Internet of Things (IOT) (EX-604 (B)) - Unit 4 Short Notes

UNIT 4: INSTRUMENTATION FOR IOT SYSTEMS – EXAM-DRIVEN NOTES

I. CATHODE RAY OSCILLOSCOPES (CROs) & WAVEFORM DISPLAY

CRT Fundamentals & Electrostatic Deflection

• Construction: Electron gun (cathode, control grid, focusing anode, accelerating anode), deflection system (electrostatic plates), phosphor-coated screen.

• Electrostatic Deflection: Beam deflection due to voltage on plates. Deflection in X-direction: $$\displaystyle D_x = \frac{L l_e V_d}{2 d_a V_a} $$, where $L$ = plate to screen distance, $$\displaystyle l_e $$ = plate length, $d$ = plate separation, $$\displaystyle V_d $$ = deflecting voltage, $$\displaystyle V_a $$ = anode voltage.

• Deflection Sensitivity ($S$): $$\displaystyle \boxed{S = \frac{D}{V_d} = \frac{L l_e}{2 d_a V_a}} $$ (cm/V) – deflection per volt.

• Deflection Factor ($G$): $$\displaystyle \boxed{G = \frac{1}{S} = \frac{2 d_a V_a}{L l_e}} $$ (V/cm) – voltage required for 1 cm deflection.

• Post-Deflection Acceleration:

  • Increases beam velocity after deflection, reducing spot size and improving brightness.

  • Increases effective anode voltage $$\displaystyle V_a $$, thus reducing sensitivity $S$ (since $$\displaystyle S \propto 1/V_a $$).

  • Essential for high-brightness, focused displays.

General Purpose CRO – Block Diagram

[Input] → [Vertical Amplifier] → [Delay Line] → [Vertical Deflection Plates]

                             ↓

[Trigger Circuit] ← [Sweep Generator] → [Horizontal Amplifier] → [Horizontal Deflection Plates]

                             ↓

                       [Power Supply]

• Vertical Amplifier: Amplifies input signal.

• Horizontal Amplifier: Amplifies sweep voltage.

• Time Base (Sweep Generator): Generates sawtooth waveform for horizontal sweep.

• Trigger: Synchronizes sweep to input for stable display.

• Applications: Voltage/time measurement, frequency measurement, phase comparison, distortion analysis, waveform display.

Time Base Circuits & Synchronization

• Time Base Circuit: Generates linear sweep voltage (sawtooth). Key: linear charging of capacitor through constant current source.

• Sweep Synchronization:

  • Trigger signal derived from input to start sweep at a consistent phase.

  • Effect on Accuracy: Without synchronization, waveform drifts; with synchronization, stationary display, accurate time measurement.

Multi-Trace/Beam Oscilloscopes

Graticules & Lissajous Patterns

• Graticules: Grid on CRT screen. Types: Internal (etched), External (glass plate), Digital (generated).

• Lissajous Patterns:

  • Formed by applying sinusoidal signals to X and Y plates.

  • Frequency Comparison: $$\displaystyle \boxed{\frac{f_y}{f_x} = \frac{\text{No. of horizontal tangencies}}{\text{No. of vertical tangencies}}} $$

  • Used for frequency and phase measurement.

Specialized Oscilloscopes

• Sampling Oscilloscope:

  • Principle: Sample input signal at high speed, reconstruct waveform.

  • Multi-Input Sampling: Multiple channels sampled sequentially.

  • Applications: Very high-frequency signals (>1 GHz).

  • Precautions: Aliasing, need for trigger synchronization, sample rate must exceed Nyquist rate.

• Wobbly Scope:

  • Frequency-modulates sweep oscillator with input signal.

  • Application: Audio-frequency response testing, distortion measurement.

II. AC BRIDGES FOR IMPEDANCE & PARAMETER MEASUREMENT

Bridge Fundamentals

• General Balance Condition: $$\displaystyle \boxed{Z_1 Z_3 = Z_2 Z_4} $$ (product of opposite arms equal).

• Sources of Errors:

  • Stray capacitances/inductances.

  • Non-ideal components (parasitic resistance, loss).

