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RF Field Probe Calibration and Usage

RF field probes are a required piece of test equipment used for measuring the intensity of radiated RF fields. The use of a field probe may seem straightforward; however, there are numerous aspects of using field probes that can affect the accuracy of RF measurements. Probe mounting positioning and orientation, and sources of additional field contributions should be considered when making field measurements. Understanding these factors is important to allow achieving the best accuracy in field measurements.

Calibrations and Correction Factors are also important concepts to understand to achieve accurate field probe measurements. Improper, or lack of use of Correction Factors, will affect the accuracy of the field measurement. Since the field probe is used to establish the field intensity, poor measurements could lead to over-testing of the test object, where the test object is subjected to higher fields than intended. Conversely, poor measurements could cause the test object to be under-tested, where the test object is tested to field intensities less than required by the test standard. Understanding the factors that affect field probe accuracy is crucial to achieving the best results in making RF field measurements.


Field Probe Calibration


Calibration Overview

Ideally, a field probe would have a flat response curve across its rated bandwidth; however, the reality is that there will be frequency-dependent variations that deviate from nominal across the band. Calibration by a qualified calibration lab is necessary to determine the compensation needed to correct for these variations of the probe response. The calibration lab will measure the probe output in an RF field of known intensity. It will then determine the Correction Factor (CF) necessary to correct the probe field measurement at this frequency. This process will be performed at multiple frequency steps across the band, resulting in a table of CFs at multiple frequencies. CFs are an important aspect of probe usage and will be discussed in greater detail in a separate section.

Other probe characteristics may be part of the probe calibration, dependent upon calibration lab and/or customer requirements. These can include isotropy, the measurement of variation when the probe is rotated about an axis, linearity, the measurements at various field levels, or various combinations.

Probe calibration should be performed regularly. This interval is ultimately determined by the customer’s metrology or quality policy, based on the probe manufacturer’s recommended calibration interval. In the absence of a defined requirement, annual calibrations are commonly employed.

Calibration Lab Accreditation

When discussing probe calibration, references to ISO/IEC 17025 or NCSL/ANSI Z540 are often encountered. These two terms refer to standards of technical requirements for calibration labs. These standards provide guidelines and requirements for calibration labs to follow to standardize processes and procedures in performing calibrations to ensure consistent results. While very similar, these two standards do have some differences and as a result, some calibration labs may conform to one over the other or both.

Labs can self-declare conformance to these standards. However, many customers will require that their probe calibrations be performed at calibration labs accredited to one of these standards by a third-party accreditation body. Accreditation bodies such as A2LA, ANAB or NVLAP (in the US), among others, provide such accreditation services. Accreditation is a formal third-party auditing of the calibration lab’s processes and procedures to ensure that the lab conforms to a standard's requirements. Calibrations at accredited labs are often referred to as an ‘accredited calibration’ or ‘A2LA calibration’, however these phrases are not technically correct as the calibration itself is not what is being accredited. These calibrations would more accurately be described as ‘a calibration performed by a lab A2LA-accredited to ISO/IEC 17025’, as an example.

Accreditation bodies exist worldwide providing calibration lab accreditation to the same standards (such as ISO/IEC 17025); however, customers are unlikely to be familiar with these bodies outside of their home countries. This situation led to the International Laboratory Accreditation Cooperation (ILAC) establishment to develop international cooperation for facilitating trade by promoting the acceptance of accredited test and calibration results. Accreditation bodies signatories to the ILAC establish a Mutual Recognition Arrangement (MRA), which allows an accredited calibration to be accepted as comparable and acceptable between accreditation bodies, thus easily allowing the international flow of calibrated products.

Additionally, a calibration can be classified as ‘NIST-traceable’ (National Institute of Standards and Technology). In the United States, a NIST-traceable calibration is a calibration that has been carried out with equipment whose calibration can be traced back to a National Institute of Standards and Technology (NIST) calibration. Note that this refers to the lab’s equipment’s calibration, not the field probe's calibration. Other countries may have their own nationally recognized calibration lab for traceability, for example, PTB in Germany or NPL in England. Traceability to a national or international standards lab is a requirement of ISO/IEC 17025.

Calibration Lab Measurement Uncertainty

When reviewing calibration results, the user must be aware of and keep in mind the Expanded Measurement Uncertainty (EMU) of the lab, which will be listed on the probe calibration certificate from an accredited lab. EMU is provided in a ±dB or % format and describes the accuracy range of measurements, with the probe calibration data falling within the EMU of the true value. The EMU is determined by considering many possible sources of variations involved in creating a known RF field level and measurement of the field and applying statistical analysis. The EMU may be defined in multiple frequency bands dependent upon equipment used. The calibration lab EMU has a trickle-down effect and impacts the EMC lab's measurement uncertainty using the field probe.

