Electronic Instrumentation Lab (EX-605) - Unit 4 Short Notes

UNIT 4: ADVANCED MEASUREMENT TECHNIQUES & DIGITAL INSTRUMENTATION

4.1 Advanced Oscilloscope Techniques

4.1.1 Triggering Modes (Pulse, Video, Pattern, Setup & Hold)

• Purpose: Stabilize complex or repetitive waveforms for clear analysis.

• Key Modes:

  • Pulse Trigger: Triggers on pulse width violations (too narrow/wide). Essential for debugging timing circuits.

  • Video Trigger: Synchronizes to standard video line/frame rates (e.g., NTSC, PAL) for analyzing video signals.

  • Pattern Trigger (Logic Trigger): Triggers when a specific combination of logic levels (on multiple channels) is met. Used for digital bus events.

  • Setup & Hold Trigger: Triggers on violations of setup time (data must be stable before clock edge) or hold time (data must be stable after clock edge) in digital systems.

• [!TIP] Exam Focus: Be able to select the correct trigger for a given problem (e.g., use Setup & Hold for debugging a flip-flop, Pulse for catching glitches).

4.1.2 Advanced Cursor Measurements & Automated Parameter Measurements

• Manual Cursors: Measure time, voltage, or frequency between two on-screen markers.

• Automated Measurements: Built-in algorithms measure parameters like:

  • Vertical: Vpp, Vrms, Max, Min, Amplitude.

  • Horizontal: Frequency, Period, Rise Time, Fall Time, Duty Cycle.

  • Mixed: Phase, Delay.

• [!CAUTION] Automated measurements assume a stable, noise-free waveform. For noisy signals, use cursors or filtering.

4.1.3 Using Oscilloscope Math Functions (FFT, Addition, Subtraction, Multiplication)

• FFT (Fast Fourier Transform): Converts time-domain signal to frequency-domain spectrum. Key for analyzing harmonic content, noise, and modulation.

  • Controls: Span (total frequency width), Center Frequency, Resolution Bandwidth (RBW).

• Basic Math (A+B, A-B, A×B):

  • A-B: Useful for differential measurements (e.g., subtract two single-ended probes).

  • A×B: Used for power analysis (instantaneous power = V × I).

• [!TIP] FFT magnitude is often in dBV or dBm. Use proper scaling and windowing (e.g., Hanning) to reduce spectral leakage.

4.1.4 Single-shot Capture & Pre-triggering for Transient Analysis

• Single-shot (Single Sequence): Captures one triggered event then stops. Critical for non-repetitive transients (e.g., power-up glitch, ESD event).

• Pre-triggering: Captures signal data before the trigger event. Essential to see what caused a glitch.

  • Set Trigger Position to 10-20% for pre-trigger data.

• [!CAUTION] Ensure sufficient record length (samples) to capture the entire transient of interest.

4.1.5 Protocol Decoding (I2C, SPI, UART) using Mixed-Signal Oscilloscopes (MSOs)

• MSO Advantage: Combines analog channels (2-4) with digital logic channels (8-16).

• Decoding Process:

  1. Connect logic probes to digital bus lines (e.g., SCL, SDA for I2C).

  2. Set correct threshold voltage for logic high/low.

  3. Select protocol (I2C, SPI, UART) and configure parameters (baud rate for UART, clock polarity for SPI).

  4. Oscilloscope overlays decoded bytes/words on the waveform.

• [!TIP] Use the digital channels for clean clock and data lines. Correlate decoded data with analog signals (e.g., power rail noise) on the same timebase.

4.2 Spectrum & Signal Analysis

4.2.1 Principles of Spectrum Analyzers (swept-tuned vs. FFT-based)

• Swept-Tuned (Traditional): A variable local oscillator sweeps across frequencies; a narrow IF filter (RBW) measures power at each point. Good for high dynamic range and spurious analysis.

• FFT-Based (Real-Time): Digitizes a wide bandwidth, then uses FFT. Enables real-time spectrum analysis and captures transient signals. Limited by ADC sampling rate and processing speed.

• [!TIP] Modern analyzers often blend both: use a wide FFT for overview, then a swept-tuned zoom for detail.

4.2.2 Key Parameters: Resolution Bandwidth (RBW), Video Bandwidth (VBW), Span, Sweep Time

• RBW (Resolution Bandwidth): The bandwidth of the IF filter. Narrower RBW = better frequency resolution, slower sweep, higher noise floor.

