EMI Lab (EC-306) - Unit 3 Short Notes

UNIT 3: EMI/EMC Measurement Techniques, Standards, and Test Setups

3.1 Test Environments and Facilities

3.1.1 Open Area Test Sites (OATS)

• Definition: An outdoor, flat, paved area (minimum 30m x 30m) with a metallic ground plane (minimum 2m x 2m mesh or solid plate) to simulate a perfect ground.

• Key Parameter: Site Attenuation (SA). Validates the site's suitability. Measured SA must be within ±4 dB of theoretical free-space SA from 30 MHz to 1 GHz.

  • Theoretical SA (for 10m distance): $$\displaystyle SA_{th} = 20\log_{10}\left(\frac{4\pi d}{\lambda}\right) $$

  • Where $d$ = distance (m), $\lambda$ = wavelength (m).

• Validation: Performed with a tuned dipole antenna pair. Ambient signals must be at least 20 dB below the limit line.

3.1.2 Shielded Enclosures

• Reverberant Chamber: Uses mode stirrers to create a statistically uniform field. Used for high-level immunity testing. Normalized Site Attenuation (NSA) or Turntable Factor (TTF) used for validation.

• Anechoic Chamber: Lined with RF absorbing material (pyramidal ferrite tiles) to minimize reflections. Field Uniformity is validated per standards (e.g., IEC 61000-4-20) using a grid of points.

• Absorbing Materials: Typically carbon-loaded foam, effective from ~80 MHz upwards. Performance degrades at lower frequencies.

3.1.3 TEM/GTEM Cells

• Principle: A tapered transmission line (TEM cell) or a "Gigahertz TEM" (GTEM) cell that provides a controlled, calibrated EM environment for pre-compliance testing.

• Operation: The DUT is placed inside; the cell converts the internal field to a voltage at its coaxial output, measured by a receiver.

• Advantages: Repeatable, low cost, no ambient noise, good for 10 kHz - 1 GHz+.

• Limitations: Small DUT size, limited to conducted and low-level radiated emissions, not suitable for final compliance.

3.1.4 Anechoic Chamber vs. OATS

[!TIP] Exam Focus: Know the validation parameters: SA for OATS, Field Uniformity for anechoic, NSA/TTF for reverberant.

3.2 Essential Test Equipment and Instrumentation

3.2.1 Antennas

• Calibration: Must be calibrated with known gain/d factors. Antenna Factor (AF) is critical: $$\displaystyle E (V/m) = V_{rec} (V) \times AF (1/m) $$.

3.2.2 Line Impedance Stabilization Networks (LISN)

• Purpose: Provides a standardized, low-impedance RF noise source reference for the EUT's power port. Isolates ambient noise from the mains.

• Types: Artificial Mains Network (AMN). Common configurations: 50Ω/50μH (for CE 150kHz-30MHz). Creates a known impedance (50Ω) for the measuring receiver.

• Role in CE: Connected between EUT mains and the AC source. The EUT's conducted emissions are measured across the LISN's 50Ω output.

3.2.3 EMC Receivers & Spectrum Analyzers

• Key Specs:

  • IF Bandwidths: 9 kHz (CISPR), 120 kHz (CISPR), 200 kHz (FCC). Must be selectable.

  • Detectors:

    • Peak (PK): Captures maximum signal.

    • Quasi-Peak (QP): Models human annoyance to impulsive noise. Has specific charge/discharge time constants.

    • Average (AV): Average power, often used with RMS detectors for wideband noise.

  • Preamplifier: Low-noise amplifier near the antenna to improve sensitivity.

  • Preselector: Tunable filter to reject strong out-of-band signals.

3.2.4 Current Probes & Clamps

• Use: Measure common-mode (CM) currents on cables for RE debugging. Used for conducted immunity (CS) by injecting current.

• Calibration: Transfer Impedance (Zt) is the key parameter: $$\displaystyle I (A) = V_{rec} (V) / Zt (\Omega) $$.

3.2.5 Transient Generators

• EFT/Burst (IEC 61000-4-4): Waveform: 5 ns rise, 50 ns pulse width, 300 ns pulse period, burst rate 15 kHz. Coupled via CDN or capacitive clamp.

• Surge (IEC 61000-4-5): Waveform: 1.2/50 μs (open circuit) & 10/700 μs (closed circuit). High energy. Coupled via CDN or gas-discharge tube.