  • Frequency instability.

  • Detector sensitivity.

• Error Reduction:

  • Shielding, guarding, three-terminal connections.

  • Use high-Q components.

  • Operate at optimal frequency.

  • Balance close to detector null.

Wien Bridge

• Circuit: $$\displaystyle R_1, R_2 $$ ratio arms; $$\displaystyle C_1, C_2 $$ in opposite arms.

• Balance Equations:

  $$\displaystyle \frac{R_1}{R_2} = \frac{C_2}{C_1} $$, $$\displaystyle \omega^2 = \frac{1}{R_1 R_2 C_1 C_2} $$

• Frequency Determination: If $$\displaystyle R_1=R_2=R $$, $$\displaystyle C_1=C_2=C $$, then $$\displaystyle \boxed{f = \frac{1}{2\pi RC}} $$

• Wien Bridge Oscillator: Uses positive feedback through Wien network; frequency set by $R$ and $C$.

Maxwell Bridge

• Circuit: Unknown $$\displaystyle L_x $$ with series $$\displaystyle R_x $$; arms: $$\displaystyle R_1, R_2, R_3, C_1 $$.

• Balance Equations:

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

• Merits: Direct reading of $$\displaystyle L_x $$ and $$\displaystyle R_x $$.

• Demerits: Requires precise $$\displaystyle C_1 $$; not suitable for low $Q$ coils ($$\displaystyle Q < 1 $$).

• Applicability: Coils with storage factor $Q$ between 1 and 10.

Schering Bridge

• Circuit: Unknown $$\displaystyle C_x $$ with series $$\displaystyle R_x $$; arms: $$\displaystyle R_4 $$, $$\displaystyle C_2 $$, $$\displaystyle R_3 \parallel C_3 $$.

• Balance Equations:

  $$\displaystyle \boxed{C_x = C_2 \frac{R_3}{R_4}} $$, $$\displaystyle \boxed{R_x = R_4 \frac{C_3}{C_2}} $$

  Dissipation factor: $$\displaystyle \boxed{D = \tan \delta = \frac{1}{\omega R_3 C_3}} $$

  Power factor: $$\displaystyle \cos \phi = \frac{1}{\sqrt{1+D^2}} \approx D $$ (for small $D$).

• High Voltage Schering Bridge:

  • Special features: Guard electrodes, high-voltage capacitors, reduced stray effects.

  • Used for insulating oil, cable testing.

• Relation to Q-factor: $$\displaystyle Q = \frac{1}{D} $$ for capacitor.

Other Bridges & Q-Meter

• De Sauty's Bridge:

  • Simple capacitance comparison: $$\displaystyle C_x = C_2 \frac{R_4}{R_3} $$.

  • Assumes no dielectric loss; unsuitable for lossy capacitors.

  • vs Schering: Schering measures loss; De Sauty does not.

• Anderson Bridge:

  • Modified Maxwell for low $Q$ coils.

  • Uses additional resistor-capacitor network; more complex but extends range.

• Q-Meter:

  • Circuit: Series $L$-$C$ circuit with known $C$, variable $L$, voltmeter across $C$.

  • Working: At resonance, $$\displaystyle V_C = Q \cdot V_{source} $$.

  • $$\displaystyle \boxed{Q = \frac{1}{R} \sqrt{\frac{L}{C}}} $$ (for coil with series $R$).

  • Applications: Measure $Q$, $L$, $R$ of coils.

Overview of AC Bridges

III. TRANSDUCERS & SENSORS (CORE IOT COMPONENT)

Resistance-Based Transducers

Strain Gauges

• Principle: Piezoresistive effect – resistance change with strain.

• Gauge Factor ($GF$):

  $$\displaystyle \boxed{GF = \frac{\Delta R / R}{\epsilon} = 1 + 2\nu + \frac{\Delta \rho / \rho}{\epsilon}} $$

  where $\epsilon$ = strain, $\nu$ = Poisson's ratio, $\Delta\rho/\rho$ = resistivity change.

