AC Voltage Standards With Quantum...

AC Voltage Standards With Quantum Traceability

Kevin Marshall, Dale Henderson, Pravin Patel and Jonathan Williams.

NPL Electromagnetics day 29/11/2007

Tuesday, 11 December 2007

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• Existing AC Voltage metrology also well established.

• SI Traceability also provided via maintained physical realisation of the Volt…… DC volt!

• AC voltage metrology requires extra links in traceability chain – AC/DC conversion.

• Challenge – Improve efficiency and accessibility of traceability to the SI for AC voltage metrology/measurement/calibration

Background To Quantum Voltage Metrology

• Existing DC Voltage metrology well established.

“ 1 Volt is the electromotive force produced between two points of a wire carrying 1 ampere of current, such that the energy dissipated between those two points is exactly equal to 1 Watt”

•Traceable to the SI system via a maintained physical representation of the Volt.

•Traceability provided by physical effects, in part, governed by the fundamental constants – link to the SI - stability, accuracy, reproducibility.

“Quantum metrology”


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Background To Quantum Voltage Metrology

•DC voltage metrology based on the Josephson effect in superconductors.

•Relationship between frequency, fundamental constants and voltage provides stable and accurate SI traceability. - (Although does not define the SI volt!)

VMHzKeKKnfV JJ

J

/9.483597290 === −

h

• f microwave : accuracy ~ better than 1 part in 1010

• For Practical purposes KJ-90 is defined, with no stated uncertainty.

Microwave Frequencyf

Josephson Junctione, ħ V(f,e, ħ)

Bias CurrentI


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Background To Quantum Voltage Metrology – DC Voltages

DC Ibias

IC- IC~ 1-2 mA

~ 1-2 mA

V

Ibias

n f / KJ-90

•Current / Voltage characteristics of a microwave irradiated Josephson junction displays voltage steps

f ∼ 70GHz

•Width of current steps removes need for high level current accuracy.

•Practical Josephson arrays contain 1000s of Junctions.

•Typical voltages produced for a whole array are ~ 1.18V.

•Moderate fabrication difficulty.

•Long working life.

•Established techniques for incorporation into automated calibration system!

VHzKeKKnfV JJ

J

/9.83597.4290 === −

h

144μV


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Background To Quantum Voltage Metrology – DC Voltages

70GHz

Zener1

Zener2

Zener3

Zener4

10MHzPrimary

Bias Control

HP33420A

Voltage Measurement Unit

4.2°K

PCOPTICAL

ISOLATION

10V Cell - 0.02ppm 1V, 1.018V Zener - 0.14ppm Standard Cell- 0.09ppm


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Background To Quantum Voltage Metrology – AC Voltages

• To measure/calibrate, must physically realise quantity being measured!

• DC Voltages realised via Josephson Effect: V = nf / KJ-90

•AC Voltages Cannot be realised in the same way

VP

-VP

Vrms

fRelevant parameters cannot be generated with values based on fundamental constants.

•Link to the SI Provided by Heating Power

•Compare heating power delivered by DC Josephson voltage with heating power from AC voltage under test.

AC/DC transfer technique using Thermal Converters


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Background To Quantum Voltage Metrology – AC VoltagesAC/DC Transfer –

•Measurements traceable to DC voltage.

•Frequency range 10Hz to 1MHz (LF).

•Equivalence of DC and AC heating power – Same EMF output from Thermal converter

EMFACTC EMFDCTC

Ideal case: VAC=VDC, EDC=EAC, δAC-DC=0

Few mV EMF

Good Frequency flatness up to GHz

Accuracy ~ 1ppm

Low thermo electric effects.

Well optimised around 1kHz

Accuracy ~ 0.1ppm


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Direct Synthesis of AC voltages

•Synthesise waveforms waveforms via high precision D to A conversion.

•Calibrate all bit levels against quantum DC source.

•Demonstrated with precision of 1ppm.

•Synthesise D to A waveforms with Bits that are quantum voltage sources!

• Principle known as Binary Waveform Synthesis or Quantum Waveform Synthesis.

•Josephson junction array is divided into binary sections with individual bias current control.


