Internet of Things (IOT) (EX-604 (B)) - Unit 3 Short Notes

UNIT 3: Electronic Instrumentation (IOT Focus)

1.0 Cathode Ray Oscilloscopes (CROs)

Core Function: A CRO is a voltage-sensitive display device that plots an electrical signal graphically, showing amplitude vs. time.

A. Types of CROs (Past Paper Focus)

B. CRT Fundamentals

• Deflection: Electrostatic (plates) for low power, Electromagnetic (yokes) for high power/large screens.

• Post-Deflection Acceleration (PDA):

  • Purpose: Accelerates the electron beam after deflection plates.

  • Effect on Beam Velocity: Increases final beam velocity significantly.

  • Effect on Spot Size: Reduces spot size (improves focus & brightness) because electrostatic lens aberrations are less critical at higher velocities.

  • Trade-off: Reduces deflection sensitivity (needs higher deflecting voltage for same deflection).

C. Deflection Parameters

• Deflection Sensitivity (S): Electrostatic deflection. Physical deflection on screen per unit deflecting voltage.

$$S = \frac{L \cdot l}{2 \cdot d \cdot V_a} \quad \text{(cm/V)}$$

Where, `L` = screen distance from plate center, `l` = plate length, `d` = plate spacing, `V_a` = anode voltage.

• Deflection Factor (G): Reciprocal of sensitivity. Electromagnetic deflection.

$$G = \frac{1}{S} \quad \text{(V/cm)}$$

> [!TIP] **Common Pitfall:** Do not confuse `S` (cm/V) with `G` (V/cm). `S` is for electrostatic, `G` for magnetic.

D. Time Base Circuits (Sweep Generator)

• Function: Generates a linear sawtooth voltage to move the beam horizontally at a constant speed.

• Synchronization: The sweep circuit is triggered by the input signal (or internally). This locks the start of each sweep to a specific point on the input waveform.

  • Effect on Accuracy: Proper synchronization stabilizes the display. Without it, the waveform appears to drift or roll. Over-synchronization (too high trigger level) can distort the waveform's leading edge.

E. Graticules and Lissajous Patterns

• Graticule: Illuminated grid on CRT face for measurement. Types: Internal (etched on glass), External (separate plate).

• Lissajous Patterns: Result of applying two sinusoidal signals to X and Y plates (no internal sweep).

  • Stationary Pattern Condition: $$\displaystyle \frac{f_y}{f_x} = \frac{\text{Number of horizontal tangencies (H)}}{\text{Number of vertical tangencies (V)}} = \frac{H}{V} $$

  • Frequency Determination: $$\displaystyle f_y = f_x \times \frac{H}{V} $$. (For rational frequency ratios).

  [!TIP] Exam Trick: Count tangencies carefully. A "loop" counts as 2 tangencies.

F. Applications of CRO

1. Voltage/time measurement (amplitude, period, frequency, phase).

2. Debugging digital circuits (timing diagrams).

3. Comparing phase relationships (Lissajous).

4. Measuring hysteresis loops (magnetic materials).

5. Displaying characteristics of transducers.

G. Accessories and Safety

• Wagener's Earthing Device: A safety probe with a high-value resistor (e.g., 1 MΩ) in series with the probe tip. Prevents short-circuits and protects both the user and the CRO input circuitry when probing live circuits.

2.0 AC Bridge Circuits for Impedance Measurement

Core Principle: Balance condition: $$\displaystyle Z_1 Z_3 = Z_2 Z_4 $$. At balance, detector (galvanometer/oscilloscope) shows null.

A. Maxwell Bridge

• Purpose: Measure unknown inductance (Lx) with series resistance (Rx).

• Circuit: Unknown inductor Lx-Rx in one arm. Known capacitor C1 in adjacent arm. Two known resistors R2, R3 in other arms.

• Balance Equations:

$$R_x = \frac{R_2 R_3}{R_1}, \quad L_x = R_2 R_3 C_1$$

• Merits: Simple, balance equations independent of frequency.

• Demerits: C1 must be large for low Lx measurement; not suitable for very low Q coils.

• Applicability: Coils with storage factor (Q) 1–10. (Storage factor Q = ωL/R).

B. Schering Bridge

• Purpose: Measure unknown capacitance (Cx) and its dissipation factor (D) or power factor (pf). Widely used for dielectric loss measurement in capacitors/insulation.

• Circuit: Cx with leak resistance Rx in parallel (or series equivalent). Known C1, R1 (adjustable), R2, R3.

• Balance Equations:

$$C_x = \frac{C_1 R_2}{R_3}, \quad D = \omega C_1 R_1 = \tan \delta$$

Where `D` = Dissipation factor = `1/Q` = `tan δ`.