• VBW (Video Bandwidth): Post-detection low-pass filter. Smoothes the displayed trace. Narrower VBW = less noise, slower response to signal changes.

• Span: Total frequency range displayed (e.g., 0-100 MHz).

• Sweep Time: Time to complete one frequency sweep. Inversely proportional to RBW² and Span.

$$ \text{Sweep Time} \propto \frac{\text{Span}}{\text{RBW}^2} $$

• [!CAUTION] Setting RBW too wide merges close spectral lines; too narrow misses fast transient signals.

4.2.3 Measuring Harmonic Distortion, Intermodulation, and Signal-to-Noise Ratio (SNR)

• Total Harmonic Distortion (THD): Ratio of RMS voltage of all harmonics to RMS fundamental.

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

*   Use spectrum analyzer's **harmonic marker** or **THD measurement** function.

• Intermodulation Distortion (IMD): Spurious signals at frequencies like $$\displaystyle 2f_1 - f_2 $$, $$\displaystyle 2f_2 - f_1 $$ from non-linear mixing.

• SNR: Ratio of signal power to noise power (in dB).

$$ \text{SNR (dB)} = 10 \log_{10} \left( \frac{P_{\text{signal}}}{P_{\text{noise}}} \right) $$

*   Measure noise in a "quiet" band adjacent to the signal.

• [!TIP] Ensure RBW is consistent when comparing signal and noise power for accurate SNR.

4.2.4 Demodulation of AM/FM Signals

• AM Demodulation: Use the analyzer's AM demod mode to listen to the audio or display the modulation envelope.

• FM Demodulation: Displays frequency deviation vs. time. Useful for measuring modulation index and detecting spurious modulation.

• [!CAUTION] Demodulation requires the carrier to be within the span and properly tuned.

4.2.5 Introduction to Real-Time Spectrum Analyzers for capturing rare events

• Concept: Digitizes a wide bandwidth continuously with no gaps (using high-speed ADC and processing). Can capture intermittent or short-duration signals (e.g., radar pulses, frequency hops) that a swept analyzer might miss.

• Key Spec: POI (Probability of Intercept) – the minimum signal duration and amplitude that can be detected.

• [!TIP] Use max-hold or average traces to visualize rare events over time.

4.3 Digital Logic & Mixed-Signal Analysis

4.3.1 Introduction to Logic Analyzers: Timing vs. State Analysis

• Timing Analysis: Displays logic levels vs. continuous time. Used for measuring timing relationships (setup/hold, pulse width, clock skew). Sample clock is asynchronous to the device under test (DUT).

• State Analysis: Displays logic states at specific clock edges. Sample clock is synchronous to the DUT's clock. Used for analyzing data flow and state machine transitions.

• [!TIP] Use timing mode for debugging speed issues; use state mode for protocol analysis.

4.3.2 Setup for Capturing Digital Buses (Setup/Hold Time violations)

1. Connect logic probes to all relevant signals (clock, data, enable).

2. Set threshold voltage correctly (usually 1.8V, 3.3V, or 5.0V TTL/CMOS).

3. Use a timing display.

4. Trigger on the clock edge.

5. Zoom in on the data transition relative to the clock edge. Measure the time from clock edge to data stable (setup) and from data stable after clock edge (hold).

• [!CAUTION] Probe capacitance and loading can distort fast edges and cause setup/hold violations. Use low-capacitance probes (<2pF).

4.3.3 Correlating Digital Bus Activity with Analog Signals (using MSOs)

• Core Advantage of MSO: Time-correlate digital bus events (e.g., SPI transaction) with analog phenomena (e.g., power supply droop, sensor output).

• Method:

  1. Connect analog channels to power/analog signals.

  2. Connect digital channels to bus lines.

  3. Use a common trigger (e.g., a specific digital pattern or analog edge).

  4. Observe cause-and-effect on the same timebase.

• [!TIP] Use color-coded buses and protocol decoding to quickly identify which transaction caused an analog event.

4.3.4 Protocol Analysis for Common Communication Buses (SPI, I2C, UART, CAN)

• SPI (Serial Peripheral Interface): Synchronous, full-duplex. Master drives clock (SCK) and chip-select (CS). Decode by identifying CS active, then clock/data edges.

• I2C (Inter-Integrated Circuit): Synchronous, multi-master, 2-wire (SCL, SDA). Decode by recognizing START/STOP conditions and ACK/NACK bits.

• UART (Universal Asynchronous Receiver/Transmitter): Asynchronous. Decode by setting correct baud rate, identifying start bit, data bits, stop bit(s).

