Next generation of atomic clocks to provide improved performance, stability and durability for U.S. Department of Defense
ColdQuanta has been awarded a 5-year subcontract to develop portable atomic clocks for the Office of Naval Research. ColdQuanta will serve as a subcontractor to Vescent Photonics, which secured the $15.6 million total award.
Under the Compact Rubidium Optical Clock (CROC) program, ColdQuanta will provide the physics package with development inputs from the Atomic Devices and Instrumentation Group at the National Institute of Standards and Technology (NIST). The program began in November 2021 and will span three phases through 2026.
As part of the CROC program, ColdQuanta and its partners will design, build and deliver a new generation of high-performance atomic clocks ready for field deployment at a high technology readiness level (TRL). Specifically, the program will interrogate a two-photon optical clock transition in a warm vapor of rubidium atoms to achieve improved stability and performance. The clocks will also offer reduced size, weight and power consumption.
ColdQuanta is participating in the project alongside Vescent, which will provide optical frequency comb technology, and Octave Photonics and the Quantum Nanophotonics Group at NIST, which will supply crucial advances in non-linear nanophotonics. The outcome of the program will be 10 prototype field-deployable optical clocks at or above TRL 6 that exhibit long-term instability to better than three parts in 100 trillion and offer >50% reduction in power consumption.
The CROC program will be conducted in three phases:
Phase 1: All critical technology elements advanced to TRL 6 and demonstrated in a modular clock.
Phase 2: Engineering and verification efforts to integrate the individual components into prototype clocks.
Phase 3: Manufacturing 10 final prototype clocks for ONR evaluation in relevant platforms.
Register today to attend the ION’s co-located International Technical Meeting (ITM) and the Precise Time and Time Interval (PTTI) Systems and Applications Meeting, being held January 25–27, 2022, at the Hyatt Regency Long Beach in Long Beach, California, with technical presentations available for on-demand viewing at ion.org.
Plenary and Keynote Sessions
The ITM/PTTI 2022 keynote addresses, “Traffic Jams, Autonomy, and Lagrangian Control” and “The Future of Industrial Atomic Clocks,” taking place on Tuesday, January 25 will be recorded live and uploaded for on-demand viewing through the ITM/PTTI 2022 virtual meeting portal.
Technical Sessions
Individual technical presentations will be pre-recorded and uploaded with slides to the ITM/PTTI 2022 virtual meeting portal for viewing at a time of your choosing, and will remain available for 30 days. Attendees will have the option to submit questions to each presenter. View the full online Technical Program now!
Exhibit Experience
ITM/PTTI 2022 features industry partners with expanded exhibitor profiles, that allow attendees to review the latest PNT-related technologies, products, and product demonstration videos.
Sandia National Laboratories scientist Peter Schwindt, left, and postdoctoral scientist Bethany Little examine the vacuum package held in a yellow, 3D-printed mount. (Photo: Bret Latter/Sandia)
The compact, fieldable device could provide means to navigating without GPS
News from Sandia National Laboratory
Don’t let the titanium metal walls or the sapphire windows fool you. It’s what’s on the inside of this small, curious device that could someday kick off a new era of navigation.
For more than a year, the avocado-sized vacuum chamber has contained a cloud of atoms at the right conditions for precise navigational measurements. It is the first device that is small, energy-efficient and reliable enough to potentially move quantum sensors — sensors that use quantum mechanics to outperform conventional technologies — from the lab into commercial use, said Sandia National Laboratories scientist Peter Schwindt.
Sandia developed the chamber as a core technology for future navigation systems that don’t rely on GPS satellites, he said. It was described earlier this year in the journal AVS Quantum Science.
Countless devices around the world use GPS for wayfinding. It’s possible because atomic clocks, which are known for extremely accurate timekeeping, hold the network of satellites perfectly in sync.
But GPS signals can be jammed or spoofed, potentially disabling navigation systems on commercial and military vehicles alike, Schwindt said.
Instead of relying on satellites, Schwindt said future vehicles might keep track of their own position. They could do that with onboard devices as accurate as atomic clocks, but that measure acceleration and rotation by shining lasers into small clouds of rubidium gas like the one Sandia has contained.
