EC-302 Electronic Measurements and Instrumentation - Jun 2024 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aDraw the circuit diagram of Schering's Bridge and explains the operation of it.
   How to answer

   • Circuit diagram of Schering's bridge with four arms, standard capacitor, detector and supply
   • Balance condition derivation for unknown capacitance and dissipation factor
   • Phasor / operation explanation at balance (zero detector current)
   • Application for measurement of capacitance and dielectric loss
2. 7 Marks
   bExplain the "parallel-connection" method of using Q-meter and Obtain the expressions for resistance, reactance and Q-factor.
   How to answer

   • Connection diagram for parallel-connection method of Q-meter
   • Resonance tuning procedure with unknown connected in parallel
   • Derivation of expressions for equivalent resistance
   • Derivation of expressions for reactance and Q-factor
3. 7 Marks
   aWhat is Thermistor and explain its importance along with advantages of it.
   How to answer

   • Definition and construction/material of thermistor (NTC/PTC)
   • Resistance-temperature characteristic and sensitivity
   • Importance in temperature measurement and compensation
   • Advantages: high sensitivity, small size, low cost with limitations
4. 7 Marks
   bDraw the Linear variable differential Transducer and explain its operation in detail.
   How to answer

   • Constructional diagram of LVDT / Linear variable differential transducer
   • Primary excitation and two secondary windings on former with movable core
   • Differential output voltage vs core displacement operation
   • Null, in-phase and out-of-phase conditions for direction sensing
5. 7 Marks
   aExplain the terms: Static Error, Static Correction, Relative Error and Percentage Relative Error.
   How to answer

   • Definitions of static error and static correction with sign relation
   • Formula for relative error = absolute error / true value
   • Formula for percentage relative error
   • Example illustrating each term
6. 7 Marks
   bExplain the working of a differential amplifier using two FET's. Derive the expression for the output voltage.
   How to answer

   • Circuit of differential amplifier using two FETs with common source resistor
   • Differential and common-mode operation explanation
   • Small-signal analysis steps
   • Derivation of differential output voltage expression and CMRR
7. 7 Marks
   aExplain special purpose CRO's with examples.
   How to answer

   • Need for special purpose CROs beyond general purpose
   • Examples: sampling, storage, digital storage, dual trace/beam
   • Key feature and application of each type
   • Comparison with conventional CRO
8. 7 Marks
   bExplain the principle of Impedance measurement by Q meter.
   How to answer

   • Resonant circuit principle of Q-meter
   • Circuit and tuning for impedance measurement
   • Relation $Q = \omega L / R = 1/\omega CR$ used to get impedance
   • Procedure to obtain unknown impedance from voltage and tuning
9. 7 Marks
   aExplain the construction and working of a LVDT. Explain how the magnitude and direction of the displacement of core of an LVDT is detected?
   How to answer

   • Construction: primary, two secondaries, movable core
   • Working on mutual inductance variation with core position
   • Output characteristics magnitude proportional to displacement
   • Phase (0/180 deg) indicating direction of displacement
10. 7 Marks
    bDescribe the different modes of operation of piezo electric transducers. Define and sketch binders and twisters.
    How to answer

    • Piezoelectric effect and modes: longitudinal, transverse, shear
    • Bender (bimorph) definition, construction and bending operation
    • Twister definition, construction and torsional operation
    • Sketches of benders and twisters with applications
11. 7 Marks
    aIn Hay's bridge the four arms are arranged as under. AB is a resistance of $600\Omega$ in series with an inductor of $0.18\, H$, BC and DA are non-inductive resistances of $1200\Omega$ each and CD consists of a resistance R in series with a capacitor C. A potential difference of $3V$ at a frequency of $50\, Hz$ is applied between A and C. Determine the values of R and C. Derive the condition for bridge balance.
    How to answer

    • Hay's bridge balance condition $Z_1 Z_4 = Z_2 Z_3$
    • Separation into real and imaginary parts for R and C
    • Substitution of $R_{AB}=600\Omega$, $L=0.18\,H$, $R_{BC}=R_{DA}=1200\Omega$, $f=50\,Hz$
    • Computed values of R and C at 3V balance
12. 7 Marks
    bWhy is it preferable to have slightly under damped instrument? Explain.
    How to answer

    • Damping in indicating instruments and transient response
    • Effect of under, over and critical damping on settling
    • Reason for slightly underdamped: fast response with small overshoot
    • Trade-off between speed and oscillations
13. 7 Marks
    aShow how the Wien's bridge can be used for the measurement of frequency in audio range. Derive the equation for frequency f.
    How to answer

    • Wien's bridge circuit with series-RC and parallel-RC arms
    • General balance condition $Z_1 Z_4 = Z_2 Z_3$
    • Derivation leading to $f = 1/(2\pi\sqrt{R_1 R_2 C_1 C_2})$ and $f=1/(2\pi RC)$ for equal arms
    • Use as frequency standard / audio-range measurement procedure
14. 7 Marks
    bDiscuss the condition during which the Bridge sensitivity is maximum for a detector.
    How to answer

    • Expression for bridge sensitivity in terms of detector current per unbalance
    • Dependence on supply voltage, detector sensitivity and arm impedances
    • Condition for maximum sensitivity: equal ratio arms / optimum detector impedance
    • Practical adjustment for maximum deflection
15. 7 Marks
    aExplain in case of DVM:

    1. i)$3\frac{1}{2}$ digits
    2. ii)$5\frac{1}{2}$ digits.
    How to answer

    • Meaning of $3\frac{1}{2}$ and $5\frac{1}{2}$ digit displays (1999 and 199999 counts)
    • What half-digit can display (0 or 1) and full-digit range 0-9
    • Significance for resolution, range and accuracy of DVM
    • Example readings on different ranges
16. 7 Marks
    bExplain about the conversion logic used in case of simultaneous type of A/D DVM.
    How to answer

    • Block diagram of simultaneous / flash type A/D DVM
    • Resistor ladder and comparator bank operation
    • Priority encoder logic generating digital output simultaneously
    • Advantages: high speed with limitations of complexity