• CAN (Controller Area Network): Used in automotive/industrial. Differential signaling. Decode by identifying arbitration field and control field.

• [!TIP] Always verify the protocol settings (baud rate, clock polarity/phase) match the DUT. Incorrect settings cause garbled decode.

4.4 Data Acquisition (DAQ) Systems

4.4.1 DAQ Hardware Overview: ADC Types (SAR, Delta-Sigma), Sampling Rate, Resolution, Input Ranges

• ADC Types:

  • SAR (Successive Approximation Register): Medium speed (up to ~10 MS/s), medium resolution (12-18 bit). Good for general-purpose DAQ.

  • Delta-Sigma: High resolution (24+ bit), low-to-medium speed (<1 MS/s). Uses oversampling and noise shaping. Excellent for low-frequency, high-precision measurements (e.g., temperature, strain).

• Sampling Rate ($$\displaystyle f_s $$): Samples per second. Must satisfy Nyquist Criterion: $$\displaystyle f_s > 2 f_{\text{max}} $$ to avoid aliasing.

• Resolution: Number of bits (n). Smallest voltage step = $$\displaystyle \frac{V_{\text{range}}}{2^n} $$.

• Input Ranges: Selectable voltage spans (e.g., ±10V, ±1V). Use the smallest range that fits the signal for best resolution.

4.4.2 Single-Ended vs. Differential Measurements, Grounding, and Isolation Concepts

• Single-Ended: Signal measured relative to a common ground (GND). Simple, but susceptible to ground loops and common-mode noise.

• Differential: Signal measured as the voltage between two conductors (+, -). Rejects common-mode noise. Essential for low-level signals in noisy environments.

• Grounding: Ensure a single-point ground reference to avoid ground loops. Use differential or isolated DAQ for floating measurements.

• Isolation: Breaks the conductive path between DAQ and DUT (using opto-couplers or transformers). Protects against high-voltage transients and breaks ground loops. Specified in Volts RMS or DC (e.g., 1000 Vrms).

4.4.3 Configuring a DAQ System: Channel Selection, Sampling Clock, Triggering

• Channel Selection: Choose analog input (AI) channels, configure as single-ended or differential.

• Sampling Clock:

  • Internal: DAQ generates clock.

  • External: Clock provided by DUT (for synchronous sampling).

  • Rate: Set based on signal bandwidth ($$\displaystyle f_s \geq 10 \times f_{\text{signal}} $$ for good waveform reconstruction).

• Triggering: Start acquisition on a condition.

  • Analog Edge Trigger: On AI channel crossing a threshold.

  • Digital Edge Trigger: On digital line.

  • Software Trigger: Start on command.

4.4.4 Introduction to DAQ Software & Programming (e.g., LabVIEW, Python with NI-DAQmx)

• Typical Flow:

  1. Configure: Create task, add channels, set sample rate, range, trigger.

  2. Start: Arm the task.

  3. Read: Acquire data into buffer (synchronous or asynchronous).

  4. Stop/Clear: Release resources.

• Python Example (nidaqmx):

  
  import nidaqmx
  
  with nidaqmx.Task() as task:
  
      task.ai_channels.add_ai_voltage_chan("Dev1/ai0", min_val=-10, max_val=10)
  
      task.timing.cfg_samp_clk_timing(rate=1000, sample_mode=nidaqmx.constants.AcquisitionType.FINITE, samps_per_chan=100)
  
      data = task.read(number_of_samples_per_channel=100)
  
  

4.4.5 Practical Considerations: Aliasing, Anti-aliasing Filters, Noise

• Aliasing: High-frequency components fold back into lower frequencies if $$\displaystyle f_s < 2f_{\text{max}} $$. Prevention: Use an anti-aliasing filter (low-pass) before the ADC, with cutoff $$\displaystyle f_c < f_s/2 $$.

• Noise:

  • Quantization Noise: Inherent to ADC resolution.

  • Thermal/EMI Noise: From wiring, environment. Use shielded cables, proper grounding, and averaging.

• [!TIP] Rule of Thumb: Set $$\displaystyle f_s \geq 10 \times $$ highest frequency component of interest to allow for anti-aliasing filter roll-off.

4.5 Frequency & Time Interval Measurements

4.5.1 Using Frequency Counters & Period Counters

• Frequency Counter: Measures cycles ($N$) in a known gate time ($$\displaystyle T_g $$).

$$ f = \frac{N}{T_g} $$

*   **Resolution** = $$\displaystyle 1/T_g $$. Longer gate = better resolution, slower update.

