Josephson Array Standard Calibrations without Frequency Synchronization

The standard technique for the calibration of a voltage reference by means of a Josephson array voltage standard (JAVS) is based on the synchronization of the frequency of the microwave irradiated onto the array, typically by means of a phase locked loop. This guarantees a very accurate and stable output voltage from the Josephson array and a quite simple frequency measurement. However, the accuracy level provided by the phase lock method is not required in the calibration of electronic standards, since their intrinsic noise sets the minimum achievable calibration uncertainty several orders of magnitude above the contribution of the frequency uncertainty for a synchronized source. An alternate technique is reported in the following that makes use of the voltage difference between the array and the reference under calibration to control the array frequency. The calibrations performed with the new method agree with the results obtained with the usual one within few parts in 10^-8