Atomic accelerometers and gyroscopes already exist, but they’re too bulky and power-hungry to use in an airplane’s navigation system. That’s because they need a large vacuum system to work, one that needs thousands of volts of electricity.
A compact device designed and built at Sandia National Laboratories could become a pivotal component of next-generation navigation systems. (Photo: Bret Latter/Sandia)
“Quantum sensors are a growing field, and there are lots of applications you can demonstrate in the lab,” said Sandia postdoctoral scientist Bethany Little, who is contributing to the research. “But when you move it into the real world, there are lots of problems you have to solve. Two are making the sensor compact and rugged. The physics takes place all in a cubic centimeter (0.06 cubic inches) of volume, so anything larger than that is wasted space.”
Little said her team has shown that quantum sensing can work without a high-powered vacuum system. This shrinks the package to a practical size without sacrificing reliability.
Instead of a powered vacuum pump, which whisks away molecules that leak in and wreck measurements, a pair of devices called getters use chemical reactions to bind intruders. The getters are each about the size of a pencil eraser so they can be tucked inside two narrow tubes sticking out of the titanium package. They also work without a power source.
To further keep out contaminants, Schwindt partnered with Sandia materials scientists to build the chamber out of titanium and sapphire. These materials are especially good at blocking out gasses like helium, which can squeeze through stainless steel and Pyrex glass. Funding was provided by Sandia’s Laboratory Directed Research and Development program.
Construction took sophisticated fabrication techniques that Sandia has honed to bond advanced materials for nuclear weapons components. And like a nuclear weapon, the titanium chamber must work reliably for years.
The Sandia team is continuing to monitor the device. Their goal is to keep it sealed and operational for five years, an important milestone toward showing the technology is ready to be fielded. In the meantime, they’re exploring ways to streamline manufacturing.
Sandia National Laboratories is a multimission laboratory operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration. Sandia Labs has major research and development responsibilities in nuclear deterrence, global security, defense, energy technologies and economic competitiveness, with main facilities in Albuquerque, New Mexico, and Livermore, California.
Merger to deliver high-end performance and ultra-accurate timing and frequency products for commercial, critical infrastructure and defense markets worldwide
Orolia has entered into a definitive agreement to acquire Seven Solutions, a global innovator in White Rabbit sub-nanosecond time transfer and synchronization technology. This transaction is subject to customary closing conditions and approvals required by the Spanish government and is expected to close before the end of the year.
This is Orolia’s 11th acquisition in the past seven years.
The merger with Seven Solutions, based in Granada, will enhance Orolia’s portfolio for defense, aerospace, data centers, telecom, financial services, smart grids and other critical infrastructure industries, and will enable the next-generation applications dependent on ultra-precise, resilient timing and frequency technology.
“Orolia and Seven Solutions under one umbrella will combine our world-leading technologies to draw a new frontier in network timing to sub-nanosecond levels, delivering the most robust and accurate resilient PNT solutions for our customers,” said Orolia CEO Jean-Yves Courtois. “Seven Solutions’ long history of delivering cutting-edge time distribution solutions to sectors like telecommunications, smart grids, aerospace, defense and scientific facilities aligns perfectly with Orolia’s DNA.”
Orolia and Seven Solutions will integrate global sales, marketing, product development and operations.
Orolia’s full-scale, modular approach to resilient PNT includes atomic clocks with a combination of GNSS signals protected with interference detection and mitigation technology, together with low-Earth-orbit secure alternative signals. The addition of Seven Solutions’ products will deliver terrestrial sub-nanosecond time distribution from distant and potentially redundant locations.
“We believe the union of our companies will produce the future of time transfer and frequency distribution solutions in terms of accuracy, reliability and interoperability,” said Rafael Rodriguez, chief technology officer and co-founder of Seven Solutions. “Finance, 5G telecommunications, data centers and hyperscalers have new and upgraded functionalities requiring ultra-accurate time distribution accuracy. To maximize interoperability, our solution for time transfer is based on the White Rabbit concept that has been pushed over the last decade to become the basis of the standard high accuracy time transfer profile (within the recent release of IEEE 1588 of precision time protocol).”
Orolia and Seven Solutions are members of the Open PNT Industry Alliance. The international organization focuses on market concepts that strengthen economic and national security by supporting government efforts to implement resilient PNT capabilities for critical infrastructure.