• Period Counter: Measures time of one cycle ($T$) by counting internal clock cycles during one input cycle.

$$ T = \frac{N_{\text{clock}}}{f_{\text{clock}}} $$

*   Better for **low frequencies** where frequency counter resolution is poor.

• [!CAUTION] For period measurement, ensure the input signal is clean; jitter causes large errors.

4.5.2 Measuring Duty Cycle, Pulse Width, and Rise/Fall Times

• Duty Cycle (D): Ratio of pulse width ($$\displaystyle t_p $$) to period ($T$).

$$ D = \frac{t_p}{T} \times 100\% $$

• Pulse Width: Time between 50% (or other threshold) points on leading and trailing edges.

• Rise/Fall Time: Time for signal to transition from 10% to 90% (or 20%-80%) of amplitude. Measures edge speed.

• [!TIP] Use automated measurements on an oscilloscope for these. Ensure the trigger is on the edge of interest.

4.5.3 Time Interval Analyzers (TIA) for precise phase and jitter measurement

• Function: Measures time difference between two events (e.g., clock and data edge) with sub-nanosecond precision.

• Applications: Phase shift measurement, jitter (timing variation) analysis, skew between signals.

• Method: Uses a high-resolution time-to-digital converter (TDC). Often requires conditioning (e.g., converting analog edges to digital with comparators).

• [!TIP] For jitter, measure many intervals and compute RMS or peak-to-peak value.

4.5.4 Introduction to Phase-Locked Loops (PLLs) for frequency synthesis and measurement

• Basic PLL: Feedback system with Phase Detector (PD), Loop Filter (LF), Voltage-Controlled Oscillator (VCO).

• In Measurement: A PLL can track an input frequency. The VCO control voltage is proportional to the frequency error. Can be used in frequency synthesizers and clock recovery circuits.

• Key Specs: Lock range, capture range, loop bandwidth (determines response speed vs. noise rejection).

• [!CAUTION] PLLs can lose lock on highly noisy or frequency-sweeping signals.

4.6 Power Electronics & Power Quality Measurements

4.6.1 Measuring AC Power: True RMS Voltage/Current, Real/Reactive/Apparent Power, Power Factor

• True RMS: Accurate measurement for non-sinusoidal waveforms. Calculates $$\displaystyle \sqrt{\frac{1}{T}\int_0^T v(t)^2 dt} $$.

• Power Types (for periodic waveforms):

  • Real Power (P): Average power consumed (Watts).

$$ P = \frac{1}{T} \int_0^T v(t) i(t) dt $$

*   **Reactive Power (Q):** Power oscillating between source and reactive elements (VAr).

*   **Apparent Power (S):** Product of RMS voltage and RMS current (VA).

$$ S = V_{\text{rms}} \times I_{\text{rms}} $$

*   **Power Factor (PF):** Ratio of real to apparent power.

$$ \text{PF} = \frac{P}{S} = \cos \phi \quad (\text{for sinusoidal}) $$

• [!TIP] Use a power analyzer or oscilloscope with math (V×I) and average measurement for real power. Do not use average-responding meters on non-sinusoidal waveforms.

4.6.2 Using Power Analyzers & Current Probes (AC/DC, Hall-effect, Rogowski)

• Power Analyzer: Dedicated instrument for accurate 3-phase power, harmonics, efficiency. Uses voltage and current inputs with high isolation.

• Current Probes:

  • AC Only (CT): Clamp-on, for high AC currents. Output proportional to AC current.

  • Hall-Effect: Measures AC and DC. Lower bandwidth, needs power.

  • Rogowski Coil: Flexible, high AC current, no magnetic saturation. Output is derivative of current ($dI/dt$), requires integration.

• [!CAUTION] Rogowski output must be integrated (by probe or instrument) to get current waveform. Check probe bandwidth and saturation current.

4.6.3 Basic Power Quality Parameters: Harmonics, THD, Flicker

• Harmonics: Integer multiples of fundamental frequency (e.g., 5th harmonic = 250 Hz on 50 Hz system). Caused by non-linear loads (rectifiers, LED drivers).

• THD (Total Harmonic Distortion) of Current/Voltage:

$$ \text{THD} = \frac{\sqrt{\sum_{n=2}^{\infty} (X_n)^2}}{X_1} \times 100\% $$

where $$\displaystyle X_n $$ is RMS value of nth harmonic, $$\displaystyle X_1 $$ is fundamental.

