RADIAL (serv.man3) - JBL Audio EMC - CB Certificate (repair manual). Page 43

Nemko USA, Inc. 

 

 

In general, a “Statement of Measurement Uncertainty” means that with a certain (specified) confidence level, 

the “true” value of a measurand will be between a (stated) upper bound and a (stated) lower bound.   

 

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  ANSI Z540.2 (2002) Guide to the Expression of Uncertainty in Measurement  

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 

 (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.