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RF voltage measurements are restricted to coaxial connectors. 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. Similar to RF power measurements, the PTB's standard measuring method is based on d.c. substitution. The RF 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 RF voltage is measured. The RF voltage standard is a d.c.-coupled RF 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 RF voltage standard using a T-junction. Here, the reference plane for the voltage measurement is the connector of the sensor input (reference plane for the admittance measurement).
Calibration of a RF thermal converter (nominal voltage 1V), left-hand side, with integrated T-junction using the PTB RF voltage standard (right-hand side). The frequency response of the HF voltage is measured between 100 kHz and 1 GHz. |
The transfer standard shown in the picture utilizes an integrated T-junction, thus the reference plane is the connector interface. For transfer standards without this feature, an additional T-junction is used, and the reference plane is the center of this T-junction, and the length from the center of the T-junction to the reference plane of the DUT is reported in a calibration certificate. Thus, measurement uncertainties due to the voltage standing waves are reduced but not completely eliminated and the upper frequency limit (without any kind of mismatch correction) is approximately 2 GHz.
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Calibration and Measurement Capabilities for HF Voltage
(PDF; Chapter 11 Radio frequency measurements; 11.7 Radio frequency voltage and current)
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Publications relating to HF voltage measurement
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Susanne Weimann
Phone: +49-531-592-2434
E-Mail: Susanne Weimann