• Flicker: Perceived light intensity fluctuation from voltage fluctuations. Measured by Pst (short-term) and Plt (long-term) values per IEC standards.

• [!TIP] Use a power quality analyzer or spectrum analyzer with harmonic measurement functions. THD can be high even if individual harmonics are within limits.

4.6.4 Safe Measurement Practices for High-Voltage/High-Current Circuits

• Isolation: Use instruments/probes with high isolation voltage (e.g., 1000 Vrms). Never connect a non-isolated oscilloscope directly to mains.

• Probes: Use high-voltage probes (attenuated, e.g., 100:1) for >100V signals. Use current probes instead of breaking the circuit.

• Personal Safety: De-energize and discharge circuits before connecting. Use one hand rule. Be aware of stored energy (capacitors, inductors).

• [!CAUTION] Grounding: Ensure the instrument's ground is at a safe potential. Using an isolated oscilloscope or differential probe eliminates ground reference issues.

4.7 RF & Microwave Measurement Fundamentals (Introductory)

4.7.1 Characterizing 2-Port Devices: S-Parameters (S11, S21), VSWR, Reflection Coefficient

• S-Parameters (Scattering): Describe RF/microwave network behavior at high frequencies where impedance matching is critical.

  • S11: Input reflection coefficient. Measures impedance match at port 1 (return loss).

  • S21: Forward transmission gain. Measures signal from port 1 to port 2.

• Reflection Coefficient (Γ): Ratio of reflected to incident voltage wave.

$$ \Gamma = \frac{Z_L - Z_0}{Z_L + Z_0} $$

where $$\displaystyle Z_L $$ = load impedance, $$\displaystyle Z_0 $$ = characteristic impedance (usually 50Ω).

• VSWR (Voltage Standing Wave Ratio): Measure of impedance mismatch.

$$ \text{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|} $$

VSWR = 1:1 (perfect match), ∞ (open/short).

• [!TIP] Return Loss (RL) = $$\displaystyle -20 \log_{10} |S11| $$ (dB). Higher RL (more negative dB) = better match.

4.7.2 Using a Network Analyzer (VNA) for basic gain and return loss measurement

• Vector Network Analyzer (VNA): Measures both magnitude and phase of S-parameters.

• Basic Measurement:

  1. Perform calibration (SOLT: Short-Open-Load-Through) at the reference plane.

  2. Connect DUT.

  3. Display S11 (log magnitude) for return loss. Display S21 for gain/loss.

• [!CAUTION] Calibration is critical. Errors in calibration lead to inaccurate results. Use proper test cables and connectors.

4.7.3 Introduction to Spectrum Measurements for RF Signals (Carrier Frequency, Modulation Depth)

• Spectrum Analyzer Use:

  • Carrier Frequency: Read the peak frequency.

  • Modulation Depth (AM): Measure sideband levels relative to carrier.

$$ m = \frac{V_{\text{max}} - V_{\text{min}}}{V_{\text{max}} + V_{\text{min}}} \times 100\% $$

    In spectrum: $$\displaystyle m \approx \frac{P_{\text{sideband}}}{P_{\text{carrier}}} \times 200\% $$ (for 100% modulation).

*   **FM Deviation:** Measure frequency span between sidebands at specific modulation frequency.

• [!TIP] Use narrow RBW to resolve close-in sidebands for modulation analysis.

4.8 Instrument Interfacing & Automation

4.8.1 Standard Instrument Communication Interfaces: GPIB (IEEE-488), USB-TMC, LAN (LXI), Serial (RS-232)

• GPIB (General Purpose Interface Bus): Parallel, 8-bit. Up to 20 devices. Legacy but robust.

• USB-TMC (Test & Measurement Class): Plug-and-play, high speed. Uses standard USB cables.

• LAN (LXI - LAN eXtensions for Instrumentation): Ethernet-based. Allows remote control over network. Scalable.

• RS-232: Serial, point-to-point, slow. Mostly legacy.

• [!TIP] LXI is modern standard for modular, distributed test systems. USB-TMC is common for single-instrument PC control.

4.8.2 Concept of Remote Instrument Control & Test Automation

• Goal: Eliminate manual setup, reduce human error, increase throughput.

• Architecture: PC (Controller) → Interface (GPIB/USB/LAN) → Instruments (Devices).

• Software: Uses VISA (Virtual Instrument Software Architecture) library as an abstraction layer for communication.

