Electronic Instrumentation Lab (EX-605) - Unit 1 Short Notes

UNIT 1: FOUNDATIONS OF ELECTRONIC MEASUREMENT & LAB PRACTICE

1.0 Laboratory Safety and Professional Practices

1.1 General Lab Safety Rules

• ESD (Electrostatic Discharge) Protection: Always use an anti-static wrist strap when handling CMOS ICs and other sensitive components. Work on anti-static mats.

• Personal Protective Equipment (PPE): Wear safety glasses. Avoid loose clothing. No food or drink in the lab area.

• Fire Safety: Know the location of fire extinguishers (CO₂ type for electrical fires) and emergency exits. Never leave powered circuits unattended.

1.2 Electrical Safety

• Hazards: Both AC mains (230V, 50Hz) and high-current DC circuits are dangerous. AC is particularly hazardous as it can cause ventricular fibrillation.

• Grounding: Ensure all equipment is properly earthed (grounded). Use three-pin plugs. Never remove the ground pin.

• Safe Procedures: Always de-energize and discharge capacitors (using a resistor) before touching a circuit. Connect power last and disconnect it first.

1.3 Handling and Care of Sensitive Instruments

• Oscilloscope: Handle the CRT (if present) gently. Avoid magnetic fields near the screen. Cover when not in use.

• Multimeter: Never exceed the maximum rated voltage/current on any port. Always start with the highest range when measuring an unknown quantity.

• General: Power down instruments before connecting/disconnecting probes. Keep inputs clean and dry.

1.4 Proper Use of Test Probes and Connectors

• Oscilloscope Probes: Use 10X probes for high-frequency or high-voltage measurements to avoid circuit loading. Always compensate the probe using the oscilloscope's built-in calibration signal before taking measurements.

• Avoiding Short Circuits: Ensure probe tips and connectors do not accidentally touch adjacent circuit nodes (causing shorts). Use insulated alligator clips for secure connections.

• Connector Security: Ensure BNC, banana plug, and screw terminal connections are tight to prevent intermittent readings or arcing.

1.5 Documentation and Reporting

• Lab Notebook: Maintain a bound, numbered, permanent notebook. Record date, aim, circuit diagram (neatly drawn), component values, observations, measurements, and conclusions in ink.

• Data Tables: Pre-draw tables with clear column headings and units. Record raw data first, then calculations.

• Result Analysis: Compare measured values with theoretical/calculated values. Calculate and discuss percentage error.

[!TIP] Exam Focus: You will be marked on safety violations in practical exams. Always state your safety steps (e.g., "checked for ESD," "discharged capacitors," "used 10X probe"). A single short circuit can destroy a multimeter or power supply.

2.0 Core Measurement Instruments: Theory and Operation

2.1 Digital Multimeter (DMM)

• Principle: Uses an Analog-to-Digital Converter (ADC) to measure input. For voltage, it has a very high input impedance (typically 10 MΩ) to minimize loading effect.

• Key Measurements:

  • Voltage (DC/AC): Connected in parallel across the component. AC voltage reading is usually RMS for sine waves.

  • Current (DC/AC): Connected in series by breaking the circuit. Beware of burden voltage (voltage drop across the meter's internal shunt), which can affect low-voltage circuits.

  • Resistance: Measures by applying a small known current and measuring the resulting voltage. Never measure resistance on a live circuit.

• Other Modes: Diode Test (applies a small current, shows forward voltage drop). Continuity Test (audible beep for low resistance, typically < 50 Ω).

• Fuse Protection: The current input (usually 10A or mA port) is fused. Know the fuse rating and location.

2.2 Cathode Ray Oscilloscope (CRO) / Digital Storage Oscilloscope (DSO)

• Functional Block Diagram:

  Signal → Vertical Amplifier → Delay Line → Horizontal Sweep (Timebase) → Trigger Circuit → CRT/Display

• Critical Controls:

  • Volts/Div (Vertical): Sets vertical sensitivity. Adjust to fit the waveform on the screen without clipping.

