PRIMARYRESISTANCESTANDARDQHR2000 A NEW STANDARD IN MEASUREMENT■ Comparison of the 100 Ohm standardwith RK to 1 part in 10 8 .■ Cryogenic Current Comparator (CCC) inseparate low loss cryostat.■ Wide range of ratios for comparingresistances over the range 1 to10k Ohm.■ CCC fully shielded with turns ratioaccuracy of 10 -10 .■ 14 Tesla magnet for ease of use.■ LabVIEW® software for automatedoperation, measurement and analysis.■ Integrated system with low Heconsumption.■ Full environmental shielding, a screenedroom is not necessary.
introductionINTRODUCTIONThe Cryogenic Current Comparator (CCC) bridge isa breakthrough in the science of electrical measurement.It provides the ability to resolve and measure currentsand resistance values to an accuracy of 10 -9 .The QHR2000 series of instruments was developed tomeet the needs of standards laboratories around theworld for a new level of accuracy in the calibration andmaintenance of primary resistance standards.introductionThe technology used in the QHR also opens up newpossibilities for studying small signal effects in the presenceof large backgrounds. The QHR2000 consists of two parts.The first is a Quantum Hall Resistance reference systemwhich provides an absolute value of resistance related tothe von Klitzing constant of 25812.807 Ohms. To providethis reference, a Quantum Hall semiconducting device ismaintained at 0.3K with a He-3 refrigerator in a magneticfield of up to 14 Tesla, generated by a superconductingmagnet. Under these conditions the Quantum Hall plateauxof resistance are easily obtained.The second part is the CCC Bridge, which allows twoindependent and isolated currents of different values tobe compared and controlled with an accuracy of betterthan 10 -9 . The CCC forms part of a bridge circuit drivingcurrent into the two resistors which are to be compared.The differential voltage is detected with a sensitivenanovoltmeter.The QHR2000 can be supplied as a full turn-key systemfor metrology purposes. For all applications, componentssuch as the CCC bridge can be supplied as an independent instrument.one
CCCTHE CRYOGENIC CURRENT COMPARATOR (CCC)The CCC is a remarkable device which uses theshielding properties of a superconductor to make itpossible to balance and control DC currents to veryhigh precision.There are two geometries used in theCCC. As shown in this diagram the drivecoils are internal to the shield. There isan alternative design in which the detectorcoil is internal to the shield. For theinternal drive coil the currents are fedto two coils enclosed in the toroidalsuperconducting shield. The shield is nota closed loop but is designed in such away that the magnetic field escaping fromthe shield depends only on the ampereturns of the coil inside the shield and noton the coil’s position or size.Thus, two coils with equal and oppositeampere turns generate no external field.A Superconducting Quantum InterferenceDevice (SQUID) is used to detect thepresence of external field and feedbackis used to control the currents to ensuretheir accurate balance.With up to 24 coils enclosed in a singleshield, it is possible to produce manydifferent current ratios and to hold allof them to high precision. In the CCC,current ratios of up to 200:1 can easilybe produced. The ratios must of course bemade up from a series of integer turncoils.The standard instrument controlsand compares currents within therange of 50 milliamps to 10microamps, all with similar accuracy.Other current ranges are possible.Tests on the CCC have shown that theturns ratio errors are maintained tobetter than 10 -10 absolute accuracy.This level of absolute accuracy isunprecedented in any analogue devicewith DC currents.The CCC is supplied complete with itsown current sources and the SQUIDelectronics. The probe itself is wellshielded from magnetic and RFinterference. The isolation of all thewindings and their connections aremaintained to the very high levelrequired for metrology.The whole Cryogenic CurrentComparator is engineered to be flexible inuse, easy to understand and simple tooperate, while at the same timemaintaining the highest level ofmeasurement precision. The deviceincluding the SQUID null detector ismounted into a single probe is installed ina special low loss cryostat.To make coil selection easy the system usesreed relays to select the coils but they canalso be replaced by links for test purposes.The selector relays are mounted at thecryostat head as shown in the illustration ofthe prototype unit.The Cryogenic CurrentComparator showing the singletoroidal screen which encloses thecoils by wrapping round itself 3times without any electricalcontact between the layers.As with any SQUID system it is essential toavoid magnetic noise and RF interference.Great care is taken in the design of theCCC and its associated circuits to make itproof against externally or internallygenerated interference. As a result theQHR2000 can operate independently of ascreened room.Coil SelectortwoCCC
circuitTHE BRIDGE CIRCUITbridge circuitThe DC bridge circuit is shown in the illustration. Itconsists of two fully isolated power supplies, CS1 andCS2 providing currents to the two resistors under test.The currents in CS1 and CS2 are set to first orderfrom the computer in the appropriate ratio for theresistive elements of the bridge. Precise control withinthe limits of the CCC to 10 -9 or better is provided byfeedback from the SQUID output. The resistors can bejust two resistors of the same or different values, aresistor and a QHE device or even two QHE devicesfor comparison of their performance.Measurements are carried out under computercontrol with the currents ramped to a pre-setvalue, both positive and negative, where theyare held constant with sufficiently low noiseand drift to allow very precise measurementsto be made. The currents are normally chosento provide a voltage drop of 0.3 volts acrossthe 100 ohms test resistors. The nanovoltamplifier is capable of very low levelmeasurements so as to provide the resolutionrequired. The voltage measured across thebridge by the nanovolt meter represents thedifference in the value of the two resistorsafter allowing for the current ratios of the CCC.To obtain accurate results for metrologicalpurposes an alternative arrangement ispreferred. A third current is added to a singleturn in the CCC which gives a null voltageacross the bridge. The third