A CRITIQUE ON TRACEABILITY OF EMC MEASUREMENTS

Introduction

In a competitive market, it is imperative that laboratories engaged in product quality evaluation are suitably certified or accredited by the empowered agency. For test laboratories dealing with electrical or electronic products, ISO/ IEC 17025 standard provides the required guidelines for accreditation, similar to ISO 9000 standards for manufacturing and service sectors. One of the most important criteria for getting accreditation for the test laboratories as per the above guidelines is the trace ability of test equipment for the measurements performed by them. For trace ability, test equipment calibration procedure and the calibration interval are required to be specifically laid out to ensure minimum inaccuracy and the required confidence level of the measured data.

India has adopted the ISO/IEC 17025 standard under the NABL scheme of Department Science and Technology for accrediting calibration and test laboratories for testing of products as per relevant standards while prescribed procedures are followed. Specific guidelines are provided for calibration of test equipment which has direct bearing on the reliability and trace ability of measured data.

EMC Measurement Scenario in India

In India there are many EMC laboratories in government sectors which are carrying our EMC test on almost at types of professional equipment. Except in one or two cases, there is not much effort in the private sector to establish EMC testing capability as per international standard. For defence sector, however, the captive test laboratories follow test procedures derived from MIL-STD. Presently no general calibration philosophy is prescribed for these EMC test laboratories. While each laboratory may have adopted its own in-house calibration methodology so that test equipment perform within their stated accuracy specifications trace ability of the measured data from one test house to another is a difficult proposition. EMC compliance evaluation activity in our country is picking up recently, due to obligations for mandatory CE Marking and self-certification required for European Union market. As the self-certification route is currently being followed by most of our export-oriented units, lacking in trace ability means may create serious problems for the shipped consignments both technically and legally. Furthermore, due to large quantum of workload involved, test laboratories and also the calibration authorities resort to rule of thumb calibration intervals which may not provide the test equipment the desired level of reliability.

EMC Standards prescribe test procedures which are elaborate but nevertheless leave scope for interpretation. The test procedures have sometimes inherent uncertainty embedded into the methodology and specifications which contribute significantly to overall accuracy of the measured data. It is, therefore, all the more important that EMC test and evaluation equipment always performs within the respective performance specifications after going though regular drills of calibration.

Uncertainty of Measurement as per ISO/ IEC 17025 Standard

The International standard prescribes “General requirements for the competence of testing and calibration laboratories” for accreditation purposes for the Certification Body. Relevant extracts are given below:-

“5.4.6 Estimation of Uncertainty of Measurement

“5.4.6.1 A calibration laboratory or a testing laboratory performing its own calibrations, shall have and shall apply procedure to estimate uncertainty of measurement for all calibrations and types of calibrations.

5.4.6.2 Testing laboratories shall have and shall apply a procedure for estimating uncertainty of measurement. In certain cases the nature of the test method may preclude rigorous, metro-logically and statistically valid calculation of uncertainty of measurement. In these cases the laboratory shall at least attempt to identify all the components of uncertainty and make reasonable estimation, and shall ensure that the form of reporting of the result does not give a wrong information of the uncertainty. Reasonable estimation shall be based on knowledge of the performance of the method and on the measurement scope and shall make use of, for example, previous experience and validation data.

Note 1 : The degree of rigor needed in an estimation of uncertainty of measurements depend on factors such as:

* the requirement of test method;

* the requirements of client;

* the existence of narrow limits on which decisions on conformance to a specification are based.

Note 2 : In those cases where a well recognized test methods specifies limits to the values of the major source uncertainty of measurement and specifies the form of presentation of calculated results the laboratory is considered to have satisfied this clause by following test and reporting instructions.

5.4.6.3 When estimating the uncertainty of measurement, all uncertainty components which are of importance in the given situation shall be taken into account using appropriate methods of analysis”.

From the above we can notice the following in respect of uncertainty of measurement :

* Reporting of uncertainty is required when information on uncertainty is relevant to the validity or application of the test results, when the client requires it or when the uncertainty affects compliance to a specification limit.

* Evaluation of uncertainty of measurement is required for calibrations including those performed in house, and procedures for estimating uncertainty of measurement of tests are needed and need to be applied.

A realistic and meaningful measurement uncertainty quality is calculated taking care of tolerance acceptable to each element or parameter which is involved in the measurement event. The goal is to achieve a quantitative parameter which would indicate how well a measurement correlates between two different test laboratories. To correlate the measured data from one test laboratory to another, additional correction factors are needed. For regulatory compliance purposes this has not yet taken a legal shape although doing this exercise has been important for reposing a high degree of confidence in the measured data at various laboratories.

