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EC-306 · EMI Lab/Quick Revision Short Notes

EMI Lab (EC-306) - Unit 2 Short Notes

UNIT 2 – EMI/EMC MEASUREMENTS & TESTING

2.1. Fundamentals of EMI/EMC Measurements

  • Core Definitions:

    • EMI (Electromagnetic Interference): The undesired electromagnetic energy that degrades the performance of a device/system.

    • EMS (Electromagnetic Susceptibility): The inability of a device/system to perform without degradation in the presence of EMI.

    • EMC (Electromagnetic Compatibility): The ability of a device/system to function satisfactorily in its electromagnetic environment without causing EMI (emission) and without being affected by EMI (immunity). EMC = Emission + Immunity.

  • Emission Types:

    • Conducted Emissions: Noise propagated along power/signal lines (IEC/CISPR 14-1, FCC Part 15).

    • Radiated Emissions: Noise propagated through space as EM waves (CISPR 11/32, FCC Part 15).

  • Immunity/Susceptibility: Tests where the EUT is exposed to external RF fields, transients, etc. (IEC 61000-4 series).

  • Measurement Units & Scales:

    | Quantity | Common Unit | Description | | :--- | :--- | :--- | | Voltage (RF) | dBµV | 20 log₁₀(V / 1µV). Most common for radiated & conducted emissions. | | Power | dBm | 10 log₁₀(P / 1mW). Used in RF generator/amplifier specs. | | Field Strength | dBµV/m | 20 log₁₀(E / 1µV/m). Standard for radiated emissions limits. | | Current | dBµA | 20 log₁₀(I / 1µA). Used with current probes. |

    • Detectors (CISPR 16-1-1):

      • Quasi-Peak (QP): Simulates human annoyance to repetitive impulsive noise. Most common for compliance.

      • Average (AV): Measures average power. Used for broadband noise (e.g., digital circuits).

      • RMS: True root-mean-square. Required for complex or non-periodic signals.

      [!TIP] Exam Focus: Know the detector for the standard (e.g., CISPR 11 uses QP & AV). QP readings are higher than AV for the same impulsive signal.

  • Frequency Bands: LF (30-300 kHz), MF (300 kHz-3 MHz), HF (3-30 MHz), VHF (30-300 MHz), UHF (300 MHz-3 GHz). Standards define specific sub-bands and limits.

2.2. Test Equipment & Setup

  • Spectrum Analyzer (SA): General-purpose tool. Key settings:

    • Resolution Bandwidth (RBW): Filter bandwidth. Narrow RBW = better frequency resolution, slower sweep.

    • Video Bandwidth (VBW): Smooths display. VBW ≤ RBW for stable trace.

    • Attenuation: Prevents overload on input mixer.

    • Sweep Time: Inversely proportional to RBW².

    • Types: Swept-Tuned (TSA) vs. FFT (real-time).

  • EMI Receiver (CISPR 16-1-1 Compliant):

    • Key Difference: Has preselection (pre-filter) to prevent intermodulation/overload. Mandatory for formal compliance testing. SAs often lack this.

    • Must have: Defined IF bandwidths (e.g., 200 Hz, 9 kHz, 120 kHz), calibrated detectors (QP, AV, Peak).

  • Antennas:

    • Types & Use: Biconical (30-300 MHz), Log-Periodic (200 MHz-1 GHz), Horn (1-18 GHz), Loop (magnetic field, LF).

    • Antenna Factor (AF): Converts measured voltage (dBµV) to field strength (dBµV/m).

$$E \text{(dBµV/m)} = V_{meas} \text{(dBµV)} + AF \text{(dB/m)} + C_L \text{(dB)}$$

    Where $$\displaystyle C_L $$ is cable loss correction.

*   **Polarization:** Horizontal (H) and Vertical (V) scans required. Antenna height varied (1-4m for radiated).

  • LISN (Line Impedance Stabilization Network):

    • Purpose: Provides a standardized, low-impedance RF noise source to the measuring instrument and a high-impedance RF noise source to the EUT's power line. Isolates external noise.

    • Types: V-type (for 50 Ω/50 µH, 9 kHz-30 MHz, CISPR 11/22) and Ω-type (for 50 Ω/50 µH + 1 µF, 150 kHz-30 MHz, MIL-STD).

    • Placement: Between mains supply and EUT. Measurement port connects to SA/receiver.

  • Current Probes & Clamps: Measure common-mode (CM) currents on cables (e.g., FCC Part 15B). Clamp around cable, output to SA.

  • Calibration: Use pulse limiter (protects SA), signal generator (check SA linearity), and reference antenna (check AF).

2.3. Conducted Emissions Testing

  • Test Setup (CISPR 11/22/32, FCC Part 15):