• ESD (IEC 61000-4-2): Waveform: 0.7-1 ns rise, 30-60 ns decay. Contact Discharge (direct) & Air Discharge (indirect). Simulator output: ±2 kV to ±30 kV.

3.2.6 Signal Generators & Power Amplifiers

• Use: Generate the high-power, modulated RF signal (typically 1 kHz AM at 80%) for Radiated Immunity (RS) testing.

• Power Amplifier: Needed to achieve required field strength (e.g., 10 V/m, 30 V/m) at the DUT location.

3.2.7 Calibration & Traceability

• All equipment (antennas, LISN, probes, generators) must have a valid calibration certificate traceable to a national metrology institute (e.g., NIST, NPL).

• Measurement Uncertainty: Must be calculated and included in the test report. Combines uncertainties from equipment, setup, and environment.

3.3 Conducted Emissions (CE) Testing

3.3.1 Test Setup

[AC Mains] --> [LISN] --> [DUT]

                |

            [50Ω Output] --> [EMC Receiver]

• Reference Ground Plane: Minimum 1m x 1m metallic plane, bonded to LISN ground.

• Cables: Use shortest possible, unshielded if possible. RF-absorbing clamps (Ferrites) on cables beyond 1m from LISN.

3.3.2 Measurement Procedure

• Frequency Range: 150 kHz to 30 MHz (CISPR 11/14/22).

• Detectors: Quasi-Peak (QP) for final result. Average (AV) measured simultaneously.

• Bandwidth: 9 kHz (QP/AV) for CISPR; 10 kHz (FCC Part 15).

• Scan: Step size 1/3 to 1 of RBW. Hold time ≥ RBW time.

3.3.3 Interpretation of Results

• Limits: Defined in dBµV (QP & AV). E.g., CISPR 11 Group 1 Class A: QP 66-56 dBµV (decreasing linearly from 0.15-30 MHz).

• Margin: $$\displaystyle Margin (dB) = Limit (dBµV) - Measured Value (dBµV) $$. Positive = Pass.

• QP vs. AV: For repetitive pulses, QP > AV. If AV is below limit but QP exceeds, FAIL.

3.3.4 Common Noise Sources & Mitigation

• Sources: Switching PSUs (flyback, forward), clock circuits (microprocessors), PWM drives.

• Mitigation:

  • Filtering: Common Mode (CM) Chokes + Y-capacitors (line-to-ground). Differential Mode (DM) X-capacitors + inductors.

  • Layout: Short return paths, ground planes, separation of noisy/digital/analog sections.

[!TIP] Common Pitfall: Forgetting that the LISN's 50Ω output impedance is the reference. A high reading could be from a poor connection to the receiver, not the DUT.

3.4 Radiated Emissions (RE) Testing

3.4.1 Test Setup

• DUT Placement: Center of turntable (1m above ground for OATS/Chamber).

• Antenna: Height 1-4m (for 3m/10m distance), polarization (H/V).

• Cable Management: Use RF-absorbing ferrite clamps on all I/O cables at the point they exit the ground plane. Keep cables shortest and routed away from antenna.

• Ambient: Must be measured and subtracted if within 6 dB of EUT signal.

3.4.2 Measurement Procedure

1. Antenna Height Scan: 1-4m (or 1-2m for 3m distance) to find maximum.

2. Turntable Azimuth Scan: 0-360° in 15° increments (or finer near max).

3. Polarization: Repeat for both horizontal and vertical.

4. Final Measurement: At the worst-case height/angle/pol, use QP detector.

3.4.3 Test Distances

• 3m: Most common for commercial products (CISPR, FCC). Limits are higher than 10m.

• 10m: Used for larger equipment or when 3m limits are too restrictive. Limits are 6.67 dB lower than 3m (inverse square law: $$\displaystyle 20\log_{10}(10/3) \approx 10.5 dB $$, but standards use 6.67 dB).

• 30m: For very large installations or military (MIL-STD).

3.4.4 Interpretation of Results

• Limits: dBµV/m (QP). Vary by standard, frequency, and test distance.

• Corrections Applied:

  • Antenna Factor (AF)

  • Cable Loss (CL)

  • Preamplifier Gain (G)

  • Attenuator Loss (A)

  • Site Attenuation (SA) Correction (for OATS only).

  • $$\displaystyle E = V_{rec} + AF + CL + G + A + SA_{corr} $$

3.4.5 Common Radiating Structures & Mitigation

• Enclosure Seams/Vents: Shielding Effectiveness (SE). Use conductive gaskets, finger stock, reduce slot length < λ/20.

