Method for Ensuring Accurate AC Waveforms With Programmable Josephson Voltage Standards

The amplitudes of stepwise-approximated sine waves generated by programmable Josephson voltage standards (PJVS) are not intrinsically accurate because the transitions between the quantized voltages depend on numerous conditions. We have developed a method that ensures that the total rms output voltages of arbitrary ac waveforms synthesized by the PJVS are accurately referenced to the quantized Josephson voltages. This is accomplished by digitizing the output waveform, utilizing the quantized voltages to correct digitizer gain, noise, and nonlinearity, and then utilizing measurements of the bandwidth, rise time, and harmonic content to precisely tune the PJVS bias parameters. Our goal is to develop an AC standard source that can directly synthesize voltages with the accuracy expected of a quantum-based standard without the use of a thermal voltage converter.