EMC and Measurement Uncertainty – LAB 34 and CISPR 16-4-2

Two of the more important publications in the area of Electromagnetic Compatibility (EMC) and Measurement Uncertainty (MU) are LAB 34 and CISPR 16-4-2. EMC and Measurement Uncertainty are receiving more attention as other CISPR Product Family Standards begin to adopt MU. LAB 34 is “The Expression of Uncertainty in EMC Testing” and is published by the United Kingdom Accreditation Service (UKAS). CISPR 16-4-2 is published by the International Electrotechnical Commission (IEC) and is titled “Specification for Radio Disturbance and Immunity Measuring Apparatus and Methods – Part 4-2: Uncertainties, Statistics, and Limit Modeling – Uncertainty in EMC Measurements.” This article compares and contrasts the two MU documents.

Basis for the Documents
Both Measurement Uncertainty documents are based on the International Standards Organization (ISO) Guide to the Expression of Uncertainty in Measurement (GUM), 1993, corrected and reprinted in 1995. This publication is the grandfather of all Measurement Uncertainty documentation and is often referred to, simply, as the “GUM.” However, it should be noted that the “GUM” has been cancelled and replaced by “ISO/IEC Guide 98-3 – Uncertainty of Measurement – Guide to the Expression of Uncertainty of Measurement (GUM:1995).” The first edition of ISO/IEC Guide 98-3 was published in 2008. (Note – IEC is the International Electrotechnical Commission; a sister organization to the ISO).

When the “GUM” was first published in 1993 (after almost a 16-year development period), it introduced a new general perspective on errors, tolerances, and measurement variances. Many seminars and workshops occurred, after the initial release of the “GUM”, to help engineers understand the new concepts of Measurement Uncertainty and specifically, Measurement Uncertainty and EMC.

Within a year of the release of the GUM, the British had released an EMC Measurement Uncertainty document called NIS 81 – “The Treatment of Uncertainty in EMC Measurements”; it was published by the National Measurement Accreditation Service (NAMAS) in May of 1994. This was a first attempt to address EMC and Measurement Uncertainty. NIS 81 had a number of mistakes in it and it was replaced by LAB 34; which was first released in August of 2002.

CISPR 16-4-2 was spun-off from CISPR 16-4 (Uncertainty in EMC Measurements) in November of 2003.

So, since 2003, there have been two fairly stable documents that have addressed MU and EMC. Since both documents are based on the original “GUM”, there is a great deal of commonality between the two MU references.

This can be seen by reviewing Figure 1, the Table of Contents of both documents; LAB 34 and CISPR 16-4-2.

Figure 1: Tables of Contents

It can be seen that both documents have an Introductory paragraph, a References paragraph, a General paragraph on Concepts and/or Scope, a paragraph on Measurement Uncertainty budgets, and Examples of Measurement Uncertainty. This article will be primarily devoted to comparing and contrasting some of the Measurement Uncertainty Examples.

Comparison of Conducted Emissions
In LAB 34, the conducted disturbance (conducted emission) from 9 kHz to 150 kHz standard uncertainties are shown in Figure 2. The standard uncertainties include the Receiver Reading, the Attenuation of the Artificial Mains Network (AMN)-Receiver combination, the AMN Voltage Division Factor, the Receiver Sine Wave, the Receiver Pulse Amplitude, the Noise Floor Proximity, the AMN Impedance, a Frequency Step Error, Mismatch (Receiver Voltage Reflection Coefficient and AMN + Cable), Measurement System Repeatability, and Repeatability of the Equipment Under Test (EUT). Table A.1 of CISPR 16-4-2 includes all these values except for Frequency Step Error, Measurement System Repeatability, and Repeatability of the EUT. However, since LAB 34 assigns values of zero to Frequency Step Error and Repeatability of the EUT, the only difference between the tables and their standard uncertainties is Measurement System Repeatability with a standard uncertainty of 0.5 dB. Subtracting that value from the Combined Standard Uncertainties for LAB 34 as shown in Figure 2, we arrive at a Combined Standard Uncertainty of 2.11 dB. Assuming a k = 2 coverage factor, we arrive at a value of 4.22 dB for the Expanded Measurement Uncertainty (EMU). Comparing that to CISPR 16-4-2, we see that “16-4-2” has a value of 3.97 dB for its Expanded Measurement Uncertainty; thus, we have a difference of 0.25 dB between the two documents.

Most of this difference seems to be from Attenuation of the AMN-receiver combination which is 0.4 dB in LAB 34 and only 0.1 dB in CISPR 16-4-2. A second reduced-factor in “16-4-2” is the AMN impedance; the standard uncertainty for that in “16-4-2” is 1.37 dB while in LAB 34 it is 1.47 dB.

