Electronic Measurements & Instruments (4331102) - Winter 2023 Solution

Question 1(a) [3 marks]

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Give Definition of Accuracy, Reproducibility and Repeatability.

Answer:

Mnemonic: “ARR - Accurate Results Repeatedly”

Question 1(b) [4 marks]

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Draw and Explain Wheatstone bridge.

Answer:

Diagram:

graph LR
    A[Supply+] --- R1
    A --- R3
    R1 --- B[Output+]
    R3 --- C[Output-]
    B --- R2
    C --- R4
    R2 --- D[Supply-]
    R4 --- D

Mnemonic: “WBMP - When Balanced, Measure Precisely”

Question 1(c) [7 marks]

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Explain Principle of Q meter. Also draw and explain Practical Q Meter.

Answer:

Principle of Q Meter:

The Q-meter operates on the principle of series resonance, where Q factor is measured as the ratio of voltage across the capacitor to the applied voltage at resonance.

Diagram of Practical Q Meter:

graph LR
    A[RF Oscillator] --> B[Work Coil]
    B --> C[Series Circuit]
    C --> D[Unknown Inductor L]
    D --> E[Variable Capacitor C]
    E --> F[VTVM]
    F --> G[Q-Scale]

• Resonant Formula: f = 1/(2π√LC)
• Q Calculation: Q = Vc/Vs (voltage across capacitor / source voltage)

Mnemonic: “RIVQ - Resonance Indicates Valuable Quality”

Question 1(c OR) [7 marks]

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Draw and explain construction of Moving coil type instruments.

Answer:

Diagram:

• Operating Principle: Deflecting torque = BIlN (B-field strength, I-current, l-length, N-turns)
• Controlling Torque: Provided by springs proportional to deflection angle

Mnemonic: “MAPS-C: Magnet Acts, Pointer Shows Current”

Question 2(a) [3 marks]

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List out different Types of errors. Explain any Two.

Answer:

Explanation of Two Errors:

1. Systematic Errors:

   • Consistent and predictable deviations from actual value
   • Caused by instrument calibration, design, or method

2. Random Errors:

   • Unpredictable variations in measurements
   • Caused by noise, environmental fluctuations, or observer limitations

Mnemonic: “GSREL - Good Systems Reduce Error Levels”

Question 2(b) [4 marks]

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Draw and Explain Maxwell’s bridge.

Answer:

Diagram:

graph LR
    A[Supply] --- R1
    A --- R3
    R1 --- B[Detector]
    R3 --- C[Detector]
    B --- R2
    C --- R4
    B --- L["Unknown L"]
    C --- C1["Capacitor C"]
    R2 --- D[Ground]
    R4 --- D
    L --- D
    C1 --- D

• Balance Equation: L = CR2R3
• Resistance Equation: RL = R2R3/R4
• Application: Measures inductance with significant resistance

Mnemonic: “MBLR - Maxwell Bridge Links Resistance”

Question 2(c) [7 marks]

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Draw and explain construction of moving iron type instruments.

Answer:

Diagram:

• Working Principle: When current flows through coil, both iron pieces get magnetized with same polarity causing repulsion
• Advantages: Suitable for both AC and DC, robust construction
• Disadvantages: Non-uniform scale, higher power consumption than PMMC

Mnemonic: “IRAM - Iron Repulsion Activates Movement”

Question 2(a OR) [3 marks]

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Explain basic DC voltmeter.

Answer:

Diagram:

• Working Principle: Voltmeter is PMMC meter with series resistor
• Calculation: Rs = (V/Im) - Rm (Rs=series resistor, V=voltage, Im=full scale current, Rm=meter resistance)

Mnemonic: “SVM - Series Voltage Measurement”

Question 2(b OR) [4 marks]

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Draw and Explain Schering bridge.

Answer:

Diagram:

graph LR
    A[AC Supply] --- C1["Unknown Capacitance"]
    A --- R3
    C1 --- B[Detector]
    R3 --- C[Detector]
    B --- R1
    C --- C4["Standard C"]
    R1 --- D[Ground]
    C4 --- R4["Variable R"]
    R4 --- D

• Balance Equations: C1 = C4(R3/R1)
• Dissipation Factor: D = ωC1R1 = ωC4R4
• Application: Measurement of capacitance and dielectric loss

Mnemonic: “SCDR - Schering Capacitance Determines Resistance”

Question 2(c OR) [7 marks]

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Write shortnote on Electronic Multimeter.

Answer:

Diagram:

graph LR
    A[Input] --> B[Attenuator/Range Selector]
    B --> C[Signal Converter]
    C --> D[Amplifier]
    D --> E[Rectifier/Detector]
    E --> F[Display]

• Types: Analog electronic multimeter, Digital multimeter (DMM)
• Advantages: High input impedance, minimal loading effect, multiple functions
• Key Circuit: Input attenuator, signal converter, amplifier, rectifier, display driver

Mnemonic: “VCAR-D: Voltage, Current And Resistance - Displayed”

Question 3(a) [3 marks]

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Explain Various probes for CRO.

