1874 mEE TRANSACllONS ON APPLIED SUPERCONDUcnVITY, VOL. 3, NO.1. MARCH 1993

ACCURACY COMPARISONS OF JOSEPHSON ARRAY SYSTEMS

R. L. Steiner, A. F. Clark, C. Kisert, T.]. Witt. and D.Reymann"National Institute of Standards and Technology

GaiU,ersburg, MD 20899 USA

tNavy Primary Standards Laboratory-EastWashington, D. C. 20374 USA

· Bureau International des Poids et MesuresF 92312Sevres codex, France

Abstract -- Five Josephson-array voltage stan-dard systems were compared using several differentmethods. All of the tests were performed on site at a1.018-V level, either by'direct connection or throughsuccessive measurements of independent vollagesources. The resulting agreement between dif.ferentsys-tems measuring the same source were generally betterthan 10.0 parts in 10-9, limited by source noise and de-tector resolution. Direct array-to-array comparisonsfor independent systems achieved agreement to withinrandom uncertainties of 0.2 p~rts in 10-9.

I. INTRODUCTION

Josephson array voltage standards [IJ are nowused in many metrology laboratories, including severalnon-national standards laboratories. There are manyadvantages of using a Josephson array system as a volt-age standard; 1) less reliance upon transfers from exter-nally calibrated voltage sources and the possibility ofmore frequent "in-house" calibrations, 2) the inherentstability and repeatability of the quantum phenomena,and 3) the direct tie to the SI system by definition andthe worldwide acceptance of the Josephson constant.However, the accuracy of a local array system remainsan assumption that is difficult to prove satisfactorily,especially to within the uncertainty possible for thesesystems [2J. Assuring the accuracy of the generated voltrequires more than testing the frequency and devicehardware. In order to provide equivalence to a recog-nized voltage standard, a total system verificationmust include testing the system software, both its con-trol procedure and measurement calculations, and such

'hardware tests as ground loops, leakage, and thermalvoltages.

This work was performed in the Electricity Division, Elec-tronics and Electrical Engineering Laboratory, TechnologyAdministration, U. S. Department of Commerce, and par-tially supported by the Calibration Coordination Group of theU. S. Department of Defense. Contributions of the NationalInstitute of Standards and Technology are not protected byU. S.copyright. Manuscript received August 24, 1992.

A comparison test is the generally accepted wayto establish total system accuracy, and the most preciseway is via an on-site comparison with another arraysystem. Comparisons can be classified as either direct(array-to-array), or indirect (via an intermediary ref-erence). Each type of test has certain benefits, either inbetter precision, or in more completeness. On-site testshaving excellent precision have already been reportedin Europe [3,4J. In a,recent comparison using a standardreference shipped 'among eight U. S. government andindustrial laboratories with array systems, testing thecomplete systems proved essential [5J. Two locales re-ported their initial determination as one Josephsonvoltage step off, 15 parts in 106. Such large errorswould eventually be found, but they also highlight theneed for comprehensive tests. Tests involving ashipped reference, however, require long times to ac-quire a satisfactory number of data points and uncer-tainties will then be limited by long term noise inher-ent in the transfer standards.

This paper focuses on on-site comparisons betweenfive array systems involving the three NIST systems, asystem from the Bureau International des Poids etMesures (BIPM), and one at the Navy Primary Stan-dards Laboratory - East. Both the previously men-tioned methods were variously employed, resulting indifferent levels of uncertainty and measurement dura-tions. As expected, direct connection of the two arrays,with an analog nanovoltmeter detecting the differencebetween them, resulted in the most accurate comparisonof the output voltages. A similar but automated test us.ing normal'system software and the system's own digi~tal voltmeter (DVM) as the detector still resulted invery good precision. Indirect tests using an intermedi-ary transfer standard demonstrated a wide range of re-sulting uncertainties but allowed for more flexible sys-tem equipment requirements.

II. EXPERIMENT DESCRIPTION

The three NIST systems are nearly identical [6J.One is the primary national volt standard system

~,

1051-8223/93$03.00 e 1993 mEn"

(NIST1), one is a 10-V research system (NIST 10), anda third is referred to as a "portable" I-V system (NIST1P),because of the need to transport this array syst~mto places inside and outside of NIST. The portable ar-ray system is a rack size system, so the description"portable" applies to its occasional transport ratherthan its smaller design. The BIPM system (BIPM 1)was independently designed but, of necessity, isschematically similar [7]. The Navy I-V system(NAV 1) is based on an alternate NIST design and dif-ferent operating software [2J but with essentially thesame components. All the systems use Josephson arraychips fabricated at NIST-Boulder [IJ.