Looking at the next higher frequency range for conducted emissions, 150 kHz to 30 MHz, as shown in A2 of LAB 34 and Table A.2 of CISPR 16-4-2, we see an Expanded Measurement Uncertainty (EMU) of 3.9 dB in LAB 34 and an Expanded Measurement Uncertainty of 3.6 dB in CISPR 16-4-2. If we subtract the Measurement System Repeatability standard uncertainty from LAB 34, we arrive at an EMU of 3.7 dB thus leaving us with a difference between the two documents of only 0.1 dB for conducted emissions between 150 kHz and 30 MHz.

Radiated Emissions
There are a number of radiated emissions (radiated disturbances) that could be reviewed depending on the antenna-to-EUT distance and the horizontal versus vertical polarization of the antenna. I chose a 3-meter antenna distance for this analysis with a vertical polarization of the log-periodic antenna and a frequency range of 300 – 1000 MHz.

As seen from Figure 3, the number of standard uncertainty factors has increased from the previous conducted emission examples. The list of standard uncertainty factors includes Receiver Indication, Receiver Sine Wave, Receiver Pulse Amplitude, Receiver Pulse Repetition, Noise Floor Proximity, Antenna Factor Calibration, Cable Loss, Antenna Directivity, Antenna Factor Height Dependence, Antenna Phase Center Variation, Antenna Factor Frequency Interpolation, Site Imperfections, Measurement Distance Variation, Antenna Balance, Cross Polarization, Frequency Step Error, Mismatch, Measurement System Repeatability, and Repeatability of EUT.

LAB 34 has three elements in its table that are not in “16-4-2”; they are Frequency Step Error, Measurement System Repeatability, and Repeatability of the EUT. Both Frequency Step Error and Repeatability of the EUT are zero in LAB 34, they don’t contribute to the Combined Standard Uncertainty. However, Measurement System Repeatability is 0.5 in LAB 34; subtracting that from the Standard Uncertainty Table leaves us with an Expanded Measurement Uncertainty for 300 MHz to 1000 MHz of 5.90 dB. The equivalent number from “16-4-2” is 5.18 dB. It should be noted that the “16-4-2” table includes a factor for Table Height of 0.1 dB. If we subtract that from the “16-4-2” table, we still have a value of 5.18 dB (the factor is so small it contributes very little to the expanded measurement uncertainty). This is a difference of 0.72 dB between the two documents for vertical polarization.

The major difference maker between the two documents is antenna directivity: LAB 34 has a value of 3.0 dB while “16-4-2” has a factor of only 1.0 db for that value.

LAB 34 has an expanded measurement uncertainty (EMU) of 4.9 dB for Vertical Polarization at 10-meters from 300 MHz to 1000 MHz; if we subtract the Measurement System Repeatability factor; we have an EMU of 4.76 dB. CISPR 16-4-2 has an EMU of 5.05 dB for this same situation. Obviously, with a difference of only 0.29 dB, we have very similar numbers for 10-meter vertical radiated field strength.

For horizontal radiated emissions, with a biconical antenna, from 30 MHz to 300 MHz, LAB 34 has no examples. CISPR 16-4-2 has an EMU of 4.95 dB for 3-meters and 4.94 dB for 10-meters.

CISPR 16-4-2 actually covers the frequency range from 30 MHz to 200 MHz while LAB 34 covers the frequency range from 30 MHz to 300 MHz; both with biconical antennas. For purposes of this paper, it was assumed that CISPR 16-4-2 would be able to cover up to 300 MHz with the same Emission Measurement Uncertainty values as LAB 34.

It should be noted that the examples in both LAB 34 and CISPR 16-4-2 use typical values in their examples; an EMC Lab must generate its own measurement uncertainty values from calibration certificates, equipment manuals, or from a series of measurements for a statistical analysis (Type A).

Figure 4: Summary of Emission Measurement Uncertainty Values

Other Material Covered in the Two Documents
LAB 34
LAB 34 has the following examples of typical uncertainty budgets:

1. Conducted Disturbances (Emissions) – 9 kHz to 150 kHz using 50 ohm/50 microhenry Artificial Mains Network
2. Conducted Disturbances (Emissions) – 150 kHz to 30 MHz using 50 ohm/50 microhenry Artificial Mains Network
3. Discontinuous Emissions from 150 kHz to 30 MHz using 50 ohm/50 microhenry Artificial Mains Network
4. Radiated Field Strength – 30 dBuV/m to 60 dBuV/m –Biconical Antenna – 30 MHz to 300 MHz –Vertical Polarization at 3 meters and 10 meters
5. Radiated Field Strength – 30 dBuV/m to 60 dBuV/m –Log Periodic Antenna – 300 MHz to 1000 MHz –Vertical Polarization at 3 meters
6. Radiated Field Strength – 30 dBuV/m to 60 dBuV/m –Log Periodic Antenna – 300 MHz to 1000 MHz –Vertical Polarization at 10 meters
7. Disturbance Power – 30 MHz to 300 MHz
8. Electrostatic Discharge – Negative Discharge Current, Negative Discharge Voltage, and Negative Rise Time
9. Radiated Immunity – Re-Establishment of Pre-Calibrated Field Level and Dynamic Feedback Field Level
10. Conducted Immunity – Re-Establishment of Pre-Calibrated Conducted Field Level and Limiting of Pre-Calibrated Conducted Voltage Level by Monitor Coil
11. Internal Calibration – Insertion Loss Uncertainty Budget