The magnitude of the EMU may be surprising when first encountered to those not familiar with field probe calibrations; however, this band-of-uncertainty reflects the realities of working with field probes and needs to be kept in mind. All measurement equipment has some amount of tolerance or uncertainty in a measurement, although field probe uncertainties are of a larger magnitude than what is typically encountered with other measuring equipment. Field probe uncertainties of ±0.5db to ±1.0dB, or higher, are not uncommon.

Repeatable field-level accuracy is inherently challenging. Reflection, refraction, and antenna near field effects combine to aggravate the uncertainties. Some probe calibrations are performed in a Transverse ElectroMagnetic (TEM) cell. A TEM cell can provide a more predictable and uniform field than free-space or semi/fully anechoic chamber environments. However, even these have a degree of uncertainty and are limited to their rated frequency ranges.

More details of field probe calibrations and EMU calculations can be found in IEEE-1309 Standard for Calibration of Electromagnetic Field Sensors and Probes.


Field Probe Correction Factors


Correction Factor Basics

A theoretical ideal field probe would provide an exact reading of the test field intensity across its entire frequency range. In the real world, however, probe response varies at different frequencies. Correction Factors (CFs) need to be applied to the field value reported by the probe to compensate for this. When applied, the effect is to flatten the probe frequency response across the entire frequency range to minimize errors. CFs are generated by a calibration lab and will consist of a table of frequencies and the correction to apply at each frequency. CFs can be expressed as a linear multiplier and/or a dB adjustment value, depending on the calibration lab's format. If plotted, the CFs yield a curve that is the inverse, or mirror image, of the probe's frequency response curve.

For separable-axes probes, CFs are provided for each axis independently (X, Y, Z) for each defined frequency. For composite-only probes, a single CF is provided for each defined frequency.

It is imperative for the user to take steps to ensure CFs are being used. Probe accuracy depends upon the application of the CFs generated by the calibration lab, and neglecting to use this data may cause substantial errors depending on the magnitude of the unused CF.

It is up to the user to determine how and where to apply CFs. This is commonly accomplished by entering the CF data into automated control software, where the software will then apply the proper CF as the test progresses. Alternatively, CFs may be loaded into an external field monitor or probe, depending on equipment functionality. Regardless, it is up to the user to ensure CFs are properly loaded and enabled. It is important to ensure CFs are applied at only a single location, not in the automated control software and an external device, which would cause the CFs to be applied twice.

Field probes are broad-band devices, and as such, are not able to discriminate the operating frequency. However, CFs are frequency-dependent, thus, to apply the proper CF, the test frequency must be known. When using automated control software, the software will control the test frequency at the time of field measurement and, therefore, apply the appropriate CF when configured to do so, interpolating between CF frequencies, as necessary. If using an external field monitor or probe system with CF capability, the equipment will need to know the frequency. The test software can accomplish this by communicating the test frequency to the equipment, or the operator can enter the frequency manually if operating in a stand-alone setup.

A CF as supplied by the calibration lab will be in the form of a linear multiplier and/or a dB adjustment value at each calibration frequency, depending on the lab. Using a multiplier is the more common and simpler method to implement, as the multiplier can be directly applied to the V/m field level reported by the probe.

Applying Correction Factors

When CFs are loaded into control software or equipment and properly enabled, the correction will occur automatically, and no manual user intervention is required. However, it is helpful to understand how CFs are used, so the process of manually applying CFs as a multiplier and calculating the composite field is described below.

When working with a composite-only probe, apply the CF multiplier for the operating frequency to the probe-reported composite field, resulting in a final corrected composite field value. Example below:

$$ ReportedField*CF=CorrectedField $$ $$ 14.7V/m*1.09=16.02V/m $$

When working with separable-axis probes, individually apply the axis-specific CF at the operating frequency to each axis-reported field value (X, Y, Z). Example below:

X-axis    13.9V/m*1.1=15.29V/m
Y-axis    1.6V/m* 0.95=1.52V/m
Z-axis    2.2V/m*1.04=2.29V/m

