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3.  Practical Explanation of the Meaning of Radiated Emissions Measurement Uncertainties 
 
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 

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 +29.9 dBuV/m. In effect, this means that in the above example 

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Nemko USA, Inc. operates a comprehensive Periodic Calibration Program in order to ensure the validity of all 
test data. Nemko USA’s Periodic Calibration Program is fully compliant to the requirements of NVLAP Policy 
Guide PG-1-1988, ANSI/NCSL Z540.1-1994 (R2002), ISO 10012:2003, ISO/IEC 17025:2005, and ISO-9000: 
2000. Nemko USA, Inc.’s calibrations program therefore meets or exceeds the US national commercial and 
military requirements [N.B. ANSI/NCSL Z540-1-1994 replaces MIL-STD-45662A]. 
 
Specifically, all of Nemko USA’s primary reference standard devices (e.g. vector voltmeters, multimeters, 
attenuators and terminations, RF power meters and their detector heads, oscilloscope mainframes and plug-ins, 
spectrum analyzers, RF preselectors, quasi-peak adapters, interference analyzers, impulse generators, signal 
generators and pulse/function generators, field-strength meters and their detector heads, etc.) and certain 
secondary standard devices (e.g. RF Preamplifiers used in CISPR 11/22 and FCC Part 15/18 tests) are 
periodically recalibrated by: 
 

o

  A Nemko USA-approved  independent (third party) metrology laboratory that uses NIST-

traceable standards and that is ISO Guide 25-accredited as a calibration laboratories by NIST; or, 

o

  A Nemko USA-approved  independent (third party) metrology laboratory that uses NIST-

traceable standards and that is ISO Guide 25-accredited as a calibration laboratory by another 
accreditation body (such as A2LA) that is mutually recognized by NIST; or, 

o

  A manufacturer of Measurement and Test Equipment (M&TE), if the manufacturer uses NIST-

traceable standards and is ISO Guide 25-accredited as calibration laboratory either by NIST or by 
another accreditation body (such as A2LA) that is mutually recognized by NIST; or 

o

  A manufacturer of M&TE (or by a Nemko USA-approved  independent third party metrology 

laboratory) that is not ISO Guide 25-accredited. (In these cases, Nemko USA conducts an annual 
audit of the manufacturer or metrology laboratory for the purposes of proving traceabilty to 
NIST, ensuring that adequate and repeatable calibration procedures are being applied, and 
verifying conformity with the other requirements of ISO Guide 25). 

Nemko USA, Inc. 

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Nemko USA, Inc. 

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