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DATE
DOCUMENT NAME
DOCUMENT #
PAGE
September 17, 2004
Harman Multimedia JBL Encounter EMC Test Report
2004 090584 EMC
70 of 79
3. Practical Explanation of the Meaning of the Conducted and Radiated Emissions
Measurement Uncertainties
In the specific case of EMC Measurements in this test report, the measurement uncertainties of the
conducted emissions measurements and the radiated emissions measurements have been calculated in
accordance with the method detailed in the following documents:
o ISO Guide to the Expression of Uncertainty in Measurement (ISO, 1993)
o NIS 81:1994, The Treatment of Uncertainty in EMC Measurements (NAMAS, 1994)
o NIST Technical Note 1297(1994), Guidelines for Evaluating and Expressing the Uncertainty of
o NIS 81:1994, The Treatment of Uncertainty in EMC Measurements (NAMAS, 1994)
o NIST Technical Note 1297(1994), Guidelines for Evaluating and Expressing the Uncertainty of
NIST Measurement Results (NIST, 1994)
The calculation method used in these documents requires that the stated uncertainty of the measurements
be expressed as an “expanded uncertainty”, U, with a k=2 coverage factor. The practical interpretation of
this method of expressing measurement uncertainty is shown in the following example:
EXAMPLE: Assume that at 39.51 MHz, the (measured) radiated emissions level was equal to +26.5
dBuV/m, and that the +/- 2 standard deviations (i.e. 95% confidence level) measurement uncertainty
was +/- 3.4 dB.
In the example above, the phrase “k = 2 Coverage Factor” simply means that the measurement uncertainty
dBuV/m, and that the +/- 2 standard deviations (i.e. 95% confidence level) measurement uncertainty
was +/- 3.4 dB.
In the example above, the phrase “k = 2 Coverage Factor” simply means that the measurement uncertainty
is stated to cover +/-2 standard deviations (i.e. a 95% confidence interval) about the measurand. The
measurand is the radiated emissions measurement of +26.5 dBuV/m at 39.51 MHz, and the 95% bounds
for the uncertainty are –3.4 dB to + 3.4 dB. One can thus be 95% confident that the “true” value of the
radiated emissions measurement is between +23.1 dBuV/m and +29.5 dBuV/m. In effect, this means that
in the above example there is only a 2.5% chance that the “true” radiated emissions value exceeds +29.5
dBuV/m.
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