• Metal vs Semiconductor:

  | Property | Metal Strain Gauge | Semiconductor Strain Gauge | |-------------------|--------------------------|----------------------------| | Gauge Factor | 2–5 | 50–200 | | Temperature Sensitivity | Low (with compensation) | High (requires compensation) | | Hysteresis | Low | Higher |

• Instrumentation Amplifier with Bridge:

  • Full-bridge or half-bridge configuration.

  • Amplifier provides high gain, common-mode rejection.

  • Output $$\displaystyle V_{out} \propto \Delta R $$.

RTDs & Thermistors

• RTD (Resistance Temperature Detector):

  • Principle: Metal resistance increases with temperature ($$\displaystyle R = R_0[1+\alpha T] $$).

  • Materials: Pt, Ni, Cu.

  • Range: –200°C to 600°C (Pt).

  • Methods: 2-wire, 3-wire, 4-wire for lead compensation.

• Thermistor:

  • Principle: Semiconductor resistance decreases with temperature (NTC) or increases (PTC).

  • Materials: Metal oxides.

  • Range: –50°C to 300°C.

  • Characteristics: High sensitivity, nonlinear.

• Comparison:

  | Feature | RTD | Thermistor | |------------------|-------------------------|--------------------------| | Linearity | Good | Poor (exponential) | | Sensitivity | Moderate | High | | Stability | Excellent | Moderate | | Cost | High | Low | | IoT Use | Industrial, high-accuracy | Consumer, medical |

Inductive & Capacitive Transducers

LVDT (Linear Variable Differential Transformer)

• Construction: Primary coil, two secondary coils (series/parallel opposition), movable ferromagnetic core.

• Working Principle:

  • AC excitation on primary.

  • Core displacement changes mutual inductance, inducing voltages in secondaries.

  • Output $$\displaystyle V_{out} = V_{s1} - V_{s2} \propto $$ displacement $x$.

• Input-Output Characteristics:

  • Linear region: ± core travel (typically few mm).

  • Null point: core centered, output zero.

  • Phase indicates direction.

• Advantages:

  • Infinite resolution, non-contact, robust, long life.

  • High output, low hysteresis.

• Limitations:

  • Stray magnetic fields affect accuracy.

  • Requires AC excitation and demodulation.

  • Bulky for large displacements.

Magnetic & Hall Effect Transducers

Hall Effect Transducers

• Hall Voltage Generation:

  $$\displaystyle \boxed{V_H = \frac{I B}{n e t} \cdot k} $$

  where $I$ = current, $B$ = magnetic flux density, $n$ = charge carrier density, $e$ = electron charge, $t$ = thickness, $k$ = geometrical correction factor.

• Geometrical Correction Factor ($k$):

  • Accounts for non-ideal geometry (e.g., finite width/length ratio).

  • $$\displaystyle k = 1 $$ for ideal infinite sheet; $$\displaystyle k < 1 $$ for real samples.

  • Determined by calibration.

Thermal Transducers

Thermocouples

• Seebeck Effect: Two dissimilar metals joined at two junctions at different temperatures generate EMF.

  $$\displaystyle \boxed{E = \alpha (T_1 - T_2)} $$ (approx.), where $\alpha$ = Seebeck coefficient.

• Materials:

  • Noble metals: Pt-Rh (Type S, R) – stable, high-temperature.

  • Base metals: Chromel-Alumel (Type K), Iron-Constantan (Type J) – cheaper, wider range.

• IoT Application: Temperature sensing in harsh environments, industrial processes.

Piezoelectric Transducers

• Piezoelectric Effect:

  • Direct: Mechanical stress → charge generation.

  • Converse: Applied voltage → mechanical strain.

• Modes of Operation:

  • Transverse: Stress perpendicular to polarization, charge on electrodes perpendicular to stress.

  • Longitudinal: Stress parallel to polarization, charge on electrodes parallel to stress.

  • Shear: Shear stress, charge on electrodes perpendicular to stress.

• Applications: Pressure sensors, accelerometers, ultrasonic generators, frequency control.