• High-Voltage Schering Bridge: Uses a high-voltage capacitor as C1 and a high-voltage resistor in detector circuit. Enables testing of high-voltage insulation (power cables, transformers) at operating voltage.

C. Comparison: Schering Bridge vs De Sauty's Bridge

D. Wien Bridge

• Purpose 1: Frequency Determination (as oscillator). Balance condition gives:

$$\omega = \frac{1}{RC} \quad \text{or} \quad f = \frac{1}{2\pi RC}$$

(When `R1=R2=R` and `C1=C2=C`).

• Purpose 2: Measure unknown capacitance (if R and f known) or unknown frequency (if C known).

• Balance Equations (General):

$$Z_1 Z_3 = Z_2 Z_4 \quad \Rightarrow \quad \frac{R_1}{1+j\omega R_1 C_1} \cdot R_3 = \left( R_2 + \frac{1}{j\omega C_2} \right) R_4$$

Solve for real & imaginary parts.

E. Anderson Bridge

• Purpose: Modified Maxwell bridge to measure inductance with higher accuracy.

• Basic Topology: Adds an extra resistor (r) and a fixed capacitor (C) in a specific configuration. The balance equations are more complex but allow the use of a standard capacitor (C) instead of a variable one, improving accuracy.

F. Q-Meter

• Circuit: Series resonant circuit (Lx-Cx-Rx) with a low-loss coil (L) and a variable capacitor (C). A rf source injects current. A voltmeter across the capacitor measures voltage Vc.

• Working: At resonance ($$\displaystyle \omega L = 1/\omega C $$), impedance is minimum (Rx). Current I = V_source / Rx. Q-factor is:

$$Q = \frac{V_c}{V_{source}}$$

(Since `V_c = I * X_c = I / \omega C` and `V_source = I * R_x`).

• Applications: Measure Q-factor, inductance, self-capacitance of coils, and distributed capacitance.

G. Sources of Errors in Bridge Circuits & Reduction

3.0 Transducers and Sensors

A. Resistive Transducers

• Strain Gauges:

  • Theory: Resistance change due to strain (ε). $$\displaystyle R = \frac{\rho L}{A} $$. Strain changes L and A.

  • Gauge Factor (GF): $$\displaystyle GF = \frac{\Delta R / R}{\epsilon} $$

  • Derivation: For metallic gauge, $$\displaystyle GF = 1 + 2\nu + \frac{d\rho/\rho}{d\epsilon} $$ ≈ 1 + 2ν (ν = Poisson's ratio). For semiconductor, GF is much larger (~100) due to piezoresistive effect ($d\rho/\rho$ term dominant).

  • Metal vs. Semiconductor:

    | Property | Metal Strain Gauge | Semiconductor Strain Gauge | | :--- | :--- | :--- | | Gauge Factor | Low (~2) | Very High (~50-200) | | Temperature Sensitivity | Moderate | Very High (requires compensation) | | Non-linearity | Low | Higher |

  • Instrumentation Amplifier Adaptation: Used in Wheatstone bridge configuration. I.A. provides high common-mode rejection ratio (CMRR) to amplify small differential voltage from bridge imbalance.

• RTDs (Resistance Temperature Detectors):

  • Principle: R = R0 [1 + α(T - T0)]. Pure metals (Pt, Ni, Cu). Positive Temperature Coefficient (PTC).

  • Applications: Wide range (-200°C to +850°C). High accuracy & stability. Industrial, aerospace.

• Thermistors:

  • Principle: Semiconductor ceramic. Negative Temperature Coefficient (NTC). Highly non-linear: $$\displaystyle R = A e^{B/T} $$.

  • Applications: Narrow, high-sensitivity range (-100°C to +300°C). Temperature compensation, inrush current limiting.

B. Inductive Transducers: LVDT (Linear Variable Differential Transformer)

• Construction: Primary winding, two identical secondary windings (series/parallel opposing), movable ferromagnetic core.

• Working: AC excitation on primary. Core position determines mutual inductance to secondaries.

• Input-Output Characteristic: Linear over ~5-10% of full scale. Output voltage (V_out = V_sec1 - V_sec2) is in-phase or 180° out-of-phase with excitation, indicating direction.

• Advantages: Infinite resolution, frictionless, robust, high output.

• Limitations: Requires AC excitation & demodulation, sensitive to stray magnetic fields, limited bandwidth.

C. Capacitive Transducers

• Principle: $$\displaystyle C = \frac{\epsilon A}{d} $$. Change in plate area (A), distance (d), or dielectric (ε).