Facebook has open-sourced the design of its time card, which features the ultra-precise u‑blox ZED-F9T timing module, providing easy access to nanosecond-level timing
Photo: u-blox
Facebook has chosen the u‑bloxZED-F9T GNSS receiver module for timekeeping, according to u-blox. By improving the synchronization of networked computers, Facebook’s time card can significantly speed up the performance of its data centers and distributed databases.
By open-sourcing their designs, Facebook has bolstered the adoption of highly accurate timing solutions based on u‑blox technology. These solutions can be adopted by other industries requiring nanosecond-level timing, such as 5G cellular networks or smart power grids.
Facebook set out to create a precise timing solution that reduces the computational overhead required when synchronizing the timing between different computers in a network, u-blox said. The social media company used a u‑blox ZED-F9T multi-band GNSS receiver to sync up its solution with the highly accurate GNSS atomic clocks. To bridge possible gaps in GNSS coverage and keep clock drift to a minimum, the time card contains a backup timing source: a miniaturized atomic clock continuously synchronized with GNSS time.
To maximize the impact of the solution, Facebook decided to open-source the design of its time card, which fits onto a PCIe form factor. Anyone with experience working with microelectronics can turn any PC built on an x86 architecture and featuring a network interface controller into a nanosecond-level-accurate timing and synchronization solution, u-blox said.
Easy access to nanosecond-level timing accuracy — based on the u‑blox RCB-F9T timing board, which hosts the u‑blox ZED-F9T GNSS receiver — opens new avenues in industry segments that rely on highly synchronized signals, such as 5G network base stations that require tighter synchronization than those of previous generations, u-blox said.
As power-distribution networks become more complex to accommodate a growing share of decentralized renewable energy, they are becoming more reliant on reliable and accurate timing solutions. Data centers and computer networks will be able to modernize infrastructure management to speed up performance and reduce latencies.
Facebook has shared the GitHub repository including the specs, the schematics, the mechanics, the bill of material, and the source code in partnership with the Open Compute Project (OCP) under the Time Appliance Project (TAP).
Orolia has been awarded €70 million in two contracts to provide atomic clocks for the first 12 satellites of the Galileo Second Generation System (G2S). The first was from the European Space Agency (ESA) and the second from Leonardo.
Each of the new G2S satellites, designed to provide unprecedented accuracy worldwide, will contain three Orolia Rubidium Atomic Frequency Standards (RAFS) and two Orolia atomic clock physics packages integrated with Leonardo’s Passive Hydrogen Masers (PHM).
“We are truly honored to be selected by the European Commission, ESA and Leonardo to continue to supply our advanced space atomic clocks for the next generation of Galileo,” said Jean-Yves Courtois, CEO of Orolia. “Our dedication, hard work and innovative design for all the clocks in the current Galileo constellation have contributed to the most accurate GNSS system in service today. We look forward to continuing to support the Galileo program with the most advanced GNSS timing technology available in the world.”
Orolia’s RAFS is an ultra-stable rubidium atomic clock able to deliver a frequency stability of about 2 x 10-14 over averaging intervals of 10,000 seconds. The Leonardo PHM, with its excellent frequency stability performance, is the master clock for the Galileo satellite payload. The maser technology embedded on Galileo offers superior stability compared to all other types of clocks onboard navigation satellites, according to Orolia.
The RAFS Flight Model atomic clock will fly aboard the second generation Galileo satellites. (Photo: Orolia)
Orolia has delivered more than 140 RAFS Flight Models worldwide, with 114 flying on GNSS satellites. In addition, 100 PHM Flight Models have been delivered worldwide, and 56 are flying on the current Galileo constellation.
According to ESA, the G2S satellites will revolutionize the Galileo constellation, joining the 26 first-generation satellites currently in orbit. They will be much larger than the existing Galileo satellites, use electric propulsion for the first time, and feature a more powerful navigation antenna. The G2S constellation should achieve decimeter-scale positioning precision.
In May, the European Commission and ESA announced the selection of Orolia to provide its Skydel GNSS signal simulation core engine for the G2S radiofrequency constellation simulator.