• Quartz Crystal Calculations:

  Given strain $\epsilon$, force $$\displaystyle F = \epsilon A Y $$, charge $$\displaystyle Q = d F = d \epsilon A Y $$, voltage $$\displaystyle V = Q/C $$, with $$\displaystyle C = \epsilon_r \epsilon_0 A / t $$.

  $$\displaystyle \boxed{V = \frac{d Y t}{\epsilon_r \epsilon_0} \epsilon} $$

Optoelectronic Transducers

Phototransducers

• Photo-Voltaic (e.g., solar cell):

  • Generates voltage/current when illuminated (no bias).

  • High impedance, low noise.

• Photo-Conductive (e.g., LDR):

  • Resistance decreases with light.

  • Requires bias, slower response.

• Photo-Diode:

  • pn junction, operated in reverse bias.

  • Current proportional to light intensity.

  • Fast, linear, low capacitance.

• Suitability for Low-Intensity Light:

  • Photo-diode (in photoconductive mode) is most suitable.

  • Why: Low noise, high sensitivity with transimpedance amplifier, fast response, linear over wide range.

Transducer Fundamentals

• Definition: Device that converts physical quantity into electrical signal.

• Classification:

  • Primary: Directly senses physical quantity (e.g., thermocouple).

  • Secondary: Converts primary output into usable form (e.g., RTD with bridge).

• Input Characteristics:

  • Sensitivity: Output change per unit input.

  • Linearity: Deviation from straight-line response.

  • Hysteresis: Difference in output for increasing/decreasing input.

  • Resolution: Minimum detectable input change.

  • Dynamic Range: Ratio of maximum to minimum measurable input.

IV. SIGNAL GENERATORS & WAVE ANALYZERS

Function Generators

• Block Diagram:

  
  [VCO] → [Wave Shaper] → [Attenuator] → [Output]
  
        ↓
  
  [Frequency Control] (external voltage)
  
  

• Sine Wave Production:

  • RC oscillator (Wein bridge) or integrator (triangle to sine via diode shaping).

• Frequency Control by External Voltage (VCO):

  • Control voltage varies capacitance (varactor) or current in oscillator, changing frequency.

  • $$\displaystyle \Delta f \propto V_{control} $$.

Specialized Oscillators

• Beat Frequency Oscillator (BFO):

  • Two close-frequency oscillators (one fixed, one variable).

  • Mixer output: $$\displaystyle f_{beat} = |f_1 - f_2| $$.

  • Application: Audio-frequency generation, radio direction finding.

Sweep Generators

• Fixed-Frequency: Single output frequency.

• Sweep-Frequency: Output frequency varies linearly or logarithmically with time.

• Applications: Frequency response testing, filter characterization, spectrum analysis.

Wave & Spectrum Analyzers

• Frequency Selective Wave Analyzer:

  • Uses tunable filters (RC, crystal) to select frequency.

  • Operation: Input → amplifier → narrowband filter → detector.

  • Limitation: Limited sensitivity, slow tuning.

• Heterodyne Wave Analyzer:

  • Operation: Input mixed with local oscillator → IF amplifier → fixed filter → detector.

  • Advantages: High sensitivity, better selectivity (due to fixed IF filter).

  • Comparison:

    | Feature | Frequency Selective | Heterodyne | |------------------|---------------------|-----------------------| | Sensitivity | Low | High | | Selectivity | Moderate | High (fixed filter) | | Tuning Speed | Slow | Fast |

• Spectrum Analyzer:

  • Block Diagram: Input → attenuator → mixer → IF filter → detector → display (swept local oscillator).

  • Concept: Displays signal's frequency components in real-time.

  • IoT Importance: Signal integrity analysis, interference detection, modulation analysis.

V. DIGITAL MEASUREMENT INSTRUMENTS

Digital Voltmeters (DVMs)

• Advantages over Analog:

  • Higher accuracy, resolution, no parallax error.

  • Auto-ranging, data output, noise immunity.

• Ramp Type DVM:

  • Principle: Input voltage charges capacitor linearly; time to reach reference measured by counter.

  • $$\displaystyle \boxed{V_{in} = \frac{V_{ref} \cdot t_1}{t_2}} $$ where $$\displaystyle t_1 $$ = charge time, $$\displaystyle t_2 $$ = fixed ramp time.