Sources of EMC Measurement Uncertainty

The following are the major sources of measurement uncertainty in EMC test:

* Quality of test environment

* Accuracy of EMI Receivers/ Spectrum Analysers of other test equipment

* Type of antenna, Sensor or probes used as pickup/delivery device

* Interconnection cables & connectors

* Equipment under test orientation/ configuration

While each and every element as above may contribute an uncertainty element if all the above are not taken into account properly then the uncertainty may go as high as +/-20 dB.

The uncertainties are introduced in three main areas as given below :

* Control Factors - The factors affecting physical / electrical environment of all components of the test.

* Coupling Factors - The factors involved with the sensors and their linkage to the quantity being measured.

* Measuring Instruments - Calibration

Quality of Test Environment

Commercial EMC testing of radiated emissions following relevant standards indicate poor correlation of test data between two different test laboratories. Quality of a test site lies in the correlation between an actual site and an ideal open site with an infinite metal ground plane. Actual site may have physical reflecting objects (including absorbers in the walls of an anechoic chamber) and also reflections from the fringe of the metal ground plane. All these would contribute to the correction factors compared to an ideal open site with infinite ground plane. Site attenuation factor is the quantitative assessment of these imperfection elements which itself is based on the accuracy of all the measurement equipment as well as the site itself.

Measurement of site attention factor over the frequency range involves optimization of emissions by way of manual manipulation of sensors, cables and peripherals which is highly dependant on the personal skill of testing personnel. Taking the measurement equipment tolerances and other factors, site attenuation factor tolerances have been specified in the standards and the most acceptable figure stipulated by ANSI is + 4 dB. It is important to mention here that for shielded anechoic chambers similar standardized figures are not available. NIST at Boulder laboratories have worked in great details to establish overall site accuracy by using a calibrated spherical dipole source and comparing its performance at closed and open sites.

Role of Test Equipment

In overall measurement uncertainty, the measurement equipment plays the most significant role provided the other parameters viz. good engineering practices in the test facility, adequate skill of the testing personnel, optimization of test process etc. are taken care of. The various test equipment deployed in any EMC test laboratory, viz. spectrum analysers, EMI test receivers, signal generators, amplifiers, network analysers, power meters, electronic loads, set-top detectors etc. are required to be calibrated against a reference standard and within specified calibration interval so as to minimize measurement uncertainty. Calibration procedures are also expected to introduce some uncertainties and this can be minimized by getting more numbers of equipment calibrated in a signal calibration operation. Individual contribution from the calibration of test receiver/ spectrum analyser, antenna, antenna factor variation with height etc. are required to be combined to find the overall measurement uncertainty. While the measurement accuracy of each element does play an important role, measurement repeatability at different test laboratories is also an indication of overall uncertainty. On this account the spread of the test data is important for quantifying the repeatability aspect.

Regulatory Guidelines in Measurement Tractability

In ISO/ IEC 17025 standard the laboratory requires to have an established programme and procedure for calibration of its equipment. The programme also include a system for selecting, using, calibrating, checking, controlling, and maintaining measurement standards, reference materials used as measurement standards, and measuring and test equipment used to perform tests and calibrations.

The specific requirements are highlighted in the standard are:

* Calibration

* Testing

* Reference Standards

* Reference Materials

* Intermediate checks

* Transport and Storage

For test laboratory the following is to be kept in mind:

“The extent to which the requirements of calibration requirements should be followed depends on the relative contribution of the calibration uncertainty to the total uncertainty. If calibration is the dominant factor, the requirements should be strictly followed.

5.6.6.2.2 Where trace ability of measurements to SI units is not possible and/ or not relevant, the same requirements for trace ability, to for example, certified reference materials, agreed method and/ or consensus standards are required as for calibration laboratories.”

Conclusion

The approaches, which should be followed to maintain tractability is summarised below:

* Calibrate against national / international standard.

* Calibrate internally using calibrated standards/ reference materials.

* Calibrate through inter-comparison methods.

* Proficiency testing to evaluate the test setup and validation of test data.

While following the method of calibration as the primary tool to maintain the traceability, measurement uncertainty can be reduced by increasing the accuracy of the measurement receiver. In case an accurate field source is available, even overall measurement system can be calibrated by combining all the various factors into one overall correction factor.