10-VOLT JOSEPHSON JUNCTION ARRAY SYSTEM

 

Three novel improvements enable automation of a voltage standard system based on a 10-volt Josephson array: (1) A low thermal scanner controls switching for calibrating both Zener references and digital voltmeters (DVM), (2) A programmable attenuator is used to obtain voltage steps, and (3) Measurements of DVM noise are used to verify array stability.

This work was done by Richard Steiner and Robert Astalos at NIST, and was presented at the CPEM’90 in Ottawa (IEEE Trans. IM vol. 40, April 1991). The system is being used at NIST to calibrate and evaluate Zener standards, and to evaluate the stability of DVMs.

then taken as before. These readings are adjusted using the preliminary meter correction. Computed values are found for the 10-volt units from the least-squares-fit calculation that are biased on the known 1-volt values. The final meter correction is found by adjusting the correction so that the adjusted computed values for the 10-volt devices are equal to their adjusted measured values. By using the method described only the meter readings at 9 and 10 volts are significant in determining the final values for the 10-volt units.

The same connection scheme works as well for DVM calibrations. For this case, one scanner input channel is shorted to let the DVM be linked directly to the array. No further switching is needed.

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GENERATING AND VERIFYING STEPS
Generating array steps is an integral process requiring sensitive adjustments of the voltage bias and millimeter wave power. Mr. Steiner has found a reliable method of automating step selection using a programmable attenuator. This method is general enough for use with any array and any voltage.

Verifying array steps is of major importance. Since multiple DVM readings are always involved, the standard deviation of each series is used as the vital parameter to monitor step quality. If the standard deviation is more that about 1.5 times greater than the background noise, the computer repeats the measurement or a new step is generated.

10-VOLT JOSEPHSON ARRAY SYSTEM DESCRIPTION
A Josephson array with 19,900 junctions can provide many precise voltages over the range from 0 to +10 volts. NIST has developed a completely automatic system that engages a programmable millimeter wave attenuator to generate known voltage steps. It also controls the DVM and low thermal scanner to perform the comparisons. This system operates completely unattended. A technician, who may be unfamiliar with Josephson physics, needs only to perform a basic setup sequence for operation.

The NIST system uses a novel connection arrangement to provide the required switching. Unlike voltage references, a Josephson array need not be physically

 

10-VOLT JOSEPHSON ARRAY SYSTEM RESULTS
The Zener reference calibration is an average of four independent points, derived from sixteen DVM readings and four array polarity changes. This procedure provides a straightforward calculation of the standard deviation of the mean. The Type-A random uncertainty is about 0.004 ppm at 10 volts, and about 0.008 ppm at 1.018 volts.

For DVM calibrations the array provides voltages at various calibration points. The NIST system calculates a proportionality constant (gain error) for each polarity and deviations from this linear fit at each measured value (linearity errors). System resolution is 0.02 ppm.


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