• Dual-Slope Integrating Type:

  • Principle:

    1. Integrate input for fixed time $$\displaystyle T_1 $$ → output slope $$\displaystyle \propto V_{in} $$.

    2. Integrate reference (opposite polarity) until zero → time $$\displaystyle T_2 \propto V_{in} $$.

  • $$\displaystyle \boxed{V_{in} = V_{ref} \frac{T_2}{T_1}} $$

  • Advantage: Rejects noise, high accuracy (12–18 bits).

  • Disadvantage: Slow (multiple conversion cycles).

• Successive Approximation Type:

  • Principle: SAR compares input with DAC output, bit by bit (MSB to LSB).

  • Advantage: Fast (typically 100 kSPS).

  • Disadvantage: Less noise rejection than dual-slope.

• Comparison:

  | Feature | Dual-Slope | Successive Approximation | |------------------|-----------------------|--------------------------| | Accuracy | Very High | Moderate | | Speed | Slow (ms) | Fast (µs) | | Noise Rejection | Excellent (integrates) | Poor | | Resolution | High (up to 18 bits) | Moderate (up to 16 bits) |

Digital Frequency Meters & Tachometers

• Digital Frequency Meter:

  • Block Diagram:

    
    [Input Conditioning] → [Gate] → [Counter] → [Latch] → [Display]
    
                ↑
    
            [Time Base]
    
    

  • Working: Gate open for precise time $T$ (from time base); counts input pulses → $$\displaystyle f = \frac{N}{T} $$.

• Digital Tachometer:

  • Principle: Measures pulses per revolution from encoder or magnetic pickup.

  • $$\displaystyle N = \text{pulses/rev} $$, $$\displaystyle f = \text{pulse frequency} $$, $$\displaystyle \text{RPM} = \frac{60 f}{N} $$.

Digital Meter Specifications

• Resolution: Smallest change detectable.

  • For $n$-digit meter: $$\displaystyle \text{Resolution} = \frac{\text{Full scale}}{10^n - 1} $$.

  • 3½ digit: Max count = 1999.

    • On 10V range: $$\displaystyle \frac{10}{1999} \approx 5\,\text{mV} $$.

    • On 1V range: $$\displaystyle \frac{1}{1999} \approx 0.5\,\text{mV} $$.

• Display Examples (3½ digit DVM):

  • 11.52V on 10V range: Overrange → displays "1" or "OL".

  • 0.5234V on 1V range: 0.523V (rounded to 0.5 mV resolution).

  • 0.5234V on 10V range: 0.525V (rounded to 5 mV resolution).

VI. INTERFACING, DATA SYSTEMS & MULTIPLEXING

Standard Instrumentation Interfaces

• RS232C:

  • Role: Serial point-to-point communication.

  • Specs: Voltage levels ±3 to ±15 V, up to 115.2 kbps, distance ≤ 15 m.

  • Limitations: Slow, single-master, no multi-drop.

• IEEE-488 (GPIB):

  • Role: Parallel bus for multiple instruments.

  • Schematic: 8-bit data bus, 8 control lines, 3 handshake lines.

  • Operation: Talker/listener addressing, bus management.

  • Speed: Up to 1 Mbyte/s.

• Comparison with Modern Interfaces:

  | Feature | RS232C/GPIB | USB/Ethernet | |------------------|----------------------|-----------------------| | Topology | Point-to-point (RS232), multi-drop (GPIB) | Network, plug-and-play | | Speed | Slow (kbps–Mbps) | Fast (Mbps–Gbps) | | Distance | Short (RS232), 20 m (GPIB) | Long (Ethernet: 100 m) | | IoT Relevance| Legacy systems | Modern IoT, cloud connectivity |

Data Systems

• Data Logger:

  • Records data over time, often with internal storage, periodic download.

  • Limited real-time processing.

• Data Acquisition System (DAS):

  • Real-time acquisition, processing, display, and control.

  • High-speed, often with feedback loops.

• Key Difference: DAS is interactive and real-time; logger is passive recording.

Transducer Interfacing Techniques

• Digital Multiplexing:

  • Concept: Use analog multiplexer (e.g., CD4051) to connect multiple sensors to a single ADC.

