Measurement uncertainty in EMC
What measurement uncertainty means in EMC tests, where it comes from, how CISPR 16-4-2 and ISO/IEC 17025 treat it, and how it affects pass/fail decisions.
Every EMC measurement has an uncertainty: the result is a value with a range inside which the true value is expected to lie with a stated confidence. Uncertainty is not a mistake; it is part of an honest result.
Where it comes from
For a radiated-emission measurement the contributions include:
- the receiver (its accuracy, detector behaviour and bandwidth);
- the cable loss and the mismatch between the antenna, cable and receiver;
- the antenna factor and its calibration, including how the factor changes with height and polarisation;
- the site (the quality of the ground plane, the NSA or site-VSWR result, reflections);
- set-up effects: table height, cable layout, the position of the EUT, the distance to the antenna;
- repeatability of the EUT and of the measurement.
For conducted emissions the main items are the receiver, the cable and attenuator, the artificial-network impedance, and the coupling to the reference plane.
Standard uncertainty and expanded uncertainty
Each contribution is estimated and combined (as a root-sum-of-squares, with sensitivity coefficients) into a combined standard uncertainty. This is multiplied by a coverage factor (typically 2, for about 95 % confidence) to give the expanded uncertainty U, quoted in dB.
What the standards say
- CISPR 16-4-2 gives the framework for evaluating uncertainty of EMC emission measurements, with examples of the budgets, and defines the criterion that compares a laboratory’s expanded uncertainty with a reference value (Ucispr, which is of the order of a few dB depending on the measurement).
- Emission product standards such as CISPR 32 state how the uncertainty is to be used in the compliance decision, for example whether the measured value is compared directly with the limit (when the laboratory’s uncertainty does not exceed Ucispr) or whether the difference must be taken into account.
- ISO/IEC 17025 requires accredited laboratories to evaluate and report uncertainty. ILAC-G8 discusses decision rules: how to state conformity when the result is close to the limit (for example “shared risk” or “guard band” approaches).
What it means for a pass/fail decision
- A result well below the limit passes under any rule.
- A result close to the limit depends on the decision rule in the standard or agreed in the contract.
- A result above the limit fails, even if the uncertainty overlaps the limit in many schemes: check the applicable standard.
For immunity tests the uncertainty concerns the test level (the field strength or injected level), and the uncertainty of the calibration is part of the report.
Practical advice
- Know your laboratory’s uncertainty and the rule it applies.
- Leave a margin in design and pre-compliance (see Pre-compliance testing).
- Do not compare results from sites with different uncertainties without allowing for it.
- Calibrate antennas, receivers, cables and probes on schedule, and validate the site (see Test facilities and site validation).