EMI Lab (EC-306) - Unit 4 Short Notes

UNIT 4: EMI Measurement Techniques and Compliance Testing

4.1 Fundamentals of EMI Measurement

4.1.1 Purpose and objectives of EMI/EMC testing

• Pre-compliance Testing: Performed in-house during product development to identify and fix potential EMI issues early. Less stringent, cost-effective, faster.

• Full-compliance Testing: Formal testing at an accredited lab for regulatory certification (e.g., FCC, CE). Follows exact standards, uses calibrated equipment, and results in a formal test report.

4.1.2 Key measurement parameters: Quasi-peak, Average, RMS, Peak detectors

The detector type determines how the receiver/spectrum analyzer responds to pulsed or continuous signals. Standards mandate specific detectors for different frequency bands and limits.

[!TIP] Exam Key: QP is NOT the same as Peak. QP gives a lower reading for low repetition rate pulses. For a continuous sine wave, QP = AV ≈ 1.05 * RMS.

4.1.3 Measurement bandwidths (IF Bandwidth, RBW, VBW)

• Resolution Bandwidth (RBW): The bandwidth of the IF filter (typically 3 dB bandwidth). Narrower RBW = better frequency resolution, slower sweep, higher noise floor. Must match standard (e.g., 9 kHz for 150 kHz-30 MHz, 120 kHz for 30 MHz-1 GHz).

• Video Bandwidth (VBW): Bandwidth of the video filter after the detector. Smooths trace noise. VBW ≤ RBW is common. Setting VBW << RBW averages noise but slows response.

• Impact: RBW directly affects measured amplitude of broadband noise (wider RBW collects more power). Always state RBW/VBW on plots.

4.1.4 Measurement uncertainty

The declared margin of error in a measurement. Sources include:

1. Instrumentation: Calibration uncertainty of SA/receiver, antenna factor, cable loss, LISN impedance.

2. Setup: EUT positioning, cable routing, ground plane quality, site attenuation.

3. Environment: Ambient signals, temperature, humidity.

4. EUT Variability: Operational mode, load conditions.

Standards (e.g., CISPR 16-4-2) provide methods to calculate combined uncertainty ($$\displaystyle U_{tot} $$). Compliance is determined by: Measured Value + $$\displaystyle U_{tot} $$ ≤ Limit.

4.2 Test Equipment and Setup

4.2.1 Spectrum Analyzers (SA) and EMI Receivers

• EMI Receiver: Purpose-built for compliance. Features: Preselector (filters out-of-band signals), high dynamic range, standardized IF filters (RBW), calibrated detectors (QP, AV, Peak), trace averaging. Gold standard for full-compliance.

• Spectrum Analyzer: General-purpose. Can be used for pre-compliance if it has: selectable RBW (matching standards), QP detector (or software emulation), sufficient dynamic range, and preamplifier. Lacks preselector, may have wider noise floor.

4.2.2 Conducted Emission Measurement Setup

• LISN (Line Impedance Stabilization Network) / AMN (Artificial Mains Network):

  • Purpose: 1) Provides a standardized, stable impedance (50 Ω) to the power source for consistent measurements. 2) Isolates EUT noise from the mains supply. 3) Provides a coupling port for the measurement receiver.

  • Types: Single-phase (for 1φ EUT), Three-phase (for 3φ EUT).

  • Insertion Loss: Must be high (>20 dB) to prevent ambient noise on the mains from reaching the receiver.

  • Connection: Mains → LISN → EUT. LISN measurement port → SA/Receiver via 50 Ω coax. LISN ground must connect to reference ground plane.

4.2.3 Radiated Emission Measurement Setup

• Antennas:

  • Biconical: 30-300 MHz. Omnidirectional, linearly polarized.

  • Log-Periodic: 200 MHz-1 GHz (or higher). Directional, linearly polarized.

  • Horn: 1-18 GHz. High gain, directional, wide bandwidth.

  • Loop: 9 kHz-30 MHz (magnetic field). Small, used for low-frequency magnetic fields.

  • Calibration: Each antenna has an Antenna Factor (AF) in dB/m. E-field (dBµV/m) = Receiver Reading (dBµV) + AF (dB/m) + Cable Loss (dB).

