Often underestimated, RF field probes are critical to the implementation of a proper radiated immunity test system. All too often, system specifiers gloss over this essential element after having spent a considerable amount of time and energy selecting components required to "generate" the required RF field. After all, what good is an RF field if you can't reliably measure it?
In an effort to simplify the process of probe selection, this application note will focus on the salient specifications of RF field probes. Given a thorough understanding of how RF field probes are specified, one can then make informed decisions as to which probes are best suited for a particular application.
RF Field Probes Specifications
Frequency Response is undoubtedly the most important probe characteristic. It is defined as the frequency range the probe will respond to. Since no probe can provide a completely flat response across the entire frequency range, this spec is always accompanied by a tolerance figure, generally provided as a ±dB allowable variation band. An example of a typical frequency response is shown in Figure 1. The frequency response shown in Figure 1 is that of an actual probe designed to cover a heavily used frequency range. If the probe does not cover the entire frequency range of the test application, multiple probes may be required.
Calibration Factors are supplied with every probe and should be updated on a periodic basis, usually once a year. The calibration factors yield a curve that is the inverse, or mirror image, of the frequency response curve. These corrections are provided in terms of dB adjustments and as multiplication factors. When applied, the effect is to flatten the probe frequency response across the entire frequency range to minimize errors.
Calibration factors are usually provided for each individual axis as well as for the composite reading. Maximum field measurement accuracy is achieved when the detailed 3-axis calibration is applied. Since measurements are never absolute, calibration labs issue a statement of uncertainty that lists the anticipated error range (±dB or %) for their measured data. The calibration lab measurement uncertainty has a trickle
down effect and impacts the measurement uncertainty of the EMC lab using the field probe.
Calibrations can be offered in two different versions:
- In the USA, a NIST traceable calibration is a calibration that has been carried out with equipment that can be traced back to a National Institute of Standards and Technology (NIST) calibration. Other countries may have their own nationally recognized calibration lab for traceability; for example, PTB in Germany and NPL in England.
- An ISO 17025 accredited calibration is the newest form of calibration. It requires the calibration lab be held to very high quality standards. To comply, the lab must be audited and carry a certificate of conformance from a recognized body, such as A2LA or NVLAP in the United States. To be accepted worldwide, the recognized body must have mutual recognition agreements (MRAs) that facilitate mutual recognition of test reports. This calibration is also NIST traceable.
Sensitivity/Dynamic Range: Sensitivity determines how small an RF signal a probe can respond to accurately. The sensitivity of RF probes is especially important when low field strengths need to be measured. Some specs call for a field level of 1V/m or even less, which may be below the sensitivity of many probes, or very close to its noise floor. The most sensitive probes can accurately measure a few hundred mV/m.
Dynamic Range is the total range of RF field coverage a probe will respond to. The greater the dynamic range the better a probe is suited to address test applications that span the gamut from low to high field strengths. Example: 0.5 to
800V/m for 0.5 MHz to 6 GHz and 1.2 to 800V/m for 100 kHz to 0.5 MHz.
Linearity is the measure of deviation from an ideal response over the dynamic range of the probe. Linearity data is provided since the response of an RF probe will vary somewhat as a function of the applied field level. This slight variation introduces an additional error component that must be considered when testing at levels other than that used during calibration. For example, one might encounter a variation of ±0.5dB across a dynamic range of 0.5 to 800 V/m.