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Application of Field Probe Correction Factors

To better achieve accurate field-level measurements, it is helpful to have an understanding of field probe calibrations, the factors generated, and what’s presented as data during testing. Previous application notes have dealt with field probe selection, proper use, and configuration. This application note shows the customer how to utilize the factors presented for inclusion into test data, how to generate composite measurements, and why calibrations are performed at selected frequencies.

Field Probe Measurements and Correction Factors

Upon purchase, Amplifier Research supplies correction factors with each of our field probes. As the manuals state, the field probe factors are given in both dB and linear multiplier values. For each probe, a 3-axis correction factor is given.

For many EMC test standards, only a composite value is required. To calculate the composite field value, a simple root-sum-of-squares calculation of the field values measured in each axis is performed.

$$ Measurement_{Composite} = √(x^2+y^2+z^2) $$

Where: x, y, and z are the individual axis measurements for the specific field probe

For each calibration frequency, the three (3) separate axis factors are given. It is up to the user to calculate the composite value using a script or spreadsheet.

Correction Factor Example

The given example shows the outcome of the composite measurement as derived from the three individual axis measurements and factors:

Linear Correction Factors:
Frequency (MHz)	X-axis	Y-axis	Z-axis
80.0	0.99	0.98	0.99

Measurement Results (V/m):
Frequency (MHz)	X-axis	Y-axis	Z-axis
80.0	5.86	47.86	1.03

The equation, from above, would show the following (via a root-sum-of-squares calculation):

$$ Measurement_{Composite} = √((0.99*5.86)^2 + (0.98*47.86)^2+(0.99*1.03)^2) $$ $$ Measurement_{Composite} = \sqrt{(2234.56)} V/m $$ $$ Measurement_{Composite} = 47.27 V/m $$

When separable-axis field measurements cannot be used, and a composite factor must be used, a simple way to calculate the composite factor is to take an average of the 3-axis factors:

$$ Factor_{Composite} = \frac{x+y+z}{3} $$

So, for the example above, the composite factor would be:

$$ Factor_{Composite} = \frac{0.99+0.98+0.99}{3} = 0.9867 $$

If the uncorrected measurement above was 48.23 V/m, the corrected composite measurement is:

$$ Measure_{Composite}= 48.23*0.9867 = 47.58 V/m $$

Calibration of AR’s Field Probes

All of AR’s field probes are calibrated by an accredited calibration laboratory in accordance with ISO 17025:2017. The accreditation falls under the scope of the ILAC (International Laboratory Accreditation Corporation) MRA (Mutual Recognition Agreement), and each calibration laboratory is accredited to an approved ILAC signatory. Each calibration is traceable to NIST, the National Institute of Standards and Technology.

For calibrations performed by Amplifier Research, an approved calibration procedure in the form of a Standard Operating Procedure and Work Instruction is performed. When calibrations are performed by an outside vendor, those calibrations are performed by the vendor’s calibration procedure.

The calibration procedures performed are generated with the guidance contained within Annex I (informative) of IEC 61000-4-3, Electromagnetic compatibility (EMC) – Part 4-3: Testing and measurement techniques – Radiated, radio-frequency, electromagnetic field immunity test, and IEEE 1309, IEEE Standard for Calibration of Electromagnetic Field Sensors and Probes (Excluding Antennas) from 9 kHz to 40 GHz.

Application of Calibration Data

To apply the correction factors, refer to the manual of the specific field probe utilized. If the correction factor of the field probe needs to be determined for a given frequency, linear interpolation can be utilized between two frequency points via the following:

$$ a_x = \frac{a_0 (f_1-f_x )+a_1 (f_x-f_0)}{f_1-f_0} $$

Where:
a_x is unknown correction factor amplitude
a₀is first known correction factor amplitude
a₁ is second known correction factor amplitude
f_x is wanted frequency value of correction factor
f₀ is first known frequency
f₁ is second known frequency

This equation assumes a linear relationship between the correction factor and the frequencies of interest.

Conclusion

AR’s Field Probes can measure fields and return data via 3-orthogonal axis or composite values, depending on the application and testing performed. The procedure to calculate the composite factors is straightforward for any technical user. If you would like to learn more, feel free to contact one of our applications engineers at 800-933-8181, [email protected], or visit our website at www.arworld.us.