current thenrepresents the difference of theresistance values. Its value can be accuratelydetermined.Repeating the measurement by reversing thecurrent to positive and negative values forseveral cycles allows the elimination ofthermo-electric offsets and gives a statisticalestimate of the random error.The nanovoltmeter and all currentsources are given isolated powersources. Fibre optic cables are usedbetween the computer and thecurrent sources to ensure that thebridge is both isolated and free fromRFI and other noise sources.The nanovolt detection circuit issimilarly isolated.It is particularly important that, wherethe bridge is used for metrologyapplications, it should be possible forthe user to carry out independentchecks on all aspects of the system,to guard against unexpectedsystematic errors. For routinemeasurements, calibrating a batch ofresistors, it is appropriate to use aform of scanner circuit. Scannersallow collection of data from severalresistors automatically undercomputer control.three
standardTHE QUANTUM HALL STANDARDThe QHR2000 uses theQuantum Hall Effect(QHE) to provide anabsolute value of resistancewhich is dependent only onthe value of Planck’s constantand that of the electron charge.This value is known as thevon Klitzing constant and istaken as 25812.807 Ohms.The quantised Hall resistance as a function ofmagnetic field, with an excitation current of 10 µA.Measurements are made to characterisethe QHE devices using the standardsoftware of the QHR 2000. A typical set ofplateaux is shown. The linearity of the 2and 4 plateaux can readily be seen.For resistance characterisation the n=2 andn=4 plateaux are used giving resistanceInner vacuumjacketSorbtion pump14 teslamagnetvalues of 12906.4035 Ohms and6453.2017 Ohms.A low contact and forward resistanceis also important, as it means that thegeometry of the Hall probe leads willnot affect the Quantum Hall Resistance.The QHR2000 allowsmeasurement of the forwardvoltage along the device.At the centre of the plateaux itshould be zero.Heliumfeed to1K potInnervacuum space1K potQHEdeviceat 0.3KThe use of the two plateauxgives added security to thereliability of the measurementas it provides a furtherindependent check forsystematic errors.The QHR2000 can use anysuitable QHE device mounted ona standard connector.Cryogenic Ltd will normallyprovide a GaAs high mobilitydevice from a well establishedsource.To provide the best and mostversatile performance, theQHR2000 is equipped with aclosed cycle He-3 refrigerator toproduce an operatingtemperature of 0.3K for the QHEdevice. The steps are presentat 1K and for some samplesthis may be sufficient.standardQHR2000 He-3 refrigerator and superconductingmagnet assemblyfour
automationAUTOMATION AND SOFTWAREThe control system for the QHR2000and the CCC uses the well establishedand flexible LabVIEW® softwarerunning in a fast Pentium based computer.National Instrument IEEE and dataacquisition cards are fitted to ensure a highlevel of reliability.automationThe LabVIEW® software allows an open measurement structureto be developed. The software is graphical and is composedusing icons for functions. Virtual Instruments are called up onthe screen which graphically display the logical structure of theprogram. Program development is by drawing new logicalstructures on the screen rather than by writing line by line code.This process is far faster and more secure, as well as more userfriendly than older programming systems. It makes the QHR2000that much easier to use.The open software structure allows the metrologist to overseethe program and to control the instrument’s operation, reducingthe possibility of undetected systematic errors.All functions of the system, including operation of the He-3refrigerator can be controlled from the computer. Individual VIwindows allow the magnetic field to be set to a fixed value orswept to display the plateaux in the Quantum Hall voltage.Other VI’s such as those shown allow set-up of the system andanalysis of the data collected.five
specificationTECHNICAL SPECIFICATIONTypical System SpecificationsPrimary resistance standard:100 OhmAccuracy of determination to the von Klitzing constant: 10 -8Systematic error determination by reference toNPL standard: +/- 3x10 -8or by special option: +/- 1x10 -8System isolation better than (dry conditions 20°Cambient temperature):1000 GigaohmHelium consumption:3 litres per dayHelium hold time:30 daysTypical measurement sequence time:25 minutesCryogenic Current Comparator BridgeTypical Coil sets included: 1,1,1,2,4,8,16,16,32,32,64,64,100,100,160,160,320,320,640,640,1600,1600,2065,4130Ratio self calibration accuracy: 10 -10Gain factor to SQUID:5000 Volts/Ampere-turnsNoise level:1.5 x 10 -9 Ampere-turnsNormal operating ampere turns:96 milliamp turnsMaximum operating ampere turns (coil set):10,000 milliamp turnsMaximum available current:50 milliampSweep ramp rate (typical coil set): 50 milliamp turns /secondFor confirmation of the latest specification, please contact our sales department.Rigorous test procedures are appliedduring all stages of manufacture withparticular care given to the key electronicand cryogenic components. An errorbudget is produced and evaluated for eachsystem. As the units are produced inseries, Cryogenic applies a policy ofcontinuing product improvement anddevelopment, in collaboration with researchinstitutes such as the UK NationalPhysical Laboratory.Formal tests are carried out in our worksfollowed by a full evaluation at the NationalPhysical Laboratory (NPL) in London.The systematic and random errors areassessed. Comparisons are also made tothe NPL calibrated values. Clients arewelcome to attend these tests to have thebest understanding of the system and toreceive initial training in its operation.All systems are installed at the customersfacility by our own engineers who providetraining for local personnel and ensure thatthe system is fully operational on-site tothe standards required formetrological applications.specificationsix
For further information or a comprehensivequotation, please contact our Sales Department:-Cryogenic Ltd,Unit 30,Acton Park Industrial Estate,The Vale,London W3 7QE,United KingdomInternational Telephone:(+44) 020 8743 6049International Facsimile:(+44) 020 8749 5315E-mail: email@example.comVisit our website at:http://www.cryogenic.co.ukJune 2002designed and produced by GIBSON BLANC design