Some of the design differences in these systemsarose from early comparison experiments. Attempts toconnect arrays directly proved unsuccessful because ofoffsets of several microvolts and increased noise. Theproblem was traced to multiple ground paths; through

. bias supplies, oscilloscopes, and voltmeters. It wassolved with better array isolation from ground, eitherby adding a switch to disconnect the array from thebias after a step had been generated (BIPM 1), or by us-ing commercial voltage calibrators as bias supplies(NIST1). Eliminating ground loop offsets enabled di-rectcoupling of the array output lines from one "source"system to a second "measurement" system. This wasimportant in eliminating the effects of thermal emf

. voltages of the connection leads. Th.e "measurement"system's voltage detector, either an analog nanovolt-meter or a DVM, can be used to measure the voltage dif-ference. The millimeter wave frequency sources of allsystems used the same external frequency reference. Asimplified schematic diagram for "these direct compar-isons is shown in Figure la.

In the most precise experiment, a direct compari-son between NIST 1 and BIPM 1, each system was runmanually to seleCt repeatedly the same voltage steps.The analog nanovoltmeter of the BIPM system wasused, making this a t~st of the complete BIPM system.The frequency on NIST 1 was varied to compensate forthermal offsets in order to maintain a differencewithin the range of the detector of less than 0.3 JiV.The amplified voltage difference was recorded in 30-60second traces on a strip chart recorder for each polarityof the detector. A single point consisted of traces com-bining two array voltage changes: one reversal and onereturn. The total time, including detector calibrationand the measurement, was about 1 h for 6 points, andsomewhat less for 4 points the next day.

A direct comparison was also devised to test acomplete NIST system. This specifically tested thesoftware and made the timing of such things as switchclosures and voltage settling as similar to normal pro-ced':1resas possible by using the programmed routine

~.

1875

(a)

(b)

(c)

Figure 1. Simplified schematics for array system com-parisons: (a) direct, (b) indirect via mercury battery(substitution method), and (c) indirect via a Zener ref-erence. Switches are not represented in full detail.

and switching. The DVM of the "measurement" system(NIST 1P) measured the output of NIST 1 as the"source" system. Similar to the NISTIBIPM procedure,selection of the NIST 1 voltage step was performedmanually and electrically reversed, the only changefrom the usual procedure of physically switching aZener reference. Both systems were so well isolatedfrom ground that step jumps on the NIST IP system(from automated step selection) did not affect the otherarray. A single measurement point was the average offour array reversals (the usual procedure), taking about20 min per point.

Two variations of indirect comparison schemesare shown in Figure Ib and lc. In both, a voltage refer-ence standard was measured repeatedly and alter-nately by each of the array systems being compared.The most sensitive indirect test used a specially builtvoltage reference based on a mercury battery [4] as avery low-noise intermediary reference, as shown inFigure lb. Switches built into the thermal enclosure ofthe battery reversed the analog nanovoltmeter, themercury battery, and selected either array in a substi- .

tution method to obtain the voltage difference mea-surement between each array. Once again, minimumvoltage difference was accomplished with manual stepselection. The measurement time was about 1.5 h for 4points. Because the same detector is part of both mea-surement loops, thermal emf offsets in the wires areminimized. The voltage drift in the battery was linearat about 1 nVImin. Though this tests only the com-plete BIPM system, it is a very precise method for com-paring output voltages, especially between systemswith incompatible equipment or grounding problems.

The indirect method shown in Figure lc simplyemployec:ithe 1.018V output of a commercial Zener ref-erence standard as a short term intermediary. Each ar-ray system, with its own normal operating procedure,repeatedly measured one or more references. Twostrategies were tried with no clear advantage to one;either complete determinations by one system alter-nated with those taken by the other, or alternatepoints from each system were independently averagedover several days. Since each system has its own detec-tor, additional measurements of the thermal emf offsetfor each input circuit wire are required, a time consum-ing and error laden procedure. A data point comprisedthe averaged number of array reversals characteristicof each system (usually 2 or 4), and averaged againover the number of references measured.

