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Impedance Metrology based on Josephson Voltage Standards

Conventional Impedance bridges using inductive voltage dividers can achieve uncertainties of a few parts in 109. A major disadvantage of these bridges is the time consuming adjustment procedure, which has to be repeated for every frequency. Furthermore, the inductive dividers limit the bandwidth for precision measurements from 500 Hz to 10 kHz.


PTB’s newly developed impedance bridges simplify the adjustment procedure significantly: Amplitudes of both synthesized voltages are controlled by the microwave frequency and their phase shift can be reduced by delay electronics capable of 1 ps steps. Hence, the automated system can measure 20 frequency points in 30 minutes.

 

Schematic diagram of the quantum impedance bridge

Schematic diagram of the quantum impedance bridge. Variations in microwave frequencies f1 and f2 changes the quantum voltages and thus balancing the bridge. A delay electronic in combination with the synchronized current sources 1 and 2 eliminates any phase difference.


This new method has been tested successfully for 1:1 ratios with 10 kΩ resistors as well as 100 pF capacitors. The new impedance bridge achieved uncertainties better than 2 × 10-8 for resistor calibrations. Uncertainties for capacitors are less than 4 × 10-8 at audio frequencies increasing to 2 × 10-7 at 25 Hz.


Future work will focus on quadrature bridges, which allow for a direct comparison between capacitors and resistors. With a quantum based quadrature bridge, capacitors can be compared directly against a quantum hall resistor over a wide frequency range at the highest level of precision.

 

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