  • Improves Efficiency:

    • Reduces wiring complexity in large sensor networks (IoT).

    • Shares expensive signal conditioning/ADC.

    • Lowers cost and power consumption.

  • Precautions: Crosstalk, multiplexer on-resistance, sample-and-hold requirements.

VII. DISPLAY & RECORDING DEVICES

Display Technologies

LED (Light Emitting Diode)

• Construction/Working: pn junction; forward bias injects electrons/holes → recombination → light emission.

• Advantages: Bright, fast response, wide viewing angle, rugged.

• Disadvantages: Higher power, limited color (without RGB), generates heat.

LCD (Liquid Crystal Display)

• Theory: Liquid crystals between polarizers; electric field twists crystals → modulates light.

• Types: Twisted Nematic (TN), In-Plane Switching (IPS).

• Advantages over LED: Low power, thin, no radiation, cheap for large displays.

• Disadvantages: Slow response, viewing angle dependent, temperature sensitive.

Comparison: LED vs LCD

Electrophoretic vs Liquid Vapor Display

• Electrophoretic (E-ink):

  • Charged pigment particles in fluid move with electric field.

  • Features: Bistable (image without power), paper-like readability, low power.

  • Applications: E-readers, IoT labels.

• Liquid Vapor Display:

  • Uses volatile liquid that vaporizes/condenses with temperature.

  • Features: Reflective, low power, temperature-dependent.

  • Applications: Thermometers, battery-powered displays.

Recording Devices

X-Y Recorders

• Analog X-Y Recorder:

  • Working: Two servo motors move pen in X and Y; feedback from input voltages.

  • Circuit: Inputs → amplifiers → servo motors → pen carriage.

  • Applications: Plotting characteristics (e.g., V-I curves), control system responses.

• Digital X-Y Recorder:

  • Uses stepper/servo motors with digital control.

  • Advantages: Higher accuracy, no wear, programmable.

  • Disadvantages: Discrete steps, may alias fast signals.

• Comparison:

  | Feature | Analog | Digital | |------------------|-----------------------|------------------------| | Accuracy | Moderate (mechanical) | High | | Speed | Limited by inertia | Fast (stepper) | | Wear | Pen/paper wear | No wear | | Flexibility | Fixed | Programmable |

• Applications: Calibration curves, process monitoring, research plots.

VIII. SPECIALIZED TOPICS & MISCELLANEOUS

Waveform Quality & Distortion

• Total Harmonic Distortion (THD):

  $$\displaystyle \boxed{\text{THD} = \frac{\sqrt{V_2^2 + V_3^2 + \cdots + V_n^2}}{V_1} \times 100\%} $$

  where $$\displaystyle V_1 $$ = fundamental RMS, $$\displaystyle V_n $$ = harmonic RMS.

  • Significance: Measures signal purity; critical in audio, power systems, IoT signal integrity.

Safety & Grounding

• Wagener's Earthing Device:

  • Purpose: Prevent electric shock from oscilloscope when measuring high-voltage circuits.

  • Construction: Capacitor (typically 0.01 µF, 2 kV) in series with ground lead.

  • Operation: Capacitor blocks DC, passes HF transients, isolates user from mains earth.

Miscellaneous Applications of CROs

• Voltage and current measurement (with probes).

• Time interval and frequency measurement (using time base).

• Phase difference measurement (dual-trace, Lissajous).

• Modulation analysis (AM/FM).

• Digital circuit debugging (logic analyzers).

• Biomedical signal monitoring (ECG, EEG).

[!TIP] Exam Focus

• Numericals: Practice bridge balance equations (Maxwell, Schering), Lissajous frequency, strain gauge GF, piezoelectric calculations, DVM resolution/display.

• Comparisons: Dual-beam vs dual-trace CRO, LED vs LCD, analog vs digital X-Y recorder, DVM types.

• Diagrams: Draw and label CRO block, LVDT, bridge circuits, function generator, spectrum analyzer.

• IoT Link: Emphasize transducer interfacing (multiplexing), digital communication (USB/Ethernet), and sensor networks in answers.