• Typical Automated Sequence:

  1. Reset instrument.

  2. Configure settings (frequency, range, trigger).

  3. Arm/trigger acquisition.

  4. Read data.

  5. Process/store data.

  6. Repeat for next DUT/condition.

4.8.3 Simple Scripting for Instrument Control (e.g., SCPI commands)

• SCPI (Standard Commands for Programmable Instruments): Text-based command standard (e.g., :MEAS:VOLT:DC?).

• Example (Python with PyVISA):

  
  import pyvisa
  
  rm = pyvisa.ResourceManager()
  
  inst = rm.open_resource('USB0::0x2A8D::0x0101::MY12345678::INSTR')  # VISA address
  
  inst.write('*RST')  # Reset
  
  inst.write('FREQ 1e6')  # Set frequency to 1 MHz
  
  voltage = inst.query('MEAS:VOLT:DC?')  # Measure DC voltage
  
  print(voltage)
  
  

• [!TIP] Always check the instrument's programmer's manual for exact SCPI syntax. Use *IDN? to verify connection.

4.8.4 Building a Simple Automated Test Sequence

1. Define Test Plan: What parameters? Pass/fail criteria?

2. Write Script: Initialize instruments, loop through test steps, capture data, compare to limits, log result.

3. Error Handling: Include try/except blocks, timeouts, instrument reset on failure.

4. Reporting: Generate simple text/CSV report or use a database.

• [!CAUTION] Timing: Allow sufficient settling time after changing settings before measuring. Use inst.query('*OPC?') to wait for operation complete.

4.9 Troubleshooting & Measurement System Design

4.9.1 Systematic Troubleshooting Methodology

1. Define Problem: What is the expected vs. actual behavior?

2. Isolate Subsystem: Divide system into blocks (source, DUT, measurement). Test each block independently.

3. Check Basics: Power, connections, probe compensation, ground.

4. Signal Injection/Probing: Start from known good point (e.g., oscillator output) and move toward fault.

5. Component/Connection Check: Look for cold solder joints, damaged components, loose connectors.

6. Verify Instrument Setup: Bandwidth, coupling, scale, trigger. This is the most common lab error.

4.9.2 Identifying Ground Loops, Noise Coupling, and Loading Effects

• Ground Loops: Multiple paths to ground create unwanted current flow, inducing 50/60 Hz hum.

  • Fix: Use isolated instruments/probes, single-point grounding, or differential measurements.

• Noise Coupling: Capacitive/inductive coupling from nearby switching supplies, digital lines.

  • Fix: Use shielded cables, twisted pairs for differential signals, increase physical separation, add ferrites.

• Loading Effect: Probe or instrument input impedance (typically 1MΩ || 10-20pF) distorts the circuit, especially high-impedance nodes.

  • Fix: Use 10x probes (10MΩ || ~1pF), active probes for very high impedance.

4.9.3 Selecting the Correct Instrument and Probe for a Given Measurement Task

4.9.4 Interpreting Complex Waveforms and Spectra for Fault Diagnosis

• Oscilloscope:

  • Ringing: Inductance/capacitance mismatch, poor grounding.

  • Glitches: Setup/hold violation, bouncing contacts, noise.

  • Slow Edges: Bandwidth limitation (probe or scope), capacitive loading.

• Spectrum Analyzer:

  • Spurious Tones: Oscillator harmonics, switching supply noise, intermodulation.

  • Broadband Noise: Poor regulation, EMI, thermal noise.

  • Narrowband Peaks: Unintended oscillations, coupled clock signals.

• [!TIP] Correlate Domains: A glitch in time domain often appears as broadband noise in frequency domain. A stable tone in frequency domain is a sine wave in time.

4.9.5 Documenting Measurements: Capturing Screens, Annotating Data, Reporting Results

• Capture: Save oscilloscope/spectrum screen images (with setup information—scale, coupling, probe attenuation).

• Annotate: Use on-screen cursors and text to mark key points (e.g., "overshoot", "setup violation").

• Data Export: Save numerical data (CSV) from automated measurements for further analysis in Excel/Python.

• Report Structure:

  1. Objective

  2. Equipment & Setup (include probe type/attenuation)

  3. Observations (screenshots + description)

  4. Analysis (interpretation, calculations)

  5. Conclusion / Fault Identified

• [!CAUTION] Never submit a screenshot without probe attenuation and vertical scale clearly visible. This is a common marking deduction.