The Galileo program is managed and funded by the European Union. The European Commission, ESA and EUSPA have signed an agreement by which ESA acts as design authority and system development prime on behalf of the commission and EUSPA as the exploitation and operation manager of Galileo.
Projects will advance technology used for secure GNSS, quantum timekeeping and communications
ColdQuanta, which specializes in cold atom quantum technology, has been awarded two development contracts from U.S. government agencies worth $2.55 million. Both projects are based on the company’s Quantum Core technology, which uses atoms cooled to a temperature of nearly absolute zero and lasers to manipulate and control the atoms with extreme precision.
Prototype Atomic Clock
The Office of the Under Secretary of Defense for Research & Engineering (OUSD R&E) awarded ColdQuanta $1.8 million for the development of a prototype atomic clock that could enable reliable, highly accurate position, navigation and timing (PNT) capabilities necessary for the functioning of critical infrastructure around the world. Atomic clocks are used for GPS/GNSS systems as well as for time-distribution services that are the basis of financial networks, computer, TV and radio services and other applications.
Alternative PNT. However, services such as satellite-based GPS can be spoofed, lack encryption or other security features, and often can’t deliver the signal strength required. Because of this, the development and deployment of a PNT system that doesn’t depend on GPS is a critical need across governments and industry, according to ColdQuanta. The company said this is especially true for mobile systems such as aircraft and spacecraft that need to know their position with great precision even when GPS is unavailable.
Under this project, ColdQuanta will deliver a state-of-the-art atomic clock with “instant on” capability if a GPS signal is lost, with a timing accuracy comparable to the best commercial clocks. It will also be ruggedized, portable and compact to enable its use in aircraft, and will minimize the power draw in between periods of demand.
“High-performance atomic clocks are the backbone of the internet, electrical power grids, financial networks, and autonomous navigation. Combining this with ColdQuanta’s development of related inertial navigation devices — such as gyroscopes, accelerometers and gravimeters — will lead to the first Quantum Positioning Systems,” said Dan Caruso, executive chairman and CEO of ColdQuanta. “We’re excited to work closely with the Department of Defense to meet their urgent needs, while also advancing the capabilities needed for future, unassailable global positioning technology.”
This velocity-distribution data for a gas of rubidium atoms confirmed the discovery of the Bose–Einstein condensate in 1995. In these three snapshots in time, atoms—cooled to near absolute zero—condensed from less dense areas on the left (red, yellow, and green) to very dense areas at the center and the right (blue and white). (Image: NIST/JILA/CU-Boulder)
Miniaturized Ion Trap System
Also, the Air Force Research Laboratory (AFRL) awarded ColdQuanta $750K for the development of a high-performance miniature ion trap system. Compact ion trap systems are applicable to a spectrum of quantum applications including quantum networks, computing, metrology, and timekeeping.
ColdQuanta previously developed a prototype miniaturized ion trapping system with performance that is competitive with traditional, large-footprint vacuum systems. The system maximizes performance and robustness while minimizing size, weight, and power consumption (SWaP). The new AFRL award will build on the success of this prototype to increase performance, reduce cost, and create a robust architecture for deployable quantum platforms.
Bose-Einstein Condensate
The story of ColdQuanta began in 1924 with the discovery of the Bose-Einstein condensate (BEC) — also known as the fifth form of matter — by Satyendra Bose and Albert Einstein. Seventy years later, BEC was first synthesized at the University of Colorado at Boulder in collaboration with the National Institute of Standards and Technology (NIST), for which Eric Cornell and Carl Wieman won a Nobel Prize in 2001. Their colleague, Dana Anderson, co-founded ColdQuanta, which is using the fifth form of matter as the foundation for its cold atom quantum technology.
When atoms are cooled to a few millionths of a degree above absolute zero, they take on quantum properties. Lasers are used to arrange the atoms, hold them in place, run computations on them, and read out the results. Quantum calculations, communications and sensing are the result.
ColdQuanta is collaborating with global customers including major commercial and defense companies; the U.S. Department of Defense; national laboratories operated by the Department of Energy, NASA, and NIST; major universities; and quantum-focused technology companies to advance products and services development with Cold Atom Quantum Technology. ColdQuanta is based in Boulder, with offices in Madison, Wisconsin, and Oxford, United Kingdom.