III. RESULTS

Table 1 summarizes the comparisons. All the re-sults were within 3 times the one standard deviationestimate Type A random measurement uncertaintylisted in the table. These random uncertainties aremainly owing to the noise of the detector and irrepro-ducible thermal emfs in the switch contacts. Josephsonjunction noise from the broadband of the driving fre-quency .spectrum could not be noticed. Reference noise isa contribution to the uncertainty in the indirect mea-surements, with Zener references adding rather largeuncertainties from source noise and thermal emf mea-surements.

For some comparisons,additional Type B uncer-tainties can be up to 5 nV for the irreproducibiIity ofthermal emfs of various switch contacts. The unccr-

~

,

ri~i\

tainty of the mil~imeter wave frequencies cannot beeasily stated. The external frequency reference used forboth systems is stable to better than 0.01 x 10-9 over100s integration times, but the counter uncertainty andshort term variation of the frequency references be-twecn thc successive measurcments in the indirectmethods could be 0.3x 10-9or more. Contributions fromleakage resistance to ground or across the wires withineach array cryostat were measured to be less than0.1 x 10-9 (_1013 Q through 22 Q for NIST 1). How-ever, this type of systematic error would not be clearlyrevealed in these tests, since in some cases errors due toleakage resistance would tend to cancel.

The NIST systems have been compared to eachother scvcral times over the years in an effort to estab-lish self consistency. Some general comments can bemade about the variations in testing. Two of the NISTsystems, NIST 10 and NIST IP, were each compared tothe primary I-V array system (NIST 1) just prior to theBIPM comparisons. Test # 1 demonstrated the feasibil-ity of simply connecting two NIST systems directly tomake a comparison, for a difference of 1.1 :t7.5 x 10-9over 4 points. Test # 2 employed NIST IP's own soft-ware/hardware system to measure its difference from avolt generated by the primary NIST 1 system. Becauseof the ease and speed of this procedure, 11 points wererecorded to enhance the precision, yielding a differenceof 4.2 :t 1.9 x 10-9.

Since system comparisons are the only way to es-tablish equivalence, the authors from BIPM broughtone of their Josephson array systems to the NIST labo-ratory for comparison, both to demonstrate their meth-ods for this procedure and to provide a basis of experi-

1876

TABLE 1. Summary of array system comparisons. Rcsults and random uncertainties are expressed in partsin 109, one standard deviation estimate.

TEST DATE OF SYSTEMS REFERENCE/ TEST- DETECTOR(S) TIME/ DIFFERENCE :I:# TEST ING "SOURCE" POINTS UNCERTAINTY

10-91 7/26/91 NIST 10 Direct/Output DVM 1h/4 1.1:1:7.5

-NIST 12. 10/10/91 NIST 1P Direct/System DVM 2.5 h/11 4.2 :t 1.9

-NIST 13 10/15/91 NIST 1 Direct/Ou tput Analog Nano- 1.1h/6 . 0.16:t 0.26.

-BIPM 1 voltmeter4 10/16/91 NIST 1 Direct/Output Analog Nano- Ih/4 0.06:t 0.15

-BIPM 1 voltmeter5 10/16/91 NIST 1 Mercury Battery/ Analog Nano- 1.5h / 6 0.01 :t 0.26

-BIPM 1 Output voltmeter6 10/17/91 NIST 1 2 Zener Refercnccs/ DVM/ Analog 2.5 h / 6 -1.4 :t 6.6

-BIPM 1 System Nanovoltmeter7 10/30-31/91 NAV-l 1Zener Rcference/ DVM/DVM 2d/9 -17:t 11

-NIST IP . System

- ----

mental data for establishing levels of equivalence be-tween the various national standards laboratories.BIPMhas performed several of these comparisons with

. other national laboratories [8). The two most sensitivedirect array comparisons, tests # 3 and # 4, showed dif-

, ferences of +0.16 :t 0.26 x 10-9 for 6 points and

: +0.06:t 0.13 x 10-9 for 4 points. Weighting the data ofi the NIST-BIPMcomparisons using the reciprocal of the. type-A variance of the results leads to a difference of. 0.1:t 0.3 x 10-9, the uncertainty including both type-A

and Type-B components.