Answer:

• Selection Criteria: Bandwidth, loading effect, measurement range
• Compensation: 10X probes require compensation adjustment for accurate waveforms

Mnemonic: “PAC-S: Probes Allow Circuit Sensing”

Question 3(b) [4 marks]

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Draw and explain construction of Clamp on Meter.

Answer:

Diagram:

• Working Principle: Based on current transformer, measures current without breaking circuit
• Applications: Measuring AC current in live conductors safely

Mnemonic: “CAMP - Current Analyzed by Magnetic Principle”

Question 3(c) [7 marks]

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Write shortnote on successive approximation type DVM.

Answer:

Block Diagram:

graph LR
    A[Input] --> B[Sample & Hold]
    B --> C[Comparator]
    C --> D[SAR Logic]
    D --> E[DAC]
    E --> C
    D --> F[Digital Display]

• Working Principle: Uses binary search algorithm to find digital value matching analog input
• Conversion Time: Fixed regardless of input magnitude (8-16 clock cycles for 8-16 bit)
• Advantages: Medium speed, good resolution, consistent conversion time
• Applications: General purpose measurements where medium speed is sufficient

Mnemonic: “SACD - Sample, Approximate, Compare, Display”

Question 3(a OR) [3 marks]

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Explain PH Sensor.

Answer:

Diagram:

• Working Principle: Generates voltage proportional to hydrogen ion concentration
• Output: ~59 mV per pH unit at 25°C
• Range: 0-14 pH scale (acidic to alkaline)

Mnemonic: “PHRV - PH Related to Voltage”

Question 3(b OR) [4 marks]

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Draw and explain construction of Electronic Watt Meter.

Answer:

Block Diagram:

graph LR
    A[Current Input] --> B[Current Transformer]
    C[Voltage Input] --> D[Voltage Transformer]
    B --> E[Multiplier Circuit]
    D --> E
    E --> F[Integrator]
    F --> G[Digital Display]

• Working Principle: Power = V × I × cosθ (cosθ is power factor)
• Advantages: High accuracy, wide range, digital display
• Types: True RMS, average sensing

Mnemonic: “VIMP - Voltage & Intensity Make Power”

Question 3(c OR) [7 marks]

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Write shortnote on Integrating type DVM.

Answer:

Block Diagram:

graph LR
    A[Input] --> B[Integrator]
    B --> C[Comparator]
    D[Clock] --> E[Counter & Control]
    C --> E
    E --> F[Digital Display]

Key Features:

• Noise Rejection: Excellent rejection of power line noise (50/60Hz)
• Accuracy: High accuracy due to time averaging
• Conversion Speed: Slower than successive approximation type
• Resolution: Typically 4½ to 6½ digits

Applications: Precision measurements, noisy environments, bench instruments

Mnemonic: “TINA - Time Integration Nullifies Average”

Question 4(a) [3 marks]

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Write advantages and applications of Digital storage oscilloscope.

Answer:

• Key Benefits: Can capture one-time events, store waveforms for later analysis
• Digital Features: Automated measurements, FFT analysis, PC connectivity

Mnemonic: “SPADE - Storage, Processing, Analysis, Display, Events”

Question 4(b) [4 marks]

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Write shortnote on Electronic Energy Meter.

Answer:

Block Diagram:

graph LR
    A[Voltage Sensor] --> C[Multiplier]
    B[Current Sensor] --> C
    C --> D[Integrator]
    D --> E[Pulse Generator]
    E --> F[Counter]
    F --> G[Display]

• Working Principle: Energy = ∫P.dt (integral of power over time)
• Advantages: No moving parts, high accuracy, tamper detection
• Features: Multiple tariff support, bi-directional measurement, remote reading

Mnemonic: “VICES - Voltage & Current Energy Summation”

Question 4(c) [7 marks]

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Draw and explain Block diagram of Analog C.R.O. and working of each block in brief.

Answer:

Block Diagram:

graph LR
    A[Vertical Input] --> B[Vertical Attenuator]
    B --> C[Vertical Amplifier]
    C --> D[Vertical Deflection Plates]
    E[Trigger Circuit] --> F[Time Base Generator]
    F --> G[Horizontal Amplifier]
    G --> H[Horizontal Deflection Plates]
    I[Cathode Ray Tube] --> J[Screen]
    D --> I
    H --> I
    K[Power Supply] --> All

• Vertical System: Processes input signal, controls Y-axis deflection
• Horizontal System: Controls X-axis deflection (time base)
• Triggering: Stabilizes waveform display by starting sweep at same point
• CRT Display: Converts electrical signals to visible trace

Mnemonic: “VTHCP - Vertical, Time, Horizontal, CRT, Power”

Question 4(a OR) [3 marks]

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Draw and explain PIEZO-ELECTRIC transducer.