New atomic clock technology will improve GNSS location accuracy, as well as addressing the scalability of other quantum technologies being developed
Nanofabrication experts Kelvin Nanotechnology have teamed up with product design specialist Wideblue, the University of Strathclyde and the University of Birmingham on a UK Research and Innovation (UKRI) project funded by the Industrial Strategy Challenge Fund to develop innovative techniques in the miniaturisation of optical atomic clocks.
The new clock technology will help improve GNSS location accuracy, as well as addressing the scalability of other quantum technologies being developed by the academic partners.
“Small, low cost atomic clocks will be essential as we develop a resilient position, navigation and timing (PNT) infrastructure to support our financial, power distribution and communications services,” said Roger McKinlay, challenge director – Quantum Technologies at UKRI.
Cold atomic samples have led to profound advancements in precision metrology by measuring the frequency separation of discrete atomic energy levels. These atomic clocks are the ultimate timekeepers, with the state-of-the-art instruments providing a timing accuracy that it would neither gain nor lose a second in over 30 million years.
Because of the high level of accuracy in these instruments, atomic clocks are used to coordinate systems that require extreme precision, such as GNSS. Each satellite network contains multiple atomic clocks that contribute precision timing data, which is decoded to provide location data by effectively synchronizing each receivers’ atomic clocks with those of the satellite.
“The project is a feasibility study which aims to facilitate the miniaturization of state-of-the-art atomic clocks.” said Russell Overend, managing director of Wideblue. “To achieve such high timing resolution, the atomic clock makes use of ultra-narrow transitions in strontium atoms, providing orders of magnitude better performance than their rubidium counterparts due to narrower atomic features. In simple terms, the narrower the atomic transition the more accurate the atomic clock.
At Strathclyde, cold atom clock experiments are aided by expertise in grating magneto-optical traps (gMOTs), illustrated here. (Image: Aidan Arnold, University of Strathclyde)
An important factor in cold atomic clock technology is grating magneto-optical traps (gMOTs). With gMOTs, diffraction gratings split and steer an incoming beam into a tripod of diffracted beams, allowing trapping in the four-beam overlap volume.
Wideblue will develop the optical system that will deliver the laser light onto the gMOT chip. Kelvin Nanotechnology will manufacture the gMOT and compact collimation optics designed by Wideblue. The University of Strathclyde will design the gMOT chip, and the University of Birmingham will perform the testing of the prototype optical system.
“Atomic clocks are an integral component in modern technology and impact our daily routines from computing and financial transactions to the navigation systems we use in our phones and cars,” said James McGilligan, Kelvin Nanotechnology, “As state-of-the-art atomic clocks push new boundaries in precision measurement, we face a new challenge of bringing this complex and large physical apparatus into a compact and user-friendly system where we can make the largest societal and economic impact.
“Our current collaboration with Wideblue and our academic partners aims to address the scalability of one such atomic clock by reducing the optical constraints into scalable micro-fabricated components as a critical step to bringing laboratory performance out into real world applications,” McGilligan said.
“With support from the Quantum Technologies Challenge in UKRI — part of the UK National Quantum Technologies Programme — we are ensuring that the UK economy and society will benefit from the next generation of quantum devices and be quantum ready,” McKinlay said.
Oxford Technical Services (OxTS) has launched precision time protocol (PTP) master functionality on all of its next-generation inertial navigation systems (INS).
PTP is a network-based time synchronization protocol used to synchronize all clocks throughout a computer network. It is used in many industries, but most notably in finance to synchronize transactions, mobile-phone tower transmissions and subsea acoustic arrays.
Time synchronization
In many commercial organizations, millisecond-level device synchronization as offered with network time protocol (NTP) is sufficient. However, in surveying and automotive testing environments where there is more than one clock source (lidar and inertial navigation systems, or INS, for example), final results can suffer from time drift if millisecond — and not microsecond — synchronization is used.
Time drift becomes relevant as soon as you introduce more than one data acquisition system working in parallel. This is because each system will have its own timing error, and over time this error will grow and create drift.
For surveyors, time drift can negatively impact point clouds by making object recognition difficult, subsequently leading to blurring and double vision.
For automotive engineers, when running campaigns, analysis of events within your data may be misaligned, making the analysis more difficult and/or less efficient.
