Besides the traditional SIS Josephson junction arrays which provide the reference for DC voltages, the quantum AC voltage metrology is based on two different systems: the programmable standard (PJVS) and the pulse driven standard (ACJVS). The PJVS generates an AC voltage by timely switching the number of active Josephson junctions. The major drawback of this approach is that the voltage is not preciously defined during the switching process. These transients limit the bandwidth of the system to a kHz. The ACJVS is based on a more complicated approach which is based on a modulation of the microwave frequency driving the Josephson junctions. This system is not plagued by transient voltages and its bandwidth can extend up to a MHz. However, raising its output voltage up to a volt has been a real challenge which necessitated almost 20 years of development.
Recently the idea of using the ACJVS embedded in a Digitally Assisted Bridge (DAB) to perform impedance measurements has emerged.
The major improvements provided by the ACJVS compared to the state of the art are the following:
- -The existing DAB bridge is a multi-frequency bridge that can operate at any frequency between 50 Hz and 50 kHz. The bandwidth of the ACJVS being 1 MHz there is a huge potential to increase the bandwidth and one can think of this bridge as a starting point for the closure of the LF-RF gap.
- The bridge ratio can be set to any desired value, in particular, to 12.9:10. This is important to realise the decadic scale starting from the quantized Hall resistance and represents an important simplification in bridge technique.
- The phase between the two AC sources can be adjusted to any value meaning that any impedance in the complex plan can be calibrated with this bridge. This is an important advance in impedance metrology, since this is not possible with the traditional bridges. In particular, this bridge will replace the cumbersome quadrature bridge which is presently in use to realise the unit of capacitance from the unit of resistance.
The three major improvements mentioned above represent a significant progress in impedance metrology which will revolutionised impedance measurements in the near future.