LAB 34 also has an Appendix B which calculates a kp when random errors in a measurement-system are comparable in magnitude to the systematic errors. kp is a coverage factor greater than 2 in order to assure a 95% level of confidence. Also, Appendix C is a discussion of calculation of uncertainty in logarithmic or linear quantities.

LAB 34 also has a discussion of Compliance with Specification in Section 4. It outlines different scenarios of meeting a specification when using “uncertainty intervals.”

CISPR 16-4-2
CISPR 16-4-2 has the following examples of typical uncertainty budgets:

1. Conducted Disturbances (Emissions) from 9 kHz to 150 kHz using a 50 ohm/50 microhenry + 5 ohm Artificial Mains Network
2. Conducted Disturbances (Emissions) from 50 kHz to 30 MHz using a 50 ohm/50 microhenry Artificial Mains Network
3. Disturbance Power Measurements – 30 MHz to 300 MHz
4. Horizontally polarized radiated disturbances from 30 MHz to 200 MHz using a biconical antenna at a distance of 3 meters, 10 meters, or 30 meters
5. Vertically polarized radiated disturbances from 30 MHz to 200 MHz using a biconical antenna at a distance of 3 meters, 10 meters, or 30 meters
6. Horizontally polarized radiated disturbances from 200 MHz to 1000 MHz using a log-periodic antenna at a distance of 3 meters, 10 meters, or 30 meters
7. Vertically polarized radiated disturbances from 200 MHz to 1000 MHz using a log-periodic antenna at a distance of 3 meters, 10 meters, or 30 meters

In Annex A of CISPR 16-4-2, there is an excellent discussion of the input quantities for the examples of measurement uncertainty in 16-4-2.

Table 1 of CISPR 16-4-2 has Values of Ucispr which are referenced by other CISPR standards. These values are 4.0 dB for conducted disturbances on the mains port from 9 kHz to 150 kHz and 3.6 dB from 150 kHz to 30 MHz. Also, 4.5 dB is the value for disturbance power from 30 MHz to 300 MHz. Finally, radiated disturbance (electric field strength on an open area test site or alternative test site) for 30 MHz to 1000 MHz is 5.2 db. The radiated disturbance number is the largest of the radiated disturbance values of 5.06, 4.95, 5.18, 4.95, 5.04, 4.94, 5.05 and 4.95 in the eight examples in CISPR 16-4-2.

At the present time, the two CISPR standards that call out CISPR 16-4-2 and its Ucispr are CISPR 11, Edition 5.0 (2009-05) and CISPR 22, Edition 6.0 (2008-09).

In CISPR 11, it is covered in Clause (Paragraph) 12.5 (Measurement Uncertainty). It says the following: “Determining compliance with the limits in this standard shall be based on the results of the compliance measurements taking into account the considerations on measurement instrumentation uncertainty. Where applicable, measurement instrumentation uncertainty shall be treated as specified in CISPR 16-4-2.”

In CISPR 22, it is covered in Clause (Paragraph) 11 (Measurement Uncertainty). It says the following: “The results of measurements of emission from Information Technology Equipment (ITE) shall reference the measurement instrumentation uncertainty considerations contained in CISPR 16-4-2. Determining compliance with the limits in this standard shall be based on the results of the compliance measurement, not taking into account measurement instrumentation uncertainty. However, the measurement uncertainty of the measurement instrumentation and its associated connections between the various instruments in the measurement chain shall be calculated and both the measurement results and the calculated uncertainty shall appear in the test report.”

Summary
It can be seen that there is a close correlation between the two EMC Measurement Uncertainty documents discussed in this paper. Both LAB 34 and CISPR 16-4-2 can be used by EMC Labs as reference documents for their lab operations, lab measurement uncertainty calculations, and for accreditation purposes. As more CISPR, regional, and national standards adopt Measurement Uncertainty criteria, the two subject documents will become increasingly important for an EMC Lab.

Dan Hoolihan is the president of Hoolihan EMC Consulting and can be reached by e-mail at Danhoolihanemc @aol.com.