EC-302 Electronic Measurements and Instrumentation - Jun 2023 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
   aWhat do you think about the permissible Errors in Ammeters and Voltmeters? Explain.
   How to answer

   • Define permissible / limiting errors in indicating instruments
   • Error limits as % of full-scale for ammeters and voltmeters
   • Effect of loading, temperature, frequency and calibration class
   • Example of accuracy classes and interpretation
2. 7 Marks
   bAn electrostatic voltmeter consists of two attracted plates (movable and fixed provided with guard rings). When a potential difference of $10\,kV$ is applied between the plates, there is a pull of $5\times10^{-3}\,N$ on the movable plate. Find the change in capacitance produced due to the change in the position of the movable plate by $1\,mm$. Diameter of the movable plate is $100\,mm$.
   How to answer

   • Force in electrostatic voltmeter $F = V^2 dC/2dx$
   • Given $V=10\,kV$, $F=5\times10^{-3}\,N$
   • Find $dC/dx = 2F/V^2$
   • For $dx=1\,mm$ find $dC$; note diameter $100\,mm$ for guard-ring area context
3. 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 $Z_{AB}Z_{CD}=Z_{BC}Z_{DA}$
   • Separate real and imaginary parts for $R$ and $C$
   • Substitute $R_{AB}=600\,\Omega$, $L=0.18\,H$, $R_{BC}=R_{DA}=1200\,\Omega$, $f=50\,Hz$
   • Solve for $R$ and $C$ at $3\,V$ supply between A and C
4. 7 Marks
   bWhy is it preferable to have slightly under damped instrument? Explain.
   How to answer

   • Define damping in PMMC / indicating instruments
   • Overdamped vs underdamped vs critically damped response
   • Why slightly underdamped gives fast settling without excessive overshoot
   • Practical damping factor ~0.7-0.8
5. 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
   • Balance condition $Z_1 Z_4 = Z_2 Z_3$
   • Derivation leading to $f = 1/(2\pi RC)$ for $R_1=R_2=R$, $C_1=C_2=C$
   • Use as frequency meter / audio oscillator application
6. 7 Marks
   bDiscuss the condition during which the Bridge sensitivity is maximum for a detector.
   How to answer

   • Define bridge sensitivity $S = \Delta I_g / \Delta Z$
   • Role of source, detector impedance and bridge ratio
   • Condition for maximum power transfer / matched detector
   • Practical condition for maximum sensitivity
7. 7 Marks
   aDiscuss about the electrostatic focusing deflection system of a CRO with necessary diagrams.
   How to answer

   • CRO electron gun, focusing anode, accelerating anodes
   • Electrostatic focusing lens action and deflection plates
   • Neat labelled diagram of CRO focusing-deflection system
   • Focusing voltage control and deflection sensitivity
8. 7 Marks
   bDraw the internal structure of CRT and list its functions.

   1. i)Internal structure diagram of CRT
   2. ii)Functions of CRT parts
   How to answer

   • CRT parts: electron gun, focusing/deflection system, fluorescent screen, glass envelope
   • Labelled internal structure diagram
   • Function of each electrode and aquadag coating
   • Beam formation to light spot conversion
9. 7 Marks
   aExplain in case of DVM: i) $3\frac{1}{2}$ digits ii) $5\frac{1}{2}$ digits. What is the significance of $\frac{1}{2}$ digit?
   How to answer

   • Meaning of $3\frac{1}{2}$ and $5\frac{1}{2}$ digit displays (1999 / 199999 counts)
   • Role of most significant half-digit (0 or 1)
   • Resolution and full-scale range examples
   • Significance of $\frac{1}{2}$ digit for overrange
10. 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 A/D converter
    • Resistor ladder, comparators and priority encoder logic
    • Parallel conversion in one clock - speed vs hardware cost
    • Use in high-speed DVM / digital storage
11. 7 Marks
    aDraw a weighted resistor DAC and obtain the transfer characteristics of a 3 bit DAC.
    How to answer

    • Weighted resistor DAC circuit with op-amp summer ($R,2R,4R...$)
    • Output $V_o = -V_{ref}(b_2/2+b_1/4+b_0/8)$ for 3-bit
    • Transfer characteristics staircase with 8 levels
    • Step size $V_{ref}/8$ and drawbacks
12. 7 Marks
    bDraw and explain the circuit operation of Successive approximation ADC.
    How to answer

    • Block diagram: sample-hold, comparator, SAR, DAC
    • Binary search operation bit-by-bit from MSB to LSB
    • Timing / flowchart for N-bit conversion
    • Waveforms and conversion time advantages
13. 7 Marks
    aDraw the circuit of weighted resistor DAC and derive expression for output analog voltage Vo.
    How to answer

    • Weighted resistor DAC circuit with binary-weighted resistors and op-amp
    • Nodal analysis at virtual ground summing junction
    • Derivation $V_o = -V_{ref} \sum b_i/2^i \cdot (R_f/R)$
    • Effect of bit weights and reference polarity
14. 7 Marks
    bFind out step size and analog output for 4-bit R-2R ladder DAC, when input is 0 1 1 1 and 1 1 1 1, assume $V_{ref}=+5V$.
    How to answer

    • Step size $= V_{ref}/2^n = 5/16\,V$
    • Output formula $V_o = V_{ref} \times (decimal/2^n)$ for R-2R ladder
    • For $0111$ (7): $V_o \approx 2.1875\,V$; for $1111$ (15): $V_o \approx 4.6875\,V$
    • Interpretation of MSB/LSB weights
15. 7 Marks
    aWrite a short note on following: Square wave generator.
    How to answer

    • Op-amp / Schmitt trigger square wave generator circuit
    • Charging-discharging of timing capacitor and hysteresis
    • Frequency formula $f=1/(2RC\ln...)$
    • Waveforms and applications
16. 7 Marks
    bWrite a short note on following: What are the applications of LVDT?
    How to answer

    • LVDT principle and construction recap
    • Displacement, force, pressure and vibration measurement
    • Industrial / biomedical / aerospace applications
    • Advantages: non-contact, high sensitivity