    
Mains Supply --> LISN --> EUT

               |         |

| --> Other I/O cables (terminated with CDN if needed)

               |

               --> Measurement Port --> SA/Receiver

```

*   **Ground Plane:** 1m x 1m conductive plane, EUT placed on it, bonded to LISN ground.

  • Procedure:

    1. Connect EUT to LISN.

    2. Set SA: RBW = 9 kHz (or 200 Hz for <150 kHz), detector = QP (and AV), span = 9 kHz - 30 MHz.

    3. Scan frequency, identify peaks.

    4. Maximize peak by adjusting LISN's "artificial hand" (capacitive coupling electrode).

  • Common Noise Sources: Switch-mode power supplies (SMPS), clock oscillators, motor drivers, digital logic.

  • Mitigation:

    • Input Filter: X-capacitors (line-to-line), Y-capacitors (line-to-ground), common-mode chokes.

    • Ferrite Clamps: On cables to suppress high-frequency CM currents.

2.4. Radiated Emissions Testing

  • Test Environments:

    • OATS (Open Area Test Site): Minimum reflections, 10m/3m separation.

    • SAC (Semi-Anechoic Chamber): Absorbing cones on walls/ceiling, ground plane. Most common for formal testing.

    • GTEM Cell: For smaller EUTs, full frequency range in a controlled transmission line.

  • Measurement Procedure (OATS/SAC):

    1. Antenna Height Scan: 1-4 meters (for 3m/10m distance).

    2. Polarization Scan: Horizontal & Vertical.

    3. Turntable Rotation: 0-360° in 10°-15° steps to maximize emission.

    4. Final Measurement: At max height/polarization/angle, use QP detector, narrow RBW (e.g., 120 kHz for CISPR).

  • Common Radiated Sources:

    • Cables as Antennas: Unshielded, long I/O cables.

    • Enclosure Leakage: Slots, vents, seams > λ/20.

    • PCB Traces: High-speed clocks, unshielded loops.

  • Mitigation:

    • Shielding: Metal enclosures, EMI gaskets on seams.

    • Cable Management: Use shielded cables, terminate shields 360°, twist differential pairs.

    • Grounding: Single-point, low-impedance ground bonds.

    • Filtering: On all I/O lines.

2.5. Radiated Immunity Testing (IEC 61000-4-3)

  • Setup: Signal Generator → Power Amplifier → Transmit Antenna → Uniform Field Area (UFA).

    • Field Probe: Monitors actual field strength at EUT location.

    • EUT Monitoring: Must be operational and monitored for performance degradation.

  • Test Parameters:

    • Frequency Range: 80 MHz – 6 GHz (or 2.4/5 GHz bands for wireless).

    • Modulation: 80% AM at 1 kHz (simulates amplitude-modulated signals like radio).

    • Field Strength: Typically 10 V/m (80-1000 MHz), 3 V/m (1-6 GHz) for industrial. Higher for military/aerospace.

    • Sweep Rate: ≤ 1.5 x 10⁻³ decades/s (logarithmic sweep).

    • Dwell Time: Time at each frequency to assess EUT (e.g., 2-3 seconds).

  • Performance Criteria (IEC 61000-4):

    • A: Normal performance within specification limits.

    • B: Temporary degradation or loss of function that self-recovers.

    • C: Temporary degradation/loss requiring operator intervention or system reset.

    • D: Irreversible damage or data loss.

  • Susceptibility Sources: Poor shield seams, long unshielded cables, inadequate filtering on I/O.

2.6. Specialized Measurements & Advanced Topics

  • Time-Domain Measurements: Use oscilloscope with FFT function to view transient noise in frequency domain. Useful for EFT/Burst analysis.

  • Near-Field Scanning:

    • E-field Probe (Voltage): Scans over PCB traces/enclosure seams to locate electric field sources.

    • H-field Probe (Current): Scans to locate magnetic field sources (loops, inductors).

    • Application: Debugging, identifying noise hotspots before full radiated test.

  • Transient Tests (IEC 61000-4-4 EFT/Burst, -5 Surge):

    • EFT/Burst: Fast transients (5 ns, 50 ns) coupled via CDN or clamp. Tests I/O and power ports.

    • Surge: High-energy transients (1.2/50 µs, 8/20 µs) from lightning/inductive switching. Coupled via CDN or antenna.

  • Power Quality & Harmonics: Use power analyzer or SA with harmonic marker functions to measure mains harmonics (IEC 61000-3-2) and voltage fluctuations.

2.7. Standards, Regulations & Reporting

  • Key Standards:

    • CISPR (International): CISPR 11 (ISM equipment), CISPR 22/32 (IT equipment), CISPR 25 (vehicles).

    • FCC (USA): FCC Part 15 (unintentional radiators).

    • IEC 61000 Series (Immunity): -4-3 (RF field), -4-4 (EFT), -4-5 (Surge), -4-6 (Conducted RF).

    • MIL-STD-461 (US Military): Comprehensive limits for platforms (ground, ship, aircraft, space).

  • Test Report Essentials:

    1. EUT identification & description.

    2. Test setup photos/diagrams.

    3. List of test equipment (with calibration due dates).

    4. Measurement data tables (frequency, level, limit, margin, detector, antenna height/pol).

    5. Final statement of compliance/non-compliance.

  • Pre-Compliance vs. Formal Compliance:

    • Pre-Compliance: In-house testing with less stringent equipment (e.g., SA instead of receiver), relaxed environment. Goal: find and fix problems early.

    • Formal Compliance: Performed by accredited lab with certified equipment (CISPR receiver), calibrated anechoic chamber/OATS. Report is legally defensible.

[!TIP] Exam-Winning Strategy:

  1. Memize Key Formulas: dBµV/m conversion, Antenna Factor equation.
  1. Draw Setup Diagrams: Be ready to sketch LISN setup for conducted and antenna/turntable setup for radiated.
  1. Compare Standards: Know one difference between CISPR and FCC (e.g., detector use, frequency range).
  1. Mitigation Chain: Link source (e.g., SMPS) → coupling path (cable) → victim (receiver) → fix (filter at source, shield cable).
  1. Detector Logic: QP > AV > Peak for impulsive noise. Use QP for final compliance.
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