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DATE
DOCUMENT NAME
DOCUMENT #
PAGE
May 21, 2004
Harman Multimedia JBL OnStage EMC Test Report
2004 050310 EMC
71 of 80
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 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 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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