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HF voltage

Working Group 2.22


HF voltage measurements are restricted to coaxial lines. An important issue with respect to reproducibility is a well-defined reference plane within smallest fractions of the wavelength. Therefore, the upper frequency limit is at a few GHz. As in case of HF power measurements, the PTB's standard measuring method is based on d.c. substitution. The HF voltage to be measured causes a temperature increase of an ohmic resistor, which is proportional to the dissipated power. The latter is replaced by a d.c. voltage causing the same power absorption, i.e. rms HF voltage is measured.

The HF voltage standard is a d.c.-coupled HF power sensor with frequency-dependent input conductance determined by calibration. Its frequency-dependent effective efficiency is measured in a microcalorimeter. The voltmeter to be calibrated is connected in parallel with this HF voltage standard using a T-junction. The reference plane for the voltage measurement is the connector front plane at the sensor input (reference plane for the admittance measurement). Special electrically short T-junctions have been developed for both different transmission line and connector systems. The distance between the plane at which the standard and device under test are connected and the symmetry plane of the junction (where the HF signal is coupled in) is very small compared with the wavelength. Thus, measurement uncertainties due to the voltage standing waves are eliminated up to the upper frequency limit of 2 GHz.

Calibration of a HF thermal converter (nominal voltage 1V), left-hand side, with integrated T-junction using the PTB HF voltage standard (right-hand side). The frequency response of the HF voltage is measured between 100 kHz and 1 GHz.

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The HF voltage measurement setups are continuously enhanced.

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