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Direct Synthesis of AC voltages

Binary Array Chip

Binary Bias Source Binary Waveform Synthesis System

•16 Channel bias source.

•10ns rise-time of Sources’ D to A modules.

•Mains and optical isolation.

•Minimum sample length – 1.7μs

•16 Channel low-loss probe.

•Bespoke computer software for generation of waveforms with calculable.

•Array “quantisation time” believed to be between 10ns – 100ns.

•Undefined “transient” contributes to error in Vrms.

•NPL will undertake modelling to better understand factors affecting rise time.

•Transient effect negated if sampling techniques used – requires phase synchronisation.


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Direct Synthesis of AC voltages•Error due to transient effect increases with frequency – Ratio of transition time to quantised time increases.

•Result is frequency dependant rms value.

•Waveform’s rms value also demonstrates dependence on bias current to individual steps.

v

I

dV/dI = 0

Ib

tr tq

tr tq

Frequency: f1 f1 < f2 Frequency: f2


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Direct Synthesis of AC voltages – Calibration of a Standard

Low Pass Filter vp

-vp

vrms

Synthesised Waveform Contains Harmonics Filtered Waveform approximates “pure” tone

The resultant waveform is no longer quantum defined. →Vrms(Vq,Vr).

Not useful for SI traceability.

v

t

v

t


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Direct Synthesis of AC voltages – Calibration of a Standard

v

t

Low Pass Filter

v

t

v

t

t

v

v

t

Phase Delay Due to Filter Engineer delay to Synchronise Samples

Synchronous Discrete Sampling

Measurement of Filtered Waveform against quantised levels provides SI traceability.

Filtered Waveform becomes calibration source. – Associated uncertainty, traceable etc.

v

t


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Delta Sigma Technology For AC Voltage Metrology

NPL vision for AC Voltage metrology:

SI Traceable Source – Filtered Synthesised waveform

Waveform measurement device with direct traceability to the SI volt.

•Traceable AC measurement will be underpinned by Delta-Sigma A-D technology

•Delta-Sigma offers high resolution with a low number of bits.

•Technology will be based on Pulse-width modulation techniques – accuracy defined by timing performance.

•Very good stability – Ideal for functioning as a calibrated transfer standard.

•For high gain, accuracy limited by reference source

•Reference source can take the form of Josephson junction!

•Research underway involving collaborators: Metron Designs.


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Josephson Array acts as DAC in reverse path of Delta Sigma loop.

Voltage pulses with quantum defined levels measured at inverting input of the op-amp.

Feed-back path works so as to null Input into op-amp.

Driving circuitry for array integrated into ADC board.

Transfer function of the ADC contains only terms relating to the gain and the DAC level.


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•Quantised DAC voltage pulses occur in discrete time lengths.

•Pulse length extension means that ratio of positive to negative pulses defines output.

•Undefined rise times must cancel in order to have acceptable linearity.

•Array must be quantised within minimum pulse length!


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•To demonstrate usefulness gain must display “quantum” nature.

• Delta-Sigma output must shown plateau nature over range of array bias current settings.

v

I

Ib

Step flatness is a criterion for quantum traceability.

Current Step flatness ∼ 50ppm

Origin still not clear: Capacitive coupling of drive pulses, non-linearity of sense resistors, common mode rejection of op-amp…?

Some encouragement – Work still on-going


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•NPL will concentrate on AC Waveform measurement strategy that incorporates a range of Delta-Sigma based approaches.

•Josephson referenced Delta-sigma work is a Flagship research area.

SI TraceableJosephson Binary

SynthesisedVoltage

Waveform Source

SI TraceableJosephson Referenced

AC VoltageMeasurement

DeviceCommercial AC Voltage

Synthesisers / Power standards

Accredited Laboratories/

Manufacturers

Dissemination of Standard

Zener Referenced Delta-Sigma Converter

External Zener Referenced Delta-Sigma Converter


Manufacturers

Dissemination of Standard Primary Calibration

Primary Calibration

Impact!

Improved usability

Quicker calibrations Lower cost

Widens access to AC Voltage traceability SI traceable

Thermal converter / Other Transfer Standards /

Commercial Measurement Instruments


Manufacturers


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