by Barry Manz, President, Manz Communications, Inc.
The National Institute of Science and Technology (NIST) may be the most underappreciated player in the development and advancement of systems that rely on electromagnetic energy. Its role today is arguably as important as at any time in its 119-year history, as the wireless industry moves into uncharted territory.
For example, researchers at NIST recently used state-of-the-art atomic clocks, light detectors, and a measurement tool called a frequency comb to increase the stability of microwave signals by 100 times. This should enable much more accurate dissemination of time, improved navigation, more reliable communications, and higher-resolution imaging for radar and astronomy. The approach transfers the stability of a laboratory ytterbium lattice atomic clock operating at 518 THz to the microwave frequencies employed to calibrate electronic devices and systems that cannot in themselves directly count optical signals (Figure 1).
Specifically, photodiodes convert the light pulses into electrical currents that generate a 10-GHz microwave signal that precisely tracks the clocks with an error rate of 1 part in a quintillion (That’s a 1 followed by 18 zeros). This level of accuracy is about the same as optical clocks and 100 times more stable than the best microwave frequency sources. Ordinarily, even doubling microwave stability can take years or decades to achieve, so a 100-times improvement is a huge milestone.
Although the frequency combs and detectors are ready to be used in field applications, the ultimate goal is to use NIST’s ytterbium clocks in mobile applications, although they currently occupy large tables in controlled laboratory environments. Optically derived signals could also improve the performance of radar systems whose sensitivity for slow-moving objects is now limited by microwave noise. Using the photodiodes developed by NIST and the University of Virginia to convert the optical signals to the microwave spectrum could provide higher predictably and lower noise.
Seeking Standards for the New Spectral Frontier
As the wireless industry rolls out 5G, one of the major concerns, at least in the U.S., is the lack of spectrum at about 6 GHz and below, as well as how to supplement them by employing small cells operating at millimeter wavelengths throughout the country. This massive and expensive undertaking is complicated by the fact that there are currently no extremely accurate standards with which equipment manufacturers can verify their equipment at these high frequencies.
According to Samuel Benz, who heads NIST’s Superconductive Electronics Group, the standard must be linked to the International System of Units (SI) and repeatable, programmable, and consistently accurate, a combination that currently does not exist. That is, there are no components or metrology to address the very stringent standards that will be needed to cover this high frequency range.
To aid in this effort, NIST is developing an extremely fast waveform generator that transfers discrete, quantized units of magnetic flux (or “single flux quantum,” SFQ) along a circuit made of a series of Josephson junctions (JJs), each of which consists of two tiny superconducting electrodes separated by a very thin barrier.
An SFQ is stored as a persistent current in the superconducting loop formed by adjacent junctions and applying a current pulse in addition to the current already passing through a junction that causes the junction to switch. The result is that the SFQ is transferred to the next junction in the series and at the end of the line, the SFQ transfer creates a pulsed voltage waveform at frequencies 10 to 100 times higher than those used in a mobile phone.
To produce the required precisely quantized DC voltages, a microwave signal at tens of gigahertz is applied to the junction in the output a series of quantum-exact DC voltages in steps that depend only on the applied frequency and fundamental constants of physics. NIST researchers have continuously improved a design using more than 260,000 JJs that lets users select specific voltages.
The same general JJ design can be used to generate AC voltages, where a programmed stream of electrical pulses is applied to the junction that locks onto the applied pulses to produce AC waveforms, not just sine waves but other waveforms as well. The result is a Josephson Arbitrary Waveform Synthesizer (JAWS) codeveloped by SRI.
Previous generations of JAWS could not reach frequencies used by wireless systems, but new circuits and bias techniques have been created that reduce errors and can potentially generate quantum-accurate signals up to a few gigahertz. However, there are several other technical limitations, including the lack of an adequately fast pulse generator the ability of the system to output controllable and uniform waves dependably. The new RF JAWS design solves this problem by separating the JJs into two separate groups, one producing crests and the other troughs, with the groups combined and synchronized into a train of complete, distortion-free waveforms.
To generate quantum-accurate signals at 5G frequencies, NIST is pursuing an alternative solution by building faster superconducting digital circuits that also use JJs as the switching devices instead of transistors. These superconducting circuits work by storing and manipulating tiny, extremely fast electrical pulses.
Because the JJ switches are fast and their output is exactly quantized, SFQ technology can extend the range of synthesized waveform frequencies with quantum-exact amplitudes into the tens or hundreds of gigahertz. Since the SFQ pulses are so small, the output voltage of each circuit is tiny, and thousands must be integrated to reach desired, much higher voltages.
There will be numerous challenges, especially fabricating circuit components that have exceptionally small dimensions. The JJs on the present voltage standards measure about 7 µm, but for the SFQ circuits the JJ diameter is about 1 µm. The barriers between the superconducting electrodes of the Josephson junction are about 5 nm thick.
Another issue is that the program is faced with a lack of commercially available test equipment that can make the required measurements at hundreds of gigahertz. However, as precision timing and control is essential for the next generation of communication devices and systems, NIST hopes to develop calibration sources and a standard reference instrument for use by wireless chip and instrument manufacturers.
Making Sharing Work
Spectrum sharing has become a necessary part of 5G implementation as it’s one of the less onerous ways to provide adequate spectral resources. To that end, NIST has built its 5G Spectrum Sharing Test Bed, an adaptable network that can measure how well 5G and older systems such as WiFi, GPS, and military radar can operate without interfering with each other (Figure 2). The testbed is designed to evaluate thousands of different possible network settings and environment scenarios likely to be encountered when 5G and other services are operating in the same frequency band.
The 5G mmWave Channel Model Alliance, organized by NIST to address the need for accurate channel measurements and models, now has more than 175 participants representing 80 academic, government, and industry research organizations throughout the world. The alliance has produced dozens of models for 5G communications scenarios ranging from offices to shopping malls to outdoor areas. These resources are publicly available and used by many companies and some organizations that set telecommunications standards. The group has also developed best-practice measurement guidelines for instrumentation used at these frequencies.
The Billion-Dollar Bargain
For most government agencies, an annual operating budget of $1 billion would be lunch money, but at NIST it enables research into everything from nanoscale science to information technology, neutron research, material measurement, and physical measurements.
The agency operates four primary facilities; its headquarters in Gaithersburg, MD, as well as large facilities in Boulder and Ft. Collins, Colo, and a small one at Kauai, HI. NIST employs about 2,900 scientists, engineers, technicians, and support and administrative personnel, and supports about 1,800 NIST guest researchers and engineers from the U.S. and foreign countries, as well as partnerships with about 1,400 manufacturing specialists and staff at nearly 350 affiliated centers around the country.
Among dozens of other achievements, NIST researchers created the first radio receiver that could run on AC, pioneered military applications such as radio triangulation during World War I, and plays a critical role in timekeeping (and thus navigation) through advances in atomic clock references that today are accurate to less than 1 second in almost 20 million years. And everyone making any type of RF test measurement relies directly or indirectly on its measurement research for microwave parameters that include microwave calibration standards and services.
The Boulder laboratories, established in the 1950s, include NIST, NOAA, and the National Telecommunications and Information Administration (NTIA), and are located in the foothills of the Rockies. At this facility, research and engineering are conducted into electromagnetics, materials reliability, optoelectronics, quantum electronics and physics, time and frequency, earth systems, weather, and telecommunications.
It’s also the hub for NIST’s Communications Technology Laboratory (CTL), which was created to support radio communications research and standards programs. Since then, its efforts in communication technologies, including its current work related to “5G-and-beyond” advanced communications, are recognized as being among the best in the world.