Zero-compensation method and reduced inductive voltage error for the AC Josephson voltage standard
Kunli Zhou, Jifeng Qu, Samuel P. Benz
In the pulse-driven ac Josephson voltage standard, the low frequency component of the drive signal increases the system complexity and induces unwanted voltages through the on-chip inductances. A novel zero-compensation method for pulse-driven ac waveform synthesis is presented in this paper. The pulse train is obtained through two-level Δ-Σ modulation and then reconstructed by replacing each pulse with three pulses of negative-positive-negative pattern. The amplitudes of the reconstructed pulse train are carefully adjusted so that the positive pulses drive the Josephson junctions to the first Shapiro step, while the negative pulses keep the junctions on the zero Shapiro step and cancel the contribution of the positive pulses to the low-frequency inductive- voltage component. This new method eliminates the need for low-frequency signal reinjection required by the conventional ACJVS bias methods and also eliminates the need for dc current bias required by the recently proposed zero-compensation method. An 8 kHz sinusoidal waveform of 1 mV rms amplitude is synthesized to demonstrate the feasibility of the approach. We also demonstrate that the inductive voltage error is significantly reduced for waveforms synthesized with the new method.