Electrical Standards Research
The use of quantum standards for the DC and low-frequency electrical units provides many research opportunities. Much of our work is focused on how to use these quantum effects to extract reliable and accurate values. Recent interests include the AC quantum Hall effect and the accuracy of single electron tunneling devices.
- Josephson Array
- Single Electron Tunnelling
- Cryogenic Current Comparators
- Quantum Hall Effect
- Electrical Energy
- AC Voltage
At higher frequencies, our research in the field of RF and microwave measurement is focused on developing better methods of achieving traceability to the SI.
The superconducting device at the heart of New Zealand’s Josephson array dc voltage standard comprises thousands of Josephson junctions in series.
Each junction acts as a perfect frequency-voltage converter, converting a known microwave frequency into an exactly proportional dc voltage, with a proportionality constant equal to the ratio of fundamental physical constants.
This photo shows the rectangular microwave waveguide in the middle region, with the chip containing the junctions near the bottom. Experience gained during the development and operation of the standard resulted in an investigation into applying the exceptional accuracy of the dc voltage to the generation and measurement of ac waveforms. Now MSL is planning to use an ac Josephson voltage standard as part of the watt balance project that is aimed at replacing the present kilogram definition.
The present SI definition of the ampere is based upon 19th century physics and implicitly links to the mechanical SI unit definitions. Quantum physics and modern electronic device fabrication techniques have allowed the invention of nano-sized devices which accurately clock electrons one by one through a circuit. This presents an opportunity to redefine the ampere in terms of a defined value of the electron charge. MSL has been involved in the international effort to develop single electron tunnelling devices for this and related applications. more >>
Cryogenic current comparators (CCC's) are used for measuring current ratios. CCC's are commonly used as part of resistance bridges used for measuring resistors against quantum Hall standards. They achieve a high sensitivity by using SQUID's to detect a magnetic flux imbalance between ratio windings. High ratio accuracy is achieved by using superconducting shields to give a tight flux coupling between windings. The current sensitivity of a CCC is decreased when the CCC is placed in an overall superconducting shield. We have found that significant gains in sensitivity can be made by optimising the relative positions and dimensions of the components (see references). Our modelling techniques has allowed investigation of novel CCC's made with high Tc materials.
It took only 10 years from discovery of the quantum Hall effect in 1980 to the adoption of a conventional value of the von Klitzing constant, RK-90, by the CGPM. Adoption of this conventional value means that laboratories benefit from this highly reproducible resistance value while work continues on determining an improved value of the constant. Current work is aimed at carrying out highly accurate dc measurements on quantum Hall devices in order to establish the scale of resistance.
We have developed a standard of electrical power that is accurate to 41 ppm at 50 Hz. Electrical power is derived from the current through a load, the voltage across the load and the phase between the current and voltage. The standard is a form of power-bridge that converts the current to a voltage and compares this voltage with the voltage across the load. The bridge is adjusted with computer-switched ratio transformers to an approximate null and a sampling voltmeter is used to measure the magnitude and phase of the out of balance signal. Work is beginning on improving this watt standard by using digital sampling of scaled versions of the current and voltage waveforms.
We have developed a comparator system to allow an ac voltage scale to be built up from a pool of single range thermal converters. Previous work included a detailed theoretical analysis of the performance of these devices by obtaining a solution of the full heat flow equation. Current work focusses on understanding the performance of modern ac voltage instruments.
There is an increasing need for high-accuracy attenuation standards that operate over a broad frequency range. Work is currently underway on a novel broadband attenuation system that will operate between 50 MHz and 18 GHz. The instrument has a modular design and will be constructed from off-the-shelf components. It will produce attenuations up to about 90 dB in 9 dB steps.
The complex impedance of RF and microwave components can be measured by a Vector Network Analyser (VNA). VNAs are complicated instruments with many sources of systematic error. They can only operate effectively by continually correcting raw data for these errors, which is done internally by the instrument's microprocessor. Our research is developing new data-processing methods to deal with this issue. We have found a way of incorporating, in software, information about VNA errors so that many standard data processing algorithms can still be used. The approach conforms to international guidelines on traceability to the SI and on the evaluation of measurement uncertainty.