. Describe the working of different types of frequency distortion analyzers with the help of a block diagram. Explain the working of frequency selective and heterodyne type wave analyser. List their advantage and disadvantage. Explain the working of fundamental suppression harmonic distortion analyser, inter modulation distortion meter and difference distortion analyser.