Two indirect comparisons were performed withNIST1 and BIPM 1. Nearly as good as the direct com-

, parisons,test # 5 used the stableand quiet mercurybat-tery reference specially built by BIPM, for a result of+0.01:t 0.26 x 10-9 for 6 points. Note that detector re-versals are part of the normal operating procedure forthese systems, because of unpredictable effects; This ismentionedbecause an unusual problem arose during test# 5, when the detector high-input terminal wasaligned relative to the voltage low and was subse-quently perturbed by electromagnetic interference, re-sulting in a 1 nV offset for this polarity. This would nothave been noticed without detector reversals.

As mentioned earlier, the series of indirect com-parisons involving Zener references suffered fromhigher uncertainties. The resulting difference fromtests # 6 between NIST 1 and BIPM 1 was

-1.4:t 6.6 x 10-9 for 6 points. Test # 7 occurred at theNavy Primary Standard Laboratory-East. The large

. result and associated error of -17:t 11 x 10-9 arose froma discrepancy in measuring the thermal emfs of theZener connecting wires. There was a significant differ-ence,30 nV, between shorting the wires to a free stand-ing binding post or to one of the reference terminals,with the latter more reliable because of the elevatedtemperature of the reference's terminals. This diffi-culty in determining the thermal emfs underscores oneof the problems of using the indirect Zener referencetests. The other problem, that of Zener reference noise,can be especially severe in tests using Zener referencesat the 1.018-V level over several days. A direct array-to-array comparison was also attempted on this sys-tem, which has a grounded bias supply. Only a singlepoint was recorded in well over an hour at a reducedtest value of only 0.7 V. A difference of 9 x 10-9was ob-tained, the result of only one array reversal.

IV. CONCLUSIONS

Our basic conclusion is that Josephson-arrayvoltage standard systems can be readily transportedand tested to assure on-site equivalence. Also, thesetests can be done quickly and with high precision, lim-

~,

1877

ited by the detector noise if directly compared, or bythe transfer reference noise if done indirectly. Repre-sentative limits are shown in Table 1. System equiva-lence has been documented to well below the 0.4 x 10-6uncertainty in the Josephson constant established bythe Consultative Committee on Electricity. Assuringequivalence at various levels of uncertainty has beenshown, with significant flexibility in both equipmentand time needed for data acquisition. Although labo-ratories have demonstrated equivalence to better than1 part in 107 via shipping Zener references, major partsin 106 differences have been seen [4). At present, onlysystem-to-system comparisons can provide the inter ,

laboratory equ.ivalence at the parts in 109 or better un-certainties seen in these on-site comparisons.

REFERENCES

[1] Kautz, R. L., Hamilton, C. A. and Lloyd, F. L.,"Series-array Josephson voltage standards," IEEETrans. Magn., Vol. MAG-23, pp. 883-890, March1987.

[2] Hamilton, C. A., Burroughs, C. and Kao Chieh,"Operation of NIST Josephson Array VoltageStandards," T.Res. Natl. Inst. Stand. Technol.. Vol.95, pp. 219-235,May-June 1990.

[3) D. Reymann, J. U. Holtung, and H. D. Jensen,"Comparisons of One-Volt Josephson-ArrayVoltage Standards with Sub-Nanovolt Accuracy,"Metrologia. Vol. 28, pp. 99-102, 1991.

[4] D. Reymann, "A Practical Device for 1-nVAccuracy Measurements with JO,sephson Arrays,"Proc. CPEM 90, IEEE Trans. Instrum. Meas.. Vol.IM-40, pp. 309-311,April 1991.

[5) R. L. Steiner, S. Stahley, "MAp voltage transferbetween 10-V Josephson array systems," inNational Conference of Standards LaboratoriesProceedings,Denver, CO, August 1991,p. 205.

[6] R. L. Steiner, and R. A. Astalos, "Improvements forAutomating Voltage Calibrations using a 10-VJosephson Array, " IEEE Trans. Instrum. Meas., Vol.IM-38,pp. 1030-1036,December 1989. ,

[7] D. Reymann and T. Witt, "The New BIPM I-V. Reference Standard based of an Array of Josephson

Jusnctions," , IEEE Trans. Instrum. Meas., Vol.IM-40,pp. 309-311,April 1991. , ' ,

[8] D. Reymann and T. J. Witt, "InternationalComparisons of Josephson Array VoltageStandards," Proc. CPEM 92, IEEE Trans. Instrum.Meas.. Vol. IM-42, (to be published, April 1993).