• Test Sites:

  • Anechoic Chamber: Fully lined with RF absorber. No reflections. Ideal for indoor, controlled testing.

  • Open Area Test Site (OATS): Outdoor, flat, reflective ground plane (minimum 4m x 4m). Must be clear of ambient sources. Reference site per CISPR 16-1-4.

  • TEM/GTEM Cell: Fully enclosed, broadband transmission line cell. Used for pre-compliance and small EUTs. Provides controlled environment but limited EUT size.

  • Comparison: Anechoic = Absorbing. OATS = Reflecting (ground bounce). TEM/GTEM = Enclosed, uniform field.

• Turntable & Antenna Mast: EUT placed on non-conductive turntable (0.8m height). Rotated 360° in critical steps (e.g., 0°, 90°, 180°, 270°). Antenna mounted on mast (1-4m height) and scanned vertically to find maximum emission.

• Ground Plane: For radiated tests above 30 MHz, a minimum 4m x 4m metallic ground plane is required (for OATS/anechoic) to simulate earth and provide a reference current return.

4.3 Conducted Emissions Testing (150 kHz – 30 MHz)

4.3.1 Standard test method

• CISPR 16-2-1: International standard for conducted measurements.

• FCC Part 15.207: US regulation for unintentional radiated emissions (conducted limits apply 150 kHz-30 MHz).

4.3.2 Measurement procedure

1. Connect EUT power cord to LISN input.

2. Connect LISN measurement port to EMI receiver/SA via 50 Ω coax.

3. Ensure LISN and EUT chassis are bonded to reference ground plane.

4. Operate EUT in all normal modes.

5. Perform full scan (150 kHz-30 MHz) using QP detector with standard RBW (9 kHz for CISPR).

6. Identify frequencies where signal exceeds quasi-peak limit.

7. Re-measure those frequencies with QP and AV detectors for final comparison.

4.3.3 Identifying and measuring disturbance signals

• Use max-hold during scan to capture intermittent signals.

• Compare measured QP and AV values against the relevant QP and AV limits (often QP limit is higher than AV limit).

• Final Compliance: Measured QP ≤ QP Limit AND Measured AV ≤ AV Limit.

4.3.4 Common-mode vs. differential-mode noise

• Differential-Mode (DM): Noise current flowing between power conductors (L-N, L-L). Caused by switching power supplies. Measured as voltage across LISN's 50 Ω resistor.

• Common-Mode (CM): Noise current flowing from power conductors to ground/earth (L-G, N-G). Caused by parasitic capacitance to chassis. Primary source of radiated emissions. LISN measures CM noise effectively.

• Key: LISN measures both DM and CM noise on each line (L, N). CM noise is often the dominant concern for compliance.

4.4 Radiated Emissions Testing (30 MHz – 6 GHz / 18 GHz)

4.4.1 Standard test method

• CISPR 16-2-3: International standard for radiated measurements.

• ANSI C63.4: US standard for radiated emissions (FCC uses this by reference).

4.4.2 Full-scan (search) vs. final measurement (re-measure) phases

1. Search Phase: Quick scan over full frequency range (e.g., 30 MHz - 6 GHz) with fast RBW (e.g., 120 kHz) and QP detector. Antenna at fixed height (e.g., 1m). Identify frequencies where trace approaches within 6 dB of limit.

2. Re-measure Phase: For each identified frequency:

   • Set final RBW (e.g., 120 kHz for 30-1000 MHz, 1 MHz above 1 GHz).

   • Perform antenna height scan (1-4m) and turntable rotation (0-360°) to find maximum emission.

   • Record final QP (and sometimes AV) value at the worst-case position.

4.4.3 Antenna height and polarization scanning protocols

• Polarization: Test both horizontal and vertical antenna polarizations. EUT orientation may favor one.

• Height Scan: Raise/lower antenna in small increments (e.g., 0.5m) between 1m and 4m. Record maximum reading.

• Turntable Scan: Rotate EUT in steps (e.g., every 15° or 45°) and record maximum reading. Critical angles often occur at 0°, 90°, 180°, 270° due to EUT enclosure seams, cables, displays.

4.4.4 EUT positioning on turntable

• EUT placed centrally on turntable, non-conductive table.

• Standard Height: 0.8m above ground plane.