Calculating Composite Field

On separable-axis probes, an additional step is needed to calculate the final composite field. After CFs have been individually applied to each axis, the composite field measurement is calculated by combining the corrected field measurements from the three axes using a Root Sum of Squares (RSS) calculation as follows:

$$ CorrectedCompositeField=\sqrt{x^2 + y^2 + z^2} $$ Where:
‘CorrectedCompositeField’ is the final corrected composite field level in V/m
X is the corrected X-axis reported field level in V/m
Y is the corrected Y-axis reported field level in V/m
Z is the corrected Z-axis reported field level in V/m

Example of calculating a composite field using the individual corrected field values from the earlier example:

$$ \sqrt{15.29^2 + 1.52^2 + 2.29^2 = 15.54V/m} $$

Interpolating Correction Factors

The Correction Factor table supplied by the calibration lab provides CFs for discrete frequencies. When operating at frequencies between those provided in the probe CF calibration table, the recommended solution is to linearly interpolate between the adjacent frequencies to determine the CF as follows:

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

Where:
$$ a_x\:\mathit{is}\:{the}\:{desired}\:{CF}\\ $$ $$ a_0\:\mathit{is}\:{the}\:{first}\:{known}\:{CF}\\ $$ $$ a_1\:\mathit{is}\:{the}\:{second}\:{known}\:{CF}\\ $$ $$ f_x\:\mathit{is}\:{the}\:{frequency}\:{of}\:{the}\:{desired}\:{CF}\\ $$ $$ f_0\:\mathit{is}\:{the}\:{first}\:{known}\:{frequency}\\ $$ $$ f_1\:\mathit{is}\:{the}\:{second}\:{known}\:{frequency}\\ $$

Alternate applications of Correction Factors

Occasionally situations may arise where the format required to apply CFs does not match the available CF data.

If using a separable-axis probe that will have individual XYZ CFs per frequency but need to enter only a single CF (e.g., test software allows only a single CF per frequency), average the three CF multipliers:

$$ CF_{avg}=\frac{X_{cf}+Y_{cf}+Z_{cf}}{3} $$

An example of calculating an average CF multiplier from individual CF multipliers:

$$ \frac{1.1+.95+1.04}{3}=1.03 $$

In the opposite scenario, using a composite-only probe that will have only a single CF per frequency but need to enter XYZ corrections (e.g., test software requires entering individual XYZ CF’s), use the same CF for all axes.


Field Probe Positioning, Orientation, and Mounting


Probe Positioning

RF fields radiating from an antenna may be reflected if encountering a metallic object, such as the Unit Under Test (UUT). These reflected fields can interact with the field radiating directly from the antenna, resulting in spatial areas of high and low field levels. These regular patterns of high and low field levels are called ‘standing waves.’ Standing waves can occur at any location where RF fields are reflected, although the effect is more likely to occur in front of the UUT, where reflections can more easily interact with the radiating field. When measuring field levels in close proximity to the UUT, experiment with various locations to find a configuration that provides reliable measurements.

In non-anechoic chambers or anechoic chambers with less-than-optimal performance, standing waves may also be present, affecting optimal field probe placement. In such applications, it may be appropriate to dampen the room resonance with strategically placed RF absorbers.

Field probe measurements can be affected by items in the probe's vicinity, such as the UUT, or if multiple probes are being used simultaneously. Measurements taken under these conditions may not provide a true measurement of the intended radiated field being generated. As a rule of thumb, maintain a minimum distance of 9 inches (23 cm) between adjacent probes. The minimum distance between a probe and a UUT will be dependent on the UUT and, therefore, must be determined by experimentation.

Probe Orientation

When mounting the probe, orient it such that the probe’s metal housing is not causing reflections. This is primarily an issue with stalk-type probes with a square housing separate from the probe head. The housing body should not be located directly behind the probe head as this can cause a reflection resulting in a standing wave at the probe head. This reflection becomes more of an issue at higher frequencies in the gigahertz range.

Due to an isotropic field probe's orthogonal design, one would expect field readings to be unaffected by the field probe’s orientation. However, RF field measurement’s reality is that isotropic probes may exhibit some variation depending on the probe orientation, described as isotropic deviation. This value provides the variation that could occur as a function of probe orientation to the field. As the probe's accuracy depends on the use of the CFs generated at the calibration lab, the effects of Isotropic Deviation can be minimized by aligning the probe in the same manner as was used for calibration, which will include aligning one of the axes to the field.