  • Sec/Div (Horizontal/Timebase): Sets horizontal scale. Adjust to display 1-2 full cycles of periodic signals.

  • Trigger Level & Source: Stabilizes a repeating waveform. Trigger Source should be the channel with the cleanest, most stable signal. Trigger Level sets the voltage at which sweep begins.

• Probes: 1X (no attenuation, higher loading), 10X (attenuates signal 10x, reduces loading). Probe compensation is mandatory for accurate frequency response.

• Key Measurements from Screen:

  • Peak-to-Peak Voltage (Vpp): $$\displaystyle V_{pp} = \text{(Number of vertical divisions)} \times \text{(Volts/Div setting)} $$

  • RMS Voltage (Sine Wave): $$\displaystyle V_{rms} = \frac{V_{pp}}{2\sqrt{2}} \approx 0.3536 \times V_{pp} $$

  • Frequency/Period: $$\displaystyle T \text{ (Period)} = \text{(Number of horizontal divisions for one cycle)} \times \text{(Sec/Div setting)} $$; $$\displaystyle f = \frac{1}{T} $$

  • Time Delay/Phase Shift: Measure horizontal distance between corresponding points (e.g., rising edges) on two channels. $$\displaystyle \Delta t = \text{(Horizontal divisions)} \times \text{(Sec/Div)} $$. Phase shift $$\displaystyle \phi = \frac{\Delta t}{T} \times 360^\circ $$.

2.3 Function / Signal Generator

• Waveforms: Sine (pure tone), Square (digital clocks), Triangle (ramp), TTL (0-5V digital pulse).

• Key Parameters:

  • Amplitude (Vpp): Peak-to-peak voltage of the output waveform.

  • DC Offset: Adds a DC voltage to the AC waveform, shifting it up/down.

  • Frequency: Set in Hz or kHz. For a sine wave: $$\displaystyle V(t) = \text{Offset} + \frac{V_{pp}}{2} \sin(2\pi f t) $$.

  • Duty Cycle: For square waves, the percentage of one period the signal is high. Standard is 50%.

2.4 DC Power Supply

• Modes:

  • Constant Voltage (CV): Supply maintains set output voltage until the current limit is reached.

  • Constant Current (CC): Once the load draws the set current limit, the supply enters CC mode and voltage drops to maintain that current.

• Setting Output: First set the voltage limit (V-set). Then set the current limit (I-set) to a value higher than expected load current. The supply will operate in CV mode until I-set is hit.

• Multiple Outputs: Dual/Triple supplies provide +V, -V, and sometimes a separate +5V. Tracking mode links two outputs to provide symmetric ± voltages (e.g., ±12V).

[!TIP] Common Pitfall: Forgetting to turn on the output button on the power supply after setting voltages. Also, setting current limit too low will cause the supply to go into CC mode prematurely, giving a lower-than-expected voltage.

3.0 Fundamental Quantities and Basic Measurements

3.1 Voltage Measurement

• DMM: Best for accurate, steady-state DC or RMS AC values. High input impedance (10 MΩ) minimizes loading.

• Oscilloscope: Essential for waveform shape, noise, transients, timing relationships. Input impedance is typically 1 MΩ // 10-20 pF, which can load high-frequency/high-impedance circuits.

3.2 Current Measurement

• DMM (Series Method): Break the circuit and insert the DMM in series. Start on the highest current range to avoid blowing the internal fuse.

• Oscilloscope (Shunt/Probe Method): Insert a small, known current shunt resistor (e.g., 1 Ω) in series. Measure the voltage across it with the oscilloscope ($$\displaystyle I = V_{shunt}/R_{shunt} $$). For high currents, use a current probe (clamp-on).

3.3 Resistance Measurement

• Ohmmeter Operation: Applies a small known voltage/current and measures the resulting voltage/current.

• Critical Rule: Never measure resistance on a circuit that is powered or has components (like capacitors) that can hold a charge.