• Cables: Must be arranged to represent typical use but also to minimize unintended antenna effects. Often routed down through a hole in the turntable to the ground plane.

• Worst-Case: Must test all operational modes, peripheral connections, and cable configurations.

4.4.5 Ambient noise measurement and subtraction

• Ambient Measurement: With EUT OFF, perform full scan at the same test position to identify external signals (radio, TV, cellular).

• Handling: If an ambient signal coincides with an EUT emission, it's a false positive. Must:

  1. Verify by turning EUT OFF/ON.

  2. Report as "Ambient" or ignore if it masks a true emission.

  3. Cannot subtract ambient from EUT measurement per most standards. Must find a clear frequency or use a different test site/time.

4.5 Immunity (Susceptibility) Testing Fundamentals

4.5.1 Purpose: Types

• Radiated Immunity: EUT exposed to RF field (e.g., 80-1000 MHz, 1-6 GHz). Tests enclosure shielding.

• Conducted Immunity: RF noise injected directly onto cables (power, I/O). Tests filtering on interfaces.

• Electrical Fast Transient/Burst (EFT/B): Fast high-voltage transients on power/control lines. Simulates inductive load switching.

• Surge: High-energy transient (e.g., lightning induction). On power and external lines.

• Electrostatic Discharge (ESD): Direct/indirect discharge to EUT surfaces and coupling planes.

4.5.2 Radiated Immunity Test Setup

• Field Generation: Signal Generator → Power Amplifier → Broadband Antenna (e.g., log-periodic, horn).

• Field Calibration: Field Probe placed at EUT location to measure actual field strength (V/m). System must be calibrated to deliver required level (e.g., 10 V/m for consumer).

• Modulation: For many standards (e.g., IEC 61000-4-3), the RF carrier is 80% amplitude modulated at 1 kHz to simulate real-world signals like cellular.

• Test Levels: Defined by product standard (e.g., 3 V/m, 10 V/m, 30 V/m).

• Scanning: Antenna swept in polarization and height, EUT rotated. Field must be uniform (±3 dB) over test volume.

4.5.3 Conducted Immunity Test Setup

• Coupling/Decoupling Networks (CDN): For power lines (CDN M1, M2, M3) and I/O lines (CDN S, L, etc.). Injects disturbance in series with the cable while decoupling from the mains/network.

• Bulk Current Injection (BCI) Probe: Clamp-on probe for injecting into cable bundles over a wide frequency range (e.g., 10 kHz-400 MHz). Used when CDN not available.

• Injection Methods: Direct injection (via CDN) or Inductive coupling (via BCI probe).

• Monitoring: EUT performance monitored during injection (functional test, error count, degradation).

4.6 Test Standards and Regulations

4.6.1 Key International Standards

• CISPR (International Special Committee on Radio Interference): Sets emission limits and test methods.

  • CISPR 11: Industrial, scientific, medical (ISM) equipment.

  • CISPR 14-1: Household appliances and similar.

  • CISPR 22/32: Information technology equipment (ITE). CISPR 32 replaces CISPR 22 for multimedia equipment.

• IEC 61000-4 Series: Immunity test standards.

  • IEC 61000-4-2: ESD.

  • IEC 61000-4-3: Radiated RF immunity.

  • IEC 61000-4-4: EFT/Burst.

  • IEC 61000-4-5: Surge.

  • IEC 61000-4-6: Conducted RF immunity.

4.6.2 Regional Regulations

• FCC Part 15 (USA): Enforces limits for both conducted (15.207) and radiated (15.209) emissions. Uses AV detector for most bands. Class A (industrial) vs. Class B (residential) limits.

• CE Marking (EU): Requires compliance with EMC Directive (2014/30/EU). Uses CISPR standards with harmonized limits. QP detector is primary. Class A (industrial) vs. Class B (residential).

• ICES-003 (Canada): Similar to FCC Part 15 but with some differences in limits and test methods.

4.6.3 Product-specific vs. generic standards

• Product-Specific Standard: Tailored to a particular product family (e.g., CISPR 11 for ISM, CISPR 14 for appliances). Has specific test setups, frequencies, and often less stringent limits.

• Generic Standard: Applied when no product standard exists (e.g., IEC 61000-6-1 for residential, IEC 61000-6-2 for industrial). Covers all phenomena but may have more conservative limits. "Catch-all" standard.