Figure 3: Side View: Z-Axis Aligned with E Field

Subjecting a test object to both horizontally and vertically polarized E-fields is a common requirement of many EMC test standards. To accommodate this, some users will mount the probe such that one of the probe’s axes is horizontal; another axis is vertical, with the third axis pointing toward the transmitting antenna. See Figures 3 and 4, showing the mounting angles and orientation.

With the probe mounted in this manner, if the antenna is rotated between horizontal and vertical polarization, the radiated field will be aligned with one of the probe axes in each polarization orientation.

Figure 4: TOP View: Z-Axis Aligned with E Field

Note that orienting the probe in this manner, although convenient for the user, may differ from the orientation used when the probe was calibrated. The user should be aware of and understand that aligning the probe to the field in an orientation that differs from the orientation used when the probe was calibrated may increase the uncertainty of measurements.

Review the test standards being used for requirements or recommendations on how the probe is to be oriented. For best accuracy, the probe should be used in the same orientation to the field as was used when the probe was calibrated.

Probe Mounting

Always mount a probe on a nonconductive (nonmetallic) stand/support using only nonconductive (nonmetallic) hardware, including screws. It is imperative to keep conductive objects away from a field probe. Any such objects in the proximity of the probe may distort the field and compromise measurement accuracy.


Field Probe Usage Consideration


Field probes are broadband devices, and as such, will report the total RF field energy detected. The RF field energy includes the expected fundamental frequency during the test, harmonics of this frequency, and other environmental signals. In EMC testing, the expectation is that the carrier frequency is the primary RF energy source. The user should perform testing and/or calculations of the test setup and environment to understand the magnitude of any ambient signals and harmonics to ensure they are at acceptably low levels. Many test standards define specific testing and verification processes for these characteristics, so it is always important to consult the test standard for any requirements.

Emissions from the UUT can contribute to the field measured by the probe. Where active field monitoring is to be used during UUT operation, it is recommended to first operate the UUT and test equipment together with the probe, however without a test field to determine any ambient field. The ambient field level should be low enough such that the error contribution to the field measurement is consistent with the application requirements.

Broadband or wideband noise energy from an amplifier can also contribute to the RF field in situations such as testing at low field levels using a substantially oversized amplifier. TWT amplifiers are especially prone to producing noticeable broadband noise power, as well as harmonic content. Although the noise is at a low noise power density level, the noise from broadband RF sources accumulates over the probe's broad frequency range. This situation would most often be seen if operating an amplifier at a small percentage of its full power. Operating the amplifier with no input signal and measuring the field can help detect amplifier noise power contribution to the measured field. Using space loss or other attenuation can help alleviate this situation.

Out-of Band Response

Although field probes have a specified operating frequency range, they may respond to frequencies above and below their specified range. Users should be alert to unexplained readings that may be caused by unintended fields, whether in-band or out-of-band. Pay special attention to fields generated by a UUT or by test equipment located very close to a probe, including AC power lines and power supplies.

RMS vs. Peak

RF field probes are designed to measure the root mean square (RMS) value of the field. This is true regardless of if the probe is a CW or a modulated probe, as the probe is responding to and measuring the RF carrier.

When working with pulsed signals, signal levels are often referred to as the ‘peak’ level; however, this would more accurately be described as the ‘RMS level of the pulse.’ The instantaneous peak of an RF field can be calculated in the same manner as a peak value can be calculated of any cyclical signal, although this peak value is not used in EMC test standards.

Operation in TEM Cell

An ideal field probe would measure the RF field without having any effect on the field. Field probes, however, are physical objects, and by necessity, include conductive parts and thus will affect the field to some degree. Probes with smaller bodies and antennas reduce field distortion caused by the probe.

This sizing factor becomes an important consideration when using a probe in a confined space, such as in a small TEM cell. Since probes have metallic content, when a probe is in a TEM cell, the probe's metallic portions can act to partially ‘short’ the field, thus causing more significant disruption and effect on the field than when measuring a field in a free-space environment. The user should be aware of this effect when operating in a TEM cell. The probe's metal body is recommended to be no more than 1/3 of the septum-to-body measurement when using a probe in a TEM cell.

Conclusion

Field probes are an important part of a radiated immunity test setup. A field probe will report the field it detects at its location; however, this field can be influenced by outside factors as discussed. Things such as probe calibration, use of Correction Factors, probe mounting and orientation, and other RF energy in the area can all contribute to the accuracy of the field level measurement. Accurate field measurements are important to avoid over or under-testing the UUT. Being aware of the external factors affecting field measurements, and taking the steps necessary to evaluate each, is important to achieving accurate measurements.