• Pitfall: Measuring resistance in-circuit gives erroneous readings because other parallel paths conduct. For accurate value, desolder one end of the resistor or remove it.

3.4 Frequency and Period Measurement

• Oscilloscope: Most accurate. Use timebase to measure period $T$ directly from the screen, then $$\displaystyle f=1/T $$.

• DMM: Many have a frequency counter function. Connect the signal to the input (often the same as voltage input). Works best for clean, periodic signals within its specified range.

3.5 Phase Difference Measurement

• Dual-Trace Oscilloscope: Display both signals (CH1 and CH2) simultaneously.

  • Time-Delay Method: Measure $\Delta t$ (horizontal distance between corresponding points, e.g., zero-crossings rising). $$\displaystyle \phi = \frac{\Delta t}{T} \times 360^\circ $$.

  • X-Y Mode (Lissajous): Connect CH1 to X, CH2 to Y. The shape of the Lissajous figure indicates the phase difference and frequency ratio. For equal frequencies ($$\displaystyle f_x = f_y $$), an ellipse indicates phase shift $$\displaystyle \phi = \sin^{-1}(A/B) $$ where A and B are ellipse axes.

4.0 Basic Electronic Components and Circuit Laws

4.1 Resistors

• Color Code: Digit-Digit-Multiplier-Tolerance. Example: Yellow-Violet-Red-Gold = 47 × 10² Ω ±5% = 4.7 kΩ ±5%.

• Power Rating: $$\displaystyle P = I^2 R = \frac{V^2}{R} = V \cdot I $$. Always use a resistor with a rating higher than the calculated dissipation.

• Combinations:

  • Series: $$\displaystyle R_{eq} = R_1 + R_2 + ... $$

  • Parallel: $$\displaystyle \frac{1}{R_{eq}} = \frac{1}{R_1} + \frac{1}{R_2} + ... $$

• Voltage Divider: For two resistors in series, $$\displaystyle V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2} $$ (where $$\displaystyle R_2 $$ is the "bottom" resistor to ground).

4.2 Capacitors

• Types: Ceramic (non-polarized, small values), Electrolytic (polarized, large values, marked with - stripe).

• RC Circuit (Charging): $$\displaystyle V_C(t) = V_{final}(1 - e^{-t/\tau}) $$, where $$\displaystyle \tau = RC $$ is the time constant.

  • At $$\displaystyle t = \tau $$, capacitor charges to ~63.2% of final voltage.

  • At $$\displaystyle t = 5\tau $$, considered fully charged (>99%).

• Measurement:

  • DMM: Many have a capacitance measurement function.

  • RC Time Constant Method: Apply a step voltage (from a function generator or switch), measure time for $$\displaystyle V_C $$ to reach 63.2% of final value on oscilloscope. $$\displaystyle \tau = t_{0.632} $$, then $$\displaystyle C = \tau / R $$.

4.3 Inductors (Introduction)

• Unit: Henry (H). Opposes change in current.

• RL Circuit (Current Rise): $$\displaystyle I(t) = I_{final}(1 - e^{-t/\tau}) $$, where $$\displaystyle \tau = L/R $$.

• Symbol: Coiled line. May have a core (iron, air).

4.4 Application of Fundamental Laws

• Ohm's Law: $$\displaystyle V = I \cdot R $$. Verify by measuring V and I across a known R.

• Kirchhoff's Voltage Law (KVL): Sum of voltages around any closed loop = 0. $$\displaystyle \sum V_{drops} = \sum V_{rises} $$.

• Kirchhoff's Current Law (KCL): Sum of currents entering a node = sum leaving. $$\displaystyle \sum I_{in} = \sum I_{out} $$.

• Thevenin's Theorem: Any linear two-terminal network can be replaced by an equivalent circuit of a Thevenin Voltage ($$\displaystyle V_{TH} $$) in series with a Thevenin Resistance ($$\displaystyle R_{TH} $$).

  • $$\displaystyle V_{TH} $$ = Open-circuit voltage across terminals.

