Development of a Measurement Procedure for the Calibration of Reference Capacitors

An important part of the Metrology department’s activities is the development of new measurement procedures. As part of our research and development, we have created a new procedure for calibrating reference capacitors at frequencies up to 30 MHz.

For many years, the Metrology department has been performing accredited calibrations of LCR meters. LCR meters are instruments used to measure the inductance, capacitance, and resistance of passive electronic components. In the past, most LCR meters operated up to a measurement frequency of approximately 1 MHz. However, the evolving needs of the industry have led to the development of a new generation of LCR meters capable of operating at frequencies exceeding several tens of MHz. Calibration laboratories are keeping pace with this technological advancement by developing accredited procedures that ensure traceable calibrations even for the most advanced instruments.

To calibrate an LCR meter, the values of the reference measurement standards used in the calibration must be known with high accuracy and traceability. To this end, we use reference resistors, capacitors, and inductors, which we used to send abroad for calibration. Calibration of reference capacitors at higher frequencies is particularly demanding and time-consuming, and as such, it is carried out by only a limited number of national metrology institutes worldwide. Due to long calibration lead times and the

risks associated with international transport, we decided to develop our own calibration procedure for reference capacitors.

We began the development of the new measurement procedure with a thorough review of relevant scientific literature. Several methods suitable for capacitor calibration were identified. We selected the one that offered the lowest measurement uncertainty and could be implemented using the measurement equipment available in our laboratory. The procedure employs a theoretical model of a reference capacitor, whose capacitance at higher frequencies depends on the values of nine elements that constitute the model. The values of six of these elements were determined directly from measurements and calculations at lower frequencies. The remaining three were derived indirectly from high-frequency measurements using a vector network analyser and computer simulations.

Since capacitance values derived from high-frequency measurements are subject to considerable uncertainty, we numerically simulated the values of the three unknown parameters and adjusted the model based on the high-frequency data. This step, not previously described in scientific literature, represents our original contribution to the field and enables a significant reduction in the overall uncertainty of the final capacitance value.

After its development, the procedure was validated through comparison with a foreign national metrology institute. This enabled us to obtain accreditation for the calibration of reference capacitors up to 30 MHz, with internationally comparable low measurement uncertainty.

With this newly developed procedure, we can now offer our clients accredited calibration of their reference capacitors and more efficiently provide calibration services for LCR meters.