• Applications: Displacement (small), pressure, humidity, level sensing. High sensitivity, but non-linear if d varies. Requires high-frequency AC excitation & impedance measurement.

D. Piezoelectric Transducers

• Modes: Transverse (stress ⟂ to generated charge), Longitudinal (stress || charge), Shear.

• Quartz Crystal Calculations:

  • Given: Dimensions l, w, t (length, width, thickness). Strain ε.

  • Properties: Charge sensitivity d (C/N), Young's modulus Y (N/m²), permittivity ε_r.

  • Force (F): $$\displaystyle F = Y \cdot A \cdot \epsilon $$, where A = w * t (area perpendicular to strain).

  • Charge (Q): $$\displaystyle Q = d \cdot F = d \cdot Y \cdot A \cdot \epsilon $$

  • Voltage (V): $$\displaystyle V = \frac{Q}{C} $$, where $$\displaystyle C = \frac{\epsilon_0 \epsilon_r A}{t} $$ (for plate capacitor).

• Applications: Dynamic force/pressure/acceleration measurement (microphones, accelerometers, ultrasonic). Cannot measure static signals (charge leaks).

E. Hall Effect Transducers

• Hall Voltage ($$\displaystyle V_H $$): $$\displaystyle V_H = \frac{I B}{n e t} = R_H \frac{I B}{t} $$

  Where I = current, B = magnetic flux density, n = charge carrier density, e = electron charge, t = thickness, R_H = Hall coefficient.

• Geometrical Correction Factor (k): Accounts for non-ideal geometry. $$\displaystyle V_H = k \cdot R_H \frac{I B}{t} $$. k ≈ 1 for ideal rectangular sample.

• Applications: Magnetic field measurement, current sensing, position/speed sensing (magnetic encoders).

F. Thermoelectric Transducers: Thermocouples

• Seebeck Effect: Two dissimilar metals joined at two junctions at different temperatures (T_hot, T_cold) generate a thermo-emf proportional to the temperature difference.

• Material Requirements: High Seebeck coefficient, linear emf-T relationship, stable, reproducible, high melting point.

• Applications: Wide temperature range (-270°C to +2300°C). Industrial temperature monitoring, scientific research. Requires cold-junction compensation.

G. Photoelectric Transducers

Answer to Past Q: Photodiode (reverse-biased) is most suitable for low-intensity light detection due to low dark current, high sensitivity, and fast response when used with a low-noise transimpedance amplifier.

H. Transducer Interfacing

• Digital Multiplexing: Using a multiplexer (MUX) to connect multiple transducer outputs to a single ADC. Reduces cost & wiring in large systems (e.g., industrial process control).

• Primary vs Secondary Transducer:

  • Primary: Directly senses the physical quantity (e.g., thermocouple senses temperature, strain gauge senses strain).

  • Secondary: Converts the primary's output into a convenient form (e.g., LVDT core motion from primary displacement sensor, potentiometer from pressure).

• Input Characteristics of I.A. for Bridge: Needs high input impedance (to not load the bridge), high CMRR (to reject common-mode voltage), low offset voltage & drift.

4.0 Signal Generators and Analyzers

A. Function Generators

• Block Diagram: [Function Gen Block: 1. VCO (Voltage Controlled Oscillator) core, 2. Waveform Shaper (sine shaper, integrator for triangle, Schmitt trigger for square), 3. Amplitude Control (attenuator), 4. Output Stage]

• Sine Wave Production: From triangle wave (from VCO integrator) using diode shaping network (piecewise linear approximation) or filter (low-pass to remove harmonics).

• Voltage-Controlled Frequency (VCF): Input voltage to VCO controls oscillation frequency. Enables frequency modulation (FM) or sweep generation.

B. Beat Frequency Oscillator (BFO)

• Working: Mixes (heterodynes) a variable-frequency oscillator output with a fixed-frequency oscillator output. The difference frequency (f_variable - f_fixed) is audible (if in audio range) or measurable. Used in communication receivers (CW/SSB demodulation) and audio frequency measurement.

C. Sweep Generators

• Fixed-Frequency: Outputs a single, selectable frequency.

• Sweep-Frequency: Output frequency varies continuously over a specified range (linear or logarithmic) at a set sweep rate. Used for frequency response testing of filters, amplifiers.

D. Wave Analyzers

Comparison: Heterodyne type has superior sensitivity and selectivity due to fixed-frequency, high-Q IF filtering, but is more complex and expensive.

E. Spectrum Analyzer

• Block Diagram: [Spectrum Analyzer: 1. Input Attenuator, 2. Mixer (with LO), 3. IF Amplifier & Filter (resolution bandwidth), 4. Detector, 5. Display (CRT/LCD). LO sweeps synchronously with X-axis.]

