The National Physical Laboratory (NPL) is the United Kingdom’s national measurement institute. NPL is a world-leading centre of excellence in developing and applying the most accurate measurement standards, science, and technology available. For more than a century, NPL has developed and maintained the nation’s primary measurement standards. These standards underpin an infrastructure of traceability throughout the UK and the world that ensures the accuracy and consistency of measurement. Based in southwest London and employing more than 500 scientists, the NPL facility is internationally regarded as one of the most extensive and sophisticated measurement science facilities.
For more than 20 years, the electrical standards of voltage, current, and resistance have been based on highly reproducible quantum effects. For example, the Josephson effect relates voltage to frequency and is now used in measurement laboratories worldwide to provide the highest accuracy voltage measurements currently possible. NPL has achieved its level of quality research by designing bespoke hardware and software that interface with the delicate quantum devices. These prototype systems form the basis of future measurement infrastructure at NPL and are regularly used by other laboratories. However, to carry out our research in a timely and competitive manner, we need to develop solutions using as many commercially available tools and systems as possible, and we need to ensure these systems can be easily maintained and supported into the future.
Quantum Waveform Synthesizer
Our application is a waveform synthesizer with direct traceability to the Josephson quantum voltage reference. Digital electrical measurement is now the method of choice in the instrumentation sector since signal processing is much easier to realise in digital circuits than in analogue filters. The performance of the ADCs is crucial to the success of the digital instruments, and our synthesizer is designed to generate waveforms with high spectral purity and an impressive level of amplitude stability. These reference waveforms are used to characterize ADCs represented by the device under test (DUT) in Figure 1.
The synthesizer is based on a digital-to-analogue converter (DAC) with 20-bit resolution and linearity. The output of the DAC is passed through an anti-imaging, multipole lowpass filter. The output of the filter is compared with the Josephson quantum voltage reference by measuring a voltage difference using an amplifier with a gain of 100 and an 18-bit ADC. A waveform is typically sampled 100 times per period to generate a 1 kHz reference sine wave (Figure 2). For ADC characterisation, a sampling frequency of 100 kHz is required on the ADC. The DAC is similarly updated at 100 kHz.
Background Information on Our Chosen Technical Solution
Our first synthesizer design used an FPGA along with a microprocessor to load data into the DAC and to read data from the ADC. This system delivered a sampling frequency of 5 kHz, which was determined by the speed of the microprocessor. This limited the synthesizer to applications at power line frequencies. An upgrade of this approach to a higher sampling frequency would have needed a complete redesign of the FPGA code.
Therefore, we required the following levels of functionality from our system:
- A logic system based on an FPGA for fast data transfer to the DAC and from the ADC together with low timing jitter
- A real-time OS for the control loop, which stabilises the synthesizer output against the Josephson reference
- An Ethernet connection to a PC running LabVIEW for the user interface and data storage
This was comfortably achieved using CompactRIO embedded hardware. Aside from providing the graphical user interface and data logging, LabVIEW simplified the sharing of data between the three architectural layers described above. That, along with the short development times, meant that LabVIEW was a real advantage to us.
Our Experience With CompactRIO
Our application required a high level of electrical isolation, so we chose to use optical fibre connections (Figure 3) between the CompactRIO hardware and the synthesizer. Each sample of the waveform consisted of three 8-bit data packets, enabling a data rate across this serial link of 2.4 MHz for a sampling frequency of 100 kHz. Two NI 9402 high-speed digital I/O modules were used to provide the digital I/O for the CompactRIO hardware. Three lines were used to implement the SPI interface to the DAC and the ADC.
The built-in FPGA on the CompactRIO system continuously updated the DAC with data from memory and read data from the ADC to memory over the serial links. In addition, a timing signal was generated to synchronise the Josephson quantum voltage reference so that it was phase-locked to the synthesizer.
The CompactRIO real-time processor transferred data to and from the memory and analysed the ADC readings, which represented the difference between the synthesized voltage and the quantum reference. An algorithm on the real-time processor calculated corrections to the DAC values to adjust the synthesized voltage and stabilise it against the reference voltage. The real-time processor also averaged the data from the ADC before transferring it to the PC over Ethernet at a lower data rate.
Software written in LabVIEW on the host PC provided the user interface for the whole measurement system including configuration of the Josephson quantum voltage reference; choice of the amplitude, frequency, and number of samples in the synthesized waveform; and presentation of the data from the ADC.
Support From National Instruments
The combined LabVIEW Real-Time and LabVIEW FPGA course was essential in understanding what was possible at the higher level and to see how the system could be deployed in different situations. The knowledge and best practices learned during the course enabled us to implement the first iteration of our application in just a few days. We also received support from our local field engineer while getting started with the CompactRIO system.
The Benefits of Using the NI Platform for Our Solution
In our endeavour to stay ahead in our field, we were asked to design a quantum waveform synthesizer capable of delivering deterministic, low-jitter operation at megahertz frequencies with a built-in control loop and an intuitive user interface for configuring and recording data measurement.
By combining a CompactRIO system with our own bespoke hardware, we realised a first-generation synthesizer in a period of just two months, which could have taken at least an year in another approach we evaluated. Furthermore, the modular, software-defined solution from NI enabled rapid prototyping and provided an upgrade path for the future with minimal internal investment. The synthesizer will offer a new basis for traceability of sampled electrical measurements to a quantum standard and will be a flexible tool for the characterisation of state-of-the-art ADCs.
National Physical Laboratory
National Physical Laboratory, Hampton Road
Teddington TW11 DLW
Tel: 020 8943 6376