• I/O Cables: Act as efficient antennas. Use filtered connectors, shielded cables with 360° bonding, ferrite clamps.

• Display Panels (LCD): Radiate like slot antennas. Use conductive mesh/foil behind panel, filter display cables.

• General: Maintain high-quality bonding (low impedance) throughout the enclosure.

3.5 Immunity (Susceptibility) Testing

3.5.1 Radiated Immunity (RS) - IEC 61000-4-3

• Test Level: 3 V/m, 10 V/m, 30 V/m (80% AM, 1 kHz).

• Frequency Range: 80 MHz - 2.7 GHz (or 6 GHz).

• Setup: DUT on non-conductive table in anechoic chamber or GTEM cell. Antenna at 1m (or calibrated distance in cell).

• Performance Criteria:

  • A: Normal performance within specification.

  • B: Self-recoverable or operator-intervention recovery.

  • C: Loss of function, requires reset/repower.

3.5.2 Conducted Immunity (CS) - IEC 61000-4-6

• Injection Methods: Coupling/Decoupling Network (CDN) for power/telecom ports, EM Clamp for I/O cables, Current Probe for monitoring.

• Test Level: 3 Vrms, 10 Vrms (unmodulated or 80% AM, 1 kHz).

• Frequency Range: 150 kHz - 80 MHz.

• Monitoring: DUT performance monitored during sweep.

3.5.3 Electrical Fast Transient/Burst (EFT/B) - IEC 61000-4-4

• Waveform: Burst of 5 ns pulses (300 ns period) at 5 kHz repetition rate. Burst duration 15 ms, repetition rate 300 ms.

• Coupling: Capacitive Coupling Clamp (direct to cable) or CDN (to power port).

• Test Levels: 0.5 kV (PE), 1 kV (I/O), 2 kV (direct).

3.5.4 Surge Immunity - IEC 61000-4-5

• Waveform: Combination wave: 1.2/50 μs (voltage) & 10/700 μs (current).

• Coupling:

  • Line-to-Ground (LG): Most severe. Surge generator between line and PE.

  • Line-to-Line (LL): Between two lines (L1-L2).

• Test Levels: 0.5 kV (LG), 1 kV (LL) for equipment.

3.5.5 Electrostatic Discharge (ESD) - IEC 61000-4-2

• Test Levels: Contact: ±2, ±4, ±6 kV. Air: ±4, ±8, ±15 kV.

• Discharge Points: All accessible surfaces, connectors, gaps.

• ESD Simulator: Includes a RC network (150 pF, 330 Ω) to simulate human body model.

3.5.6 Power Frequency Magnetic Field - IEC 61000-4-8

• Test Level: 30 A/m (continuous), 300 A/m (short duration).

• Coil: Single-layer solenoid (1m diameter) generating uniform field.

• Frequency: 50/60 Hz.

3.6 Key International, Regional, and Military Standards

3.6.1 CISPR Standards

• CISPR 11: ISM equipment (industrial, scientific). Group 1 (no intentional RF), Group 2 (intentional RF). Class A (non-domestic), Class B (domestic).

• CISPR 14-1: Household appliances - Emissions.

• CISPR 14-2: Household appliances - Immunity.

• CISPR 22/32: IT equipment (radiated & conducted). CISPR 32 supersedes CISPR 22.

3.6.2 IEC 61000-4 Series (Immunity)

• Part 2: ESD.

• Part 3: EFT/B.

• Part 4: Surge.

• Part 5: Surge (communication lines).

• Part 6: Conducted RF (CS).

• Part 8: Power frequency magnetic field.

• Part 11: Voltage dips, interruptions, variations.

3.6.3 FCC Rules (Part 15)

• Unintentional Radiators: All digital/electronic devices.

• Verification: For Class B (digital devices). Manufacturer tests and declares.

• Declaration of Conformity (DoC): For Class A (commercial).

• Certification: For intentional radiators (e.g., Wi-Fi) and some Class B devices.

• Limits: 150 kHz - 30 MHz (conducted), 30 MHz - 6 GHz (radiated). QP & AV detectors.

3.6.4 MIL-STD-461

• Department of Defense (DoD) requirement for subsystems.

• Key Tests:

  • CE102: Conducted emissions, power leads, 10 kHz - 10 MHz.

  • RE101: Radiated emissions, magnetic field, 10 kHz - 100 kHz.

  • RE102: Radiated emissions, electric field, 2 MHz - 18 GHz.

