Pulse-driven AC Josephson Voltage Research Articles

Differential Sampling of AC Waveforms Based on a Commercial Digital-to-Analog Converter for Reference.

This paper introduces an innovative differential sampling technique for calibrating AC waveforms, leveraging a commercially available 16-bit digital-to-analog converter (DAC) as the reference standard. The novelty of this approach lies in its enhanced stability over traditional direct sampling methods, especially as the frequency of the AC waveform increases. Notably, this technique provides a cost-effective sampler alternative to the differential sampling methods that rely on a programmable Josephson voltage standard (PJVS). A critical aspect of this methodology is the precise measurement of the DAC's output voltage, for which a static measurement strategy is adopted to utilize the exceptional linearity and transfer accuracy of the Keysight 3458A (Santa Rosa, CA, USA) in its standard DCV mode. The differential sampling method has demonstrated good accuracy, achieving a near 1 µV/V agreement with a pulse-driven AC Josephson voltage standard (ACJVS) across a 40 Hz to 200 Hz frequency range. The method attained an expanded uncertainty (k = 2) of 1 part in 106 while measuring a 0.707107 VRMS sine wave at 50 Hz, showcasing its efficacy in precise AC waveform calibration.

Behavior Modeling of the Amplifier Nonlinearity With the AC Josephson Voltage Standard

Behavior models of electronic components are critical in many metrological and measurement areas, e.g., time-domain simulations for analog-integrated circuit design and predistortion of the amplifier to reduce the distortion. This paper describes the use of pulse-driven ac Josephson voltage standard (ACJVS) to synthesize quantum accurate waveforms with ultralow distortion and noise floor to model the amplifier nonlinearity, which is a promising way to increase the modeling accuracy. Four Volterra series models including the memory polynomial model and three submodels of the dynamic deviation reduction model are analyzed and compared in detail. Normalized mean square error is calculated to evaluate the modeling accuracy. The difference of the measured and predicted outputs of the amplifier are compared in both time and frequency domains. The result shows that high-performance models can be achieved, which demonstrates the feasibility of modeling nonlinear amplifier behavior with the ACJVS.

Mitigating voltage lead errors of an AC Josephson voltage standard by impedance matching

A pulse-driven AC Josephson voltage standard (ACJVS) generates calculable AC voltage signals at low temperatures, whereas measurements are performed with a device under test (DUT) at room temperature. The voltage leads cause the output voltage to show deviations that scale with the frequency squared. Error correction mechanisms investigated so far allow the ACJVS to be operational for frequencies up to 100 kHz. In this paper, calculations are presented to deal with these errors in terms of reflected waves. Impedance matching at the source side of the system, which is loaded with a high-impedance DUT, is proposed as an accurate method to mitigate these errors for frequencies up to 1 MHz. Simulations show that the influence of non-ideal component characteristics, such as the tolerance of the matching resistor, the capacitance of the load input impedance, losses in the voltage leads, non-homogeneity in the voltage leads, a non-ideal on-chip connection and inductors between the Josephson junction array and the voltage leads, can be corrected for using the proposed procedures. The results show that an expanded uncertainty of 12 parts in 106 (k = 2) at 1 MHz and 0.5 part in 106 (k = 2) at 100 kHz is within reach.

AC–DC Calibrations With a Pulse-Driven AC Josephson Voltage Standard Operated in a Small Cryostat

AC–DC measurements have been performed on a commercial thermal transfer standard (TTS) using a pulse-driven ac Josephson voltage standard with a small cryostat enabling shorter voltage leads. A 20 mV RMS output voltage was chosen, close to the maximum value that can be obtained without low-frequency compensation (at a sigma-delta code amplitude of about 10 %) and close to the upper limit of the TTS in its 22 mV range. The measured deviation at higher frequencies, which is due to the length of the voltage leads, was an order of magnitude smaller than was observed before with a normal cryostat. The observed square dependence on the signal frequency and cable length follows exactly the expected behavior. All results agree well with the calibrations performed using a micropot reference system. Furthermore, the influence of other cables and equipment was investigated. The measurement results can be quantitatively understood both in terms of a lumped circuit description and in terms of reflected waves.

Zero-Compensation Method and Reduced Inductive Voltage Error for the AC Josephson Voltage Standard

In the pulse-driven ac Josephson voltage standard, the low frequency (LF) component of a drive signal increases the system complexity and induces unwanted voltages through on-chip inductances. A novel zero compensation (ZC) method for pulse-driven ac waveform synthesis is presented in this paper. A pulse train is obtained through a two-level $\Delta{-}\Sigma$ modulation and then reconstructed by replacing each pulse with three pulses of a 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 quantum state, whereas the negative pulses keep the junctions on the zero quantum state and cancel the contribution of the positive pulses to the LF inductive voltage component. This new method eliminates the need for LF signal reinjection required by the conventional bias method and eliminates the need for a direct-current bias required by the recently proposed ZC method. An 8-kHz sinusoidal waveform with an amplitude of 1 mV rms 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.

Cryocooler Operation of a Pulse-Driven AC Josephson Voltage Standard at PTB

The PTB Josephson Arbitrary Waveform Synthesizer (JAWS) enables the generation of arbitrary waveforms up to voltages of 70 mVRMS (199 mVPP) using two Josephson arrays in series containing 4000 Josephson junctions each. The SNS-like double-stacked junctions are based on NbxSi1-x as barrier material. While the JAWS system is typically operated using a Dewar with liquid helium, the operation in a closed-cycle pulse-tube cryocooler at temperatures around 4.2 K range was here investigated and successfully demonstrated. For this purpose a special designed cryoprobe was used to provide high quality pulses to the Josephson arrays.

Voltage lead corrections for a pulse-driven ac Josephson voltage standard

We present measurements on a pulse-driven ac Josephson voltage standard (ACJVS) for frequencies up to 1 MHz. By automating our setup we were able to perform a systematic study of the frequency dependence of the ACJVS output. We show that the error caused by the voltage leads is the major error source when measuring at frequencies up to 1 MHz. The squared frequency dependence of the error, which is caused by a resonance in the circuit, allows us to make a correction that results in an uncertainty on the µV V−1 level for frequencies up to at least 20 kHz.

AC–DC Transfer Standard Measurements and Generalized Compensation With the AC Josephson Voltage Standard

We present AC-DC transfer standard measurements using the National Institute of Standards and Technology's pulse-driven AC Josephson voltage standard source. We have investigated the frequency dependence for several output voltages up to 200 mV for frequencies from 2.5 to 100 kHz. We found that, as the frequency increases, the ac-dc differences for the two arrays on the same chip do not agree. We explored this deviation in ac-dc difference for the two arrays by investigating different configurations of the probe cabling and wiring, chip carriers, and on-chip circuit design. We found that the circuit design produced the greatest improvement, particularly at the highest frequency (100 kHz), where the deviation in ac-dc difference was reduced by more than 60%. In this paper, we also demonstrate tenfold higher output voltages and improved operating margins for arbitrary (nonsinusoidal) waveforms. These enhancements were accomplished by implementing a more general current bias to the arrays having the same harmonic content as that of the synthesized arbitrary waveform.