AC Voltage Standards With Quantum Traceability

Transcription

1 NPL Electromagnetics day 29/11/2007 AC Voltage Standards With Quantum Traceability Kein Marshall, Dale Henderson, Prain Patel and Jonathan Williams.

2 Background To Quantum Voltage Metrology Existing DC Voltage metrology well established. 1 Volt is the electromotie 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 ia a maintained physical representation of the Volt. Traceability proided by physical effects, in part, goerned by the fundamental constants link to the SI - stability, accuracy, reproducibility. Quantum metrology Existing AC Voltage metrology also well established. SI Traceability also proided ia maintained physical realisation of the Volt DC olt! AC oltage metrology requires extra links in traceability chain AC/DC conersion. Challenge Improe efficiency and accessibility of traceability to the SI for AC oltage metrology/measurement/calibration 2

3 Background To Quantum Voltage Metrology DC oltage metrology based on the Josephson effect in superconductors. Relationship between frequency, fundamental constants and oltage proides stable and accurate SI traceability. - (Although does not define the SI olt!) V = nf 2e K J K J K = h J 90 = MHz / V f microwae : accuracy ~ better than 1 part in For Practical purposes K J-90 is defined, with no stated uncertainty. Bias Current I Microwae Frequency f Josephson Junction e, ħ V(f,e, ħ) 3

4 Background To Quantum Voltage Metrology DC Voltages Current / Voltage characteristics of a microwae irradiated Josephson junction displays oltage steps V = nf 2e K J K J K = h J ~ 1-2 ma V 144μV 90 = I C -I C ~ 1-2 ma Hz / V n f / K J-90 I bias Width of current steps remoes need for high leel current accuracy. Practical Josephson arrays contain 1000s of Junctions. Typical oltages produced for a whole array are ~ 1.18V. Moderate fabrication difficulty. Long working life. f 70GHz Established techniques for incorporation into automated calibration system! DC I bias 4

5 Background To Quantum Voltage Metrology DC Voltages PC Bias Control OPTICAL ISOLATION 70GHz HP33420A 10MHz Primary Voltage Measurement Unit Zener 1 Zener K Zener 3 Zener 4 10V Cell ppm 1V, 1.018V Zener ppm Standard Cell- 0.09ppm 5

6 Background To Quantum Voltage Metrology AC Voltages To measure/calibrate, must physically realise quantity being measured! DC Voltages realised ia Josephson Effect: V = nf / K J-90 AC Voltages Cannot be realised in the same way V P V rms -V P f Releant parameters cannot be generated with alues based on fundamental constants. Link to the SI Proided by Heating Power Compare heating power deliered by DC Josephson oltage with heating power from AC oltage under test. AC/DC transfer technique using Thermal Conerters 6

7 Background To Quantum Voltage Metrology AC Voltages AC/DC Transfer Measurements traceable to DC oltage. Frequency range 10Hz to 1MHz (LF). Equialence of DC and AC heating power Same EMF output from Thermal conerter TC EMF AC TC EMF DC Ideal case: V AC =V DC, E DC =E AC, δ AC-DC =0 Few mv EMF Good Frequency flatness up to GHz Accuracy ~ 1ppm Low thermo electric effects. Well optimised around 1kHz Accuracy ~ 0.1ppm 7

8 Direct Synthesis of AC oltages Synthesise waeforms waeforms ia high precision D to A conersion. Calibrate all bit leels against quantum DC source. Demonstrated with precision of 1ppm. Synthesise D to A waeforms with Bits that are quantum oltage sources! Principle known as Binary Waeform Synthesis or Quantum Waeform Synthesis. Josephson junction array is diided into binary sections with indiidual bias current control. 8

9 Direct Synthesis of AC oltages Binary Array Chip 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. Binary Bias Source Binary Waeform Synthesis System Bespoke computer software for generation of waeforms with calculable. Array quantisation time belieed 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. 9

10 Direct Synthesis of AC oltages Error due to transient effect increases with frequency Ratio of transition time to quantised time increases. Result is frequency dependant rms alue. Waeform s rms alue also demonstrates dependence on bias current to indiidual steps. dv/di = 0 I b I Frequency: f 1 f 1 < f 2 Frequency: f 2 t r t q t r t q 10

11 Direct Synthesis of AC oltages Calibration of a Standard Synthesised Waeform Contains Harmonics Low Pass Filter Filtered Waeform approximates pure tone p rms t t - p The resultant waeform is no longer quantum defined. V rms (V q,v r ). Not useful for SI traceability. 11

12 Direct Synthesis of AC oltages Calibration of a Standard Low Pass Filter t t Phase Delay Due to Filter Engineer delay to Synchronise Samples t t t Measurement of Filtered Waeform against quantised leels proides SI traceability. t Filtered Waeform becomes calibration source. Associated uncertainty, traceable etc. Synchronous Discrete Sampling 12

13 Delta Sigma Technology For AC Voltage Metrology NPL ision for AC Voltage metrology: SI Traceable Source Filtered Synthesised waeform Waeform measurement deice with direct traceability to the SI olt. 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 inoling collaborators: Metron Designs. 13

14 Delta Sigma Technology For AC Voltage Metrology Josephson Array acts as DAC in reerse path of Delta Sigma loop. Voltage pulses with quantum defined leels measured at inerting input of the op-amp. Feed-back path works so as to null Input into op-amp. Driing circuitry for array integrated into ADC board. Transfer function of the ADC contains only terms relating to the gain and the DAC leel. 14

15 Delta Sigma Technology For AC Voltage Metrology Quantised DAC oltage pulses occur in discrete time lengths. Pulse length extension means that ratio of positie to negatie pulses defines output. Undefined rise times must cancel in order to hae acceptable linearity. Array must be quantised within minimum pulse length! 15

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17 Delta Sigma Technology For AC Voltage Metrology I b To demonstrate usefulness gain must display quantum nature. I Delta-Sigma output must shown plateau nature oer range of array bias current settings. Step flatness is a criterion for quantum traceability. Current Step flatness 50ppm Origin still not clear: Capacitie coupling of drie pulses, non-linearity of sense resistors, common mode rejection of op-amp? Some encouragement Work still on-going 17

18 Delta Sigma Technology For AC Voltage Metrology Josephson referenced Delta-sigma work is a Flagship research area. NPL will concentrate on AC Waeform measurement strategy that incorporates a range of Delta-Sigma based approaches. SI Traceable Josephson Binary Synthesised Voltage Waeform Source Thermal conerter / Other Transfer Standards / Commercial Measurement Instruments Impact! Improed usability Quicker calibrations Lower cost Widens access to AC Voltage traceability SI traceable Accredited Laboratories/ Manufacturers Primary Calibration Zener Referenced Delta-Sigma Conerter Dissemination of Standard Accredited Laboratories/ Manufacturers SI Traceable Josephson Referenced AC Voltage External Zener Referenced Delta-Sigma Conerter Measurement Deice Primary Calibration Commercial AC Voltage Synthesisers / Power standards Dissemination of Standard Accredited Laboratories/ Manufacturers 18

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