  • RS101: Radiated susceptibility, magnetic field.

  • RS103: Radiated susceptibility, electric field.

• Limits: Often stricter than commercial, with specific band-edge restrictions.

3.6.5 ISO 11452 Series (Automotive)

• Component-level immunity tests for vehicles.

• ISO 11452-2: Radiated immunity (ALSE, 10 V/m, 200 MHz - 1 GHz).

• ISO 11452-4: Bulk current injection (BCI) for conducted immunity.

• ISO 11452-5: Transient emission (from vehicle electrical system).

3.6.6 Product-Specific vs. Generic

3.7 Test Execution, Documentation, and Reporting

3.7.1 Pre-test Planning

• Identify applicable standard(s) (CISPR, FCC, IEC 61000-4-x).

• Prepare test configuration diagram (mechanical layout, cable types/lengths).

• Finalize equipment list with calibration due dates.

• Determine performance criteria for immunity tests.

3.7.2 Test Procedure

1. Setup: Assemble per diagram. Connect monitoring equipment (if DUT has output).

2. Pre-check: Verify DUT operates normally.

3. Emissions: Perform full scan (QP). Identify frequencies exceeding margin (e.g., 6 dB below limit). Re-scan with AV. Record final QP & AV at worst points.

4. Immunity: Apply test level at each frequency/port. Monitor DUT function. Record any malfunction.

5. Post-test: Verify DUT recovers/operates normally.

3.7.3 Test Report Contents

• DUT Description: Model, serial, function.

• Test Setup: Photos & diagrams (top & side views).

• Equipment Details: Make, model, calibration date, certificate number.

• Raw Data: Plots (spectra) with markers, limits, and annotations.

• Results Table: Frequency, measured value (QP/AV or field strength), limit, margin, detector, antenna pol/height.

• Final Statement: PASS or FAIL against specific standard clauses.

• Measurement Uncertainty: Statement of expanded uncertainty (k=2) and its relevance to compliance.

3.7.4 Common Pitfalls & Troubleshooting

• False Failures:

  • Ambient Signals: Identify and exclude if >6 dB below EUT signal.

  • Cable Effects: Unfiltered cables radiating. Use ferrites, shorten cables.

  • Grounding: Poor ground plane bonding or LISN ground.

  • Test Distance: Antenna not at correct distance from DUT.

• Debugging Emissions:

  • Near-field Probe: Use H-field loop & E-field probe to locate source on PCB/cable.

  • Disconnect Cables: Identify radiating cable.

  • Add Ferrites/Filter: Temporarily to confirm source.

3.8 Advanced and Specialized Topics

3.8.1 Transient Emission Testing

• Measures spurious emissions from switching events (power on/off, relay actuation).

• Often done with a wideband receiver or oscilloscope with RF front-end.

• Standard: CISPR 14-1 requires testing for certain appliances.

3.8.2 Harmonic & Flicker Testing (IEC 61000-3-2/3)

• Purpose: Limit harmonic currents injected into mains (3-2) and voltage fluctuations causing light flicker (3-3).

• CE Setup: Power Analyzer or dedicated harmonic/flicker meter connected UPSTREAM of the LISN (or between LISN and mains).

• Limits: Class A, B, C, D equipment have different harmonic current limits per order (odd/even).

3.8.3 Power Quality Immunity (IEC 61000-4-11)

• Voltage Dips & Interruptions: Simulate mains sags/outages.

• Test Levels: 0% (interruption), 40%, 70%, 80% of nominal voltage.

• Duration: 10 ms, 500 ms, 5 s, etc.

• Performance Criteria: Critical for equipment that must not shut down (e.g., industrial controllers).

3.8.4 In-Situ/On-Site Testing

• Challenges: No controlled ground plane, uncontrolled ambient, unknown site attenuation.

• Equipment: Portable receivers, battery-powered amplifiers, broadband antennas.

• Method: Often uses comparative method vs. a known reference source or pre-compliance baseline.

3.8.5 Automotive EMI Testing Overview

• Broadband vs. Narrowband: CISPR 25 (vehicles) uses broadband limits (10 dB lower than narrowband) from 150 kHz - 30 MHz.

• Key Standards:

  • CISPR 25: Vehicle, boat, internal combustion engine - radiated & conducted emissions.

  • ISO 7637-2: Transient emissions from vehicle electrical system (pulses on power lines).

  • ISO 11452 Series: Component immunity (ALSE, BCI, etc.).

• Test Distances: Often 3m or 1m for vehicle components.