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EMC

EMI Margin Budget

Calculate EMI compliance margin accounting for measurement uncertainty and safety margin to predict pre-compliance test pass/fail.

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Formula

M_adj = (E_limit − E_meas) − U − SM

U— Measurement uncertainty (dB)
SM— Safety margin (dB)

How It Works

EMI margin budgeting determines whether a product will pass formal EMC testing by accounting for measurement uncertainty and design safety margin. The raw margin M_raw = E_limit − E_measured is the difference between the regulatory limit and the measured emission. However, pre-compliance measurements are subject to significant uncertainty (antenna calibration, LISN accuracy, site imperfections) — typically 6 dB for table-top setups and 3 dB for calibrated test sites. A design safety margin (typically 6 dB) is added to account for unit-to-unit production variation, temperature, and aging effects. The adjusted margin M_adj = M_raw − U − SM must be ≥ 0 for confidence that the product will pass. If M_adj < 0, the product needs further noise reduction before formal testing.

Worked Example

Problem
A pre-compliance scan shows a peak emission of 34 dBμV/m against a CISPR 22 Class B limit of 40 dBμV/m at 3 m. Pre-compliance uncertainty is estimated at 6 dB and a 6 dB safety margin is required. Will the product likely pass?
Solution
1. Raw margin: M_raw = 40 − 34 = 6 dB 2. Adjusted margin: M_adj = 6 − 6 − 6 = −6 dB 3. Required reduction: 6 dB Result: Despite being 6 dB below the limit in pre-compliance, the adjusted margin is −6 dB. The product is likely to FAIL the formal test. A further 6 dB reduction (halving the loop area, or reducing switching current by a factor of √2) is needed before formal testing.

Practical Tips

  • ✓Always test at worst-case operating conditions: maximum clock frequency, all I/O ports active, maximum load current.
  • ✓Use a calibrated LISN (line impedance stabilisation network) and calibrated antennas to minimise measurement uncertainty in pre-compliance.
  • ✓Budget 10 dB total margin (6 dB uncertainty + 4 dB safety) when the product is being tested on a non-compliant pre-compliance setup such as a desk or anechoic foam.

Common Mistakes

  • ✗Declaring success when raw margin is positive without accounting for measurement uncertainty — a 3 dB measured margin on a poor pre-compliance setup may become a 3 dB failure at an accredited lab.
  • ✗Using a 3 m distance measurement to predict 10 m compliance — 10 m measurements differ by more than just the geometric 1/r factor due to ground reflections.
  • ✗Ignoring worst-case operating conditions — test the product at maximum clock speed, maximum I/O activity, and maximum supply voltage for worst-case emissions.

Frequently Asked Questions

A well-calibrated pre-compliance setup with a calibrated LISN and loop antenna can achieve ±3 dB uncertainty. A desktop scan with uncalibrated probes can have ±10 dB or worse. Most practitioners budget 6 dB for a reasonably careful pre-compliance scan.
CISPR 22 has been formally replaced by CISPR 32 (Multimedia Equipment EMC Requirements) as of 2015. However, many national regulations still reference CISPR 22 limits, and they are numerically similar. Check the specific applicable standard for your market (CE: EN 55032, FCC: Part 15 Class B).
Zero adjusted margin means you have exactly compensated for uncertainty and safety — there is no additional buffer. In practice, test sample variation and temperature effects will cause some units to fail. Aim for at least 3 dB of adjusted margin after accounting for all uncertainty components.

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