4.6.4 Understanding limits and classification

• Units: Conducted: dBµV (or dBµA via current probe). Radiated: dBµV/m.

• Classification:

  • Class A / Group 1: Equipment for industrial/controlled environments. Higher limits.

  • Class B / Group 2: Equipment for residential/domestic environments. Lower limits.

• Limit Lines: Plotted on spectrum as a series of points (frequency, amplitude). Often have different slopes (e.g., 20 dB/decade) in different bands.

4.7 Test Reporting and Documentation

4.7.1 Essential elements of an EMI test report

1. Cover Page: Lab name, report number, EUT description, standard(s) tested to, conclusion.

2. Test Facility: Accreditation details (ISO/IEC 17025), site description.

3. EUT Description: Model, serial numbers, functional description, block diagram.

4. Test Configuration: Detailed diagrams of all connections, cable types/lengths, EUT orientation.

5. Test Equipment: List of all equipment (SA, antennas, LISN, probes) with calibration due dates.

6. Test Conditions: Temperature, humidity, power supply voltage/frequency.

7. Results: Tabular and graphical data for all measurements (conducted & radiated).

8. Photographs: Of EUT, test setup, inside EUT (if required).

9. Statement of Compliance/Non-Compliance.

10. Signatures: Engineer, reviewer.

4.7.2 Test configuration photographs and diagrams

• Photos: Wide shot of entire setup (EUT on turntable, antennas, cables). Close-ups of critical connections (LISN, EUT I/O). Must include scale (ruler) and identify key components.

• Diagrams: Schematic showing EUT, peripherals, power source, LISN/AMN, grounding points, cable routing paths.

4.7.3 Tabular and graphical presentation of results

• Tables: Columns: Frequency (MHz), Quasi-Peak (dBµV/m), Average (dBµV/m), Limit (QP/AV), Margin (dB), Detector, Antenna, Height, Polarity, Turntable Angle.

• Graphs (Spectrum Plots): Frequency (X) vs. Amplitude (Y). Plot measured trace and limit line on same graph. Mark peak emission frequency. Annotate with RBW, detector, antenna, height, polarization. Include ambient trace (EUT OFF) as reference.

4.7.4 Statement of compliance/non-compliance

• Compliance: "Based on the measurements documented in this report, the EUT was found to comply with the requirements of [Standard] for [Class] equipment."

• Non-Compliance: "The EUT failed to meet the [QP/AV] limit at [Frequency] MHz. Measured value was X dBµV/m vs. limit Y dBµV/m (margin -Z dB)."

• Justification: Reference specific table/graph numbers. State if failure is due to fundamental emission or spurious.

4.8 Common Measurement Pitfalls and Troubleshooting

4.8.1 False positives from ambient signals and intermodulation

• Ambient: Always perform ambient scan. If emission disappears with EUT OFF, it's ambient. Can be radio stations, airport radar, cellular.

• Intermodulation: Two strong ambient signals mixing in a non-linear device (poor cable, connector) to create a false emission at $$\displaystyle f_{im} = mf_1 \pm nf_2 $$. Turns off when one strong ambient is removed.

4.8.2 Cable routing and its effect on conducted/radiated results

• Conducted: Power cord must be as short as possible from EUT to LISN. Excess length can act as an antenna, increasing CM current.

• Radiated: Cables are primary radiators. Must be routed to minimize loop area (e.g., bundled, twisted, routed along ground plane). "Worst-case" cable routing must be tested (e.g., all cables fully extended).

4.8.3 Grounding issues and ground loops

• LISN Ground: Must be solid, low-impedance connection to reference ground plane. Poor ground raises measured noise.

• Ground Loops: Multiple ground paths create loops that pick up ambient noise. Ensure single-point ground for LISN/EUT system.

• EUT Chassis Ground: Must be properly connected to ground plane via LISN protective earth.

4.8.4 EUT operational modes and worst-case scenario identification

• Must Test All Modes: Power on/off, all functions (print, scan, communicate), all speed settings (e.g., CPU clock, motor speed), all peripheral connections.

• Identify Worst-Case: Often the mode with highest clock speed, most active I/O, or maximum load. May require iterative testing to find.

• Software: EUT software can control operational states. Use software to force worst-case modes during measurement.