A method for establishing RF power traceability below 10 MHz

The calibration of RF power measuring devices such as power sensors, ensures the establishment of traceability of RF power and the achievement of lower measurement uncertainty. Power measurement and calibration at high frequencies are less complicated than those at low frequencies. The reason lies in the characteristic impedance, which is complex and frequency-dependent at low frequencies, while at several megahertz, it becomes purely resistive. This purely resistive impedance enables direct power measurement, rather than the indirect measurement through voltage and current, typically used at low frequencies. This is why the equipment and methods used at high frequencies are fundamentally different and simpler. Consequently, RF power standards, typically microcalorimeters, thermistor power sensors, and power transfer standards, are primarily designed for frequencies above a few megahertz, often starting at 10 MHz. As a result, most metrological laboratories that calibrate measuring equipment in the radio-frequency and microwave ranges cover the range from 10 MHz to 18 GHz, or even up to 26.5 GHz. While there are some standards in the market available for low frequencies, procuring high-price standard to cover a narrow frequency range is unreasonable and expensive for most laboratories. Unlike older-generation power sensors, which do not cover frequencies below 100 kHz, there is an increasing number of contemporary sensors that can measure power from several kilohertz, or even from DC. Therefore, the issue of power sensor calibration in the low-frequency range arises, and the solution is proposed by the method presented in this paper. This method overcomes the problems of radio-frequency power calibration below 10 MHz, which are widespread in many calibration laboratories. These problems are related to the specificity of power calibration and the absence of a typical RF power standard and network analyzer in the low-frequency range. One of the primary goals of the study was to develop a method that could be applied in most metrology laboratories in the field of RF domain. The proposed method focuses on the calibration of RF power sensors as typical RF power measuring devices and the determination of their accuracy, specifically their calibration factor below 10 MHz. The crucial points in the study included the selection of an appropriate RF power standard in the low-frequency range and the realization of a measurement procedure that provides an acceptable measurement uncertainty. The traceability of RF power is established using the alternating voltage standard, specifically, the thermal converter. The method employs a resistive power splitter, the symmetry of which is determined by an alternative method involving a thermal voltage converter. The determination of symmetry significantly reduces the total measurement uncertainty by more than six times. This method ensures users the traceability of RF power within the low-frequency range and the reduces the uncertainty of RF power measurements by sensors by up to seventy percent, by applying correction factor obtained during calibration.