Answer:

Diagram:

• Applications: Pressure sensors, accelerometers, ultrasonic devices
• Advantages: High sensitivity, fast response, wide frequency range
• Limitations: High output impedance, temperature sensitive

Mnemonic: “PFVD - Pressure Forms Voltage via Displacement”

Question 4(b OR) [4 marks]

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Draw and explain Measurement of Frequency by using CRO.

Answer:

Method 1: Using Lissajous Patterns

Method 2: Using Time Base

• Time Base Method: Measure period of waveform, calculate frequency as 1/T
• Lissajous Method: Connect reference to X input, unknown to Y input
• Digital CRO: Direct frequency readout using internal counter

Mnemonic: “LTX - Lissajous or Time for X-axis”

Question 4(c OR) [7 marks]

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Draw and explain Thermistor and Thermocouple.

Answer:

Thermistor Diagram:

Thermocouple Diagram:

Thermistor Types:

• NTC: Negative Temperature Coefficient (resistance decreases with temperature)
• PTC: Positive Temperature Coefficient (resistance increases with temperature)

Thermocouple Types:

• Type K: Chromel-Alumel (-200°C to 1350°C)
• Type J: Iron-Constantan (-40°C to 750°C)
• Type T: Copper-Constantan (-200°C to 350°C)

Mnemonic: “TRT/TVJ - Temperature Resistance/Voltage Junction”

Question 5(a) [3 marks]

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Draw and Explain Velocity transducer.

Answer:

Diagram:

• Working Principle: Based on Faraday’s law of electromagnetic induction
• Output: Voltage proportional to velocity (V = Blv)
• Applications: Vibration measurement, seismic monitoring, motion control

Mnemonic: “VMMF - Velocity Makes Magnetic Flux”

Question 5(b) [4 marks]

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Give Classification of transducers and explain it.

Answer:

Explanation:

Mnemonic: “APRCI - Active Passive Resistive Capacitive Inductive”

Question 5(c) [7 marks]

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Write shortnote on LVDT.

Answer:

Diagram:

graph LR
    A[Primary Coil] --> B[Core]
    B --> C[Secondary Coil 1]
    B --> D[Secondary Coil 2]
    E[AC Excitation] --> A
    C --> F[Phase Sensitive Detector]
    D --> F
    F --> G[Output]

Working Principle:

• At null position: Equal voltage induced in both secondaries, net output zero
• Core movement: Creates imbalance in secondary voltages
• Output voltage: Proportional to displacement, phase indicates direction

Characteristics:

• Range: Typically ±0.5mm to ±25cm
• Linearity: Excellent within rated range
• Resolution: Virtually infinite (limited by readout circuit)
• Advantages: Frictionless, robust, reliable, high resolution

Mnemonic: “CPSO: Core Position Shifts Output”

Question 5(a OR) [3 marks]

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Draw and Explain block diagram of simple frequency Counter.

Answer:

Block Diagram:

graph LR
    A[Input] --> B[Input Conditioning]
    B --> C[Gate Control]
    D[Time Base] --> C
    C --> E[Counter]
    E --> F[Display]

• Working Principle: Counts pulses over precise time interval (typically 1 second)
• Frequency Calculation: f = counts/time interval
• Resolution: Determined by time base accuracy and gate time

Mnemonic: “IGTCD - Input Gated Time Counts Display”

Question 5(b OR) [4 marks]

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Draw and Explain Capacitive Transducer.

Answer:

Diagram:

Working Principle:

• Capacitance changes with physical parameter
• Signal conditioning converts capacitance to voltage/current
• High impedance output requires proper shielding

Advantages: High sensitivity, no moving contacts, low mass

Mnemonic: “CGAD - Capacitance Gap Area Dielectric”

Question 5(c OR) [7 marks]

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Draw and Explain block diagram of Function generator.

Answer:

Block Diagram:

graph LR
    A[Frequency Control] --> B[Waveform Generator]
    C[Mode Selector] --> B
    B --> D[Amplifier & Attenuator]
    D --> E[Output Buffer]
    E --> F[Output]
    G[Sweep Circuit] --> B
    H[AM/FM Modulator] --> D

Working Principle:

• Generates sine wave using RC oscillator or DDS
• Shape converters transform sine into square and triangle
• Output amplitude controlled by attenuator circuit
• Modern generators use digital synthesis techniques

Applications: Circuit testing, signal injection, filter characterization

Mnemonic: “FWMASO - Frequency Waveform Mode Amplitude Sweep Output”