• Importance: Displays amplitude vs. frequency of a signal. Essential for spectrum analysis, harmonic distortion measurement, interference identification, signal integrity.

F. Total Harmonic Distortion (THD)

• Concept: Measure of harmonic content in a periodic signal relative to the fundamental.

$$THD = \frac{\sqrt{V_2^2 + V_3^2 + V_4^2 + ...}}{V_1} \times 100\%$$

Where `V1` = RMS voltage of fundamental, `V2, V3...` = RMS voltages of harmonics.

• Measurement: Using wave analyzer (measure each harmonic) or distortion analyzer (notch filter removes fundamental, measures residual).

5.0 Digital Measurement Instruments

A. Digital Voltmeters (DVM)

• Types:

  1. Ramp Type: Integrates input for fixed time, measures reference ramp time. Medium speed, medium accuracy.

  2. Dual-Slope Integrating: Integrates input for fixed period T1, then reference of opposite slope for time T2. $$\displaystyle V_{in} = V_{ref} \frac{T_2}{T_1} $$. High accuracy, noise immunity, low speed.

  3. Successive Approximation: SAR ADC. Fast, moderate accuracy.

• Comparison (Dual-Slope vs Successive Approximation):

  | Feature | Dual-Slope | Successive Approximation | | :--- | :--- | :--- | | Accuracy | Very High (independent of component tolerances, integrates noise). | Moderate (depends on DAC linearity). | | Speed | Slow (conversion time ~ T1 + T2). | Fast (n clock cycles for n-bit). | | Noise Rejection | Excellent (power line frequency rejection). | Poor (needs sample-hold). |

• Resolution & Display (3½ Digit):

  • Resolution: $$\displaystyle \frac{1}{2^N - 1} $$ of full scale, where N = number of digits. For 3½ digit (N=14 bits max), resolution = 0.01% of full scale or 1 part in 1999.

  • Display Examples:

    • 11.52V on 10V range: Overrange → displays "1" (or "OL", "OVER").

    • 0.5234V on 1V range: Displays "0.5234" (4 digits after decimal).

    • 0.5234V on 10V range: Displays "0.523" (3 digits after decimal, last digit uncertain).

B. Digital Frequency Meters

• Block Diagram: [DFM: 1. Input Conditioning (amplifier, Schmitt trigger), 2. Gate Circuit (controlled by timebase), 3. Counter (counts pulses in gate time), 4. Timebase (crystal oscillator), 5. Display]

• Working: Input signal is shaped into pulses. A gate (opened for a precise time T from timebase) allows these pulses to enter a counter. Count N displayed. Frequency $$\displaystyle f = \frac{N}{T} $$.

C. Sampling Oscilloscopes (Multi-input)

• Applications: Measuring very high-frequency (>1 GHz) repetitive signals, eye patterns in digital communications, jitter measurement.

• Precautions:

  1. Signal must be repetitive.

  2. Sampling rate must be sufficiently high (≥ 2x highest frequency component) to avoid aliasing.

  3. Triggering must be stable.

  4. Interpolation is used to reconstruct waveform; beware of artifacts.

D. Digital Tachometers

• Working Principle: Measure time period (T) between successive pulses from a rotating encoder (optical/magnetic) or zero-crossings of an AC generator signal.

$$Speed (RPM) = \frac{60}{T} \quad \text{or} \quad Speed = \frac{60 \times \text{Pulses per revolution}}{T}$$

Uses a **microcontroller/timer** to measure `T` precisely and compute RPM. Displays digitally.

6.0 Data Interfaces and Communication Buses

A. Legacy Interfaces

B. Modern Interfaces (USB, Ethernet)

C. Data Logger vs Data Acquisition System (DAS)

7.0 Display and Recording Systems

A. Display Devices

• Electrophoretic Image Display (E-ink): Micro-capsules with charged pigment particles move under electric field. Bistable (image persists without power). Paper-like readability, ultra-low power. Used in e-readers (Kindle).

• Liquid Vapor Display (LVD): Uses thermochromic liquid crystal. Changes color with temperature. Used in thermometers, mood rings. Low power, passive.

B. Recording Systems

C. Integrated Instruments: Digital pH Meter (Example)

• Components: pH electrode (primary transducer, generates mV proportional to pH), high-impedance amplifier (to avoid loading electrode), temperature sensor (for compensation), ADC, microcontroller (applies Nernst equation, temperature compensation), LCD display.

• Working: Electrode potential E = E0 - (2.303RT/F) * pH. Microcontroller measures E and temperature T, calculates and displays compensated pH value.