  • $$\displaystyle R_{TH} $$ = Resistance seen from terminals with all independent sources killed (voltage sources shorted, current sources opened).

5.0 Practical Circuit Building and Troubleshooting

5.1 Reading Schematics

• Identify inputs, outputs, power supply connections (Vcc, GND).

• Follow the signal flow from left to right (conventional).

• Recognize standard symbols for all components.

5.2 Breadboard Best Practices

• Bus Strips: Long rows on sides marked + (red) and - (blue/black) for power/ground. Connect all points in a strip together.

• Terminal Strips: Central area with rows of 5 connected holes (A-E, F-J, etc.). Rows are connected, columns are isolated.

• Avoid Loose Wires: Ensure component leads and jumper wires are fully inserted. Use short, direct connections.

5.3 Systematic Troubleshooting Methodology

1. Visual Inspection: Check for cold solder joints, incorrect component values (read color codes!), reversed polarity (especially electrolytic caps, diodes, ICs), loose wires, solder bridges (shorts).

2. Power Check: With power ON but circuit unloaded, measure power supply voltages at the breadboard bus strips and at key Vcc/GND pins of ICs.

3. Signal Tracing: Start at the input. Use oscilloscope to probe signal at each stage's output. Use DMM for DC bias voltages at transistor/op-amp pins.

4. Divide and Conquer: Isolate sections. If an amplifier fails, test the input stage separately from the output stage. Disconnect downstream loads.

5.5 Common Faults

• Open Circuit: Broken wire, loose connection, blown fuse, component lead not inserted.

• Short Circuit: Solder bridge, wire touching two nodes, component inserted incorrectly (e.g., resistor across power rails).

• Incorrect Component Value: Wrong resistor/capacitor installed.

• Miswired: Schematic not followed correctly, especially power and ground connections.

[!TIP] Golden Rule: "Power off, inspect, then power on and measure." Never probe a live circuit with an ohmmeter. Always verify power supply voltages at the board before checking signal paths.

6.0 Measurement Uncertainty and Error Analysis (Introductory)

6.1 Key Definitions

• Accuracy: Closeness of measurement to the true value.

• Precision: Closeness of repeated measurements to each other (repeatability).

• Resolution: Smallest change in input that produces a detectable change in output (e.g., 0.1 mV on a 3.5-digit DMM).

6.2 Instrument Specifications

• Read as: ±(a% of reading + b digits).

• Example: A DMM reads 10.00 V with spec ±(0.5% + 2 digits). Last digit = 0.01 V.

  • % Error = 0.5% of 10.00 V = 0.05 V.

  • Digit Error = 2 × 0.01 V = 0.02 V.

  • Total Uncertainty = ±(0.05 V + 0.02 V) = ±0.07 V.

  • True value lies between 9.93 V and 10.07 V.

6.3 Types of Errors

• Systematic (Bias): Consistent, repeatable errors. Causes: Instrument calibration error, parallax (reading analog scale from angle), zero error. Can be corrected if known.

• Random (Noise): Unpredictable fluctuations. Causes: Electrical noise, reading variation. Reduced by taking multiple readings and averaging.

6.4 Basic Uncertainty Calculation

For a single instrument reading: Uncertainty = ±(Specified % error + Digit error).

When combining measurements (e.g., calculating power $$\displaystyle P=VI $$), combine relative uncertainties:

$$\frac{\Delta P}{P} = \frac{\Delta V}{V} + \frac{\Delta I}{I}$$

where $\Delta V, \Delta I$ are the absolute uncertainties of V and I.

6.5 Significant Figures

• The result of a measurement should be reported with the same number of significant figures as the least precise instrument used.

• Example: Measuring 10.0 V (3 sig fig) and 0.253 A (3 sig fig), power = 2.53 W (3 sig fig), not 2.530 W.

[!TIP] Exam Trap: Always calculate the absolute uncertainty (in the same units as the measurement) from the instrument spec. Never report a result like "10.00 ± 0.07 V" as "10.07 V" or "9.93 V". The uncertainty defines the range of true value.