Stamp out time drift
To stamp out time drift, it is important to use the most accurate clock source available.
A key component of an INS is the GNSS receiver. The GNSS receiver acquires data, including timing information, directly from multiple GNSS constellations (GPS, GLONASS, BeiDou and Galileo). The GNSS receiver, coupled with the inertial measurement unit within the INS, allows users to benefit from the centimeter-level position accuracy that is so important in surveying and automotive testing environments.
These satellite systems house the most accurate time source possible — atomic clocks — meaning that devices connected to a network that includes an INS can take advantage of this time source owing to the GNSS receiver within the INS.
Simpler setup for lidar use
By migrating from a traditional PPS hardware set-up, which involves connecting and wiring multiple cables, to a PTP setup, which is essentially an Ethernet “plug-and-play” solution, users can also make day-to-day use of the equipment simpler and more efficient.
Without PTP – using PPS setup. (Image: OxTS)An example PPS hardware setup with a PTP-enabled network. (Image: OxTS)
This much-improved hardware setup allows surveyors and automotive test engineers to be up and running in a much shorter time frame than previously possible.
Adding value to the automotive industry
The addition of PTP also adds value for automotive users. With cars-under-test incorporating multiple sensors (lidars, cameras, etc.), synchronizing all that data can help support accurate analysis after the test is complete.
OxTS is continuing to develop its PTP solution by working on PTP slave functionality and improving the configuration process, which will provide greater flexibility in typical automotive setups that use data acquisition (DAQ) for larger sensor networks.
Summary
PTP as a time synchronization method is becoming more popular, particularly in the lidar industry, with manufacturers such as Ouster and Hesai enabling PTP on their sensors.
The shorter “time to survey” gives customers a much-enhanced user experience, and the higher quality final output on offer means that many users will demand their sensors are PTP-compatible before considering them for their projects.
Manufacturers of complimentary sensors, such as INS, need to build the capability into their product sets to allow them to be fit for the future.
Various OxTS INS are available to use PTP, including the new xNAV650, the company’s new small, lightweight and affordable INS for applications where payload size and weight matter. Learn more about the xNAV650 INS.
Users can also find out more about OxTS and its range of PTP-enabled devices by visiting its dedicated landing page, OxTS PTP-enabled INS devices.
A new miniature atomic clock offers improvements to temperature sensitivity and long-term drift, which correlate to longer holdover durations. Features important to mobile applications —warm-up characteristics, gravity sensitivity, and shock and vibration — as well as new 1 pulse-per-second (PPP) input and output signals are highlighted.
By William Krzewick, Jamie Mitchell, John Bollettiero, Peter Cash, Kevin Wellwood, Igor Kosvin and Larry Zanca
The miniature atomic clock (MAC) was developed out of the same size and power-reducing technology, known as coherent population trapping (CPT), as the venerable chip-scale atomic clock (CSAC). By implementing low-power lasers as opposed to traditional lamp designs, this technology allows for unparalleled performance versus power consumption in the commercial oscillator domain.
Since its initial release in 2009, the MAC has been well-suited for telecom applications as a holdover reference oscillator in GNSS-denied environments. Now, with advances in field-programmable gate array (FPGA) design, signal processing and electronics miniaturization, and by leveraging more than 40 years of atomic clock design at Microchip Technology, the next generation MAC is designed to meet a variety of applications with demanding mission scenarios.
In this article, we discuss improvements to temperature sensitivity and long-term drift, which correlate to longer holdover durations. We also discuss warm-up characteristics, gravity (g)-sensitivity, and shock and vibration, which are important for mobile applications. Finally, several new features will be introduced including a 1 pulse-per-second (1PPP) input and output signal.
INTRODUCTION
Low-drift performance over time and frequency stability during temperature changes have enabled small atomic oscillators to maintain precise time and frequency in the absence of a primary reference such as GNSS. The MAC-SA5X rubidium (Rb) miniature atomic clock has advanced the design of the legacy MAC-SA.3Xm with a wider operating temperature range, additional features and improvement in frequency drift and temperature stability to enable longer holdover durations. Measuring 2 × 2 × 0.72 inches (5.08 × 5.08 × 1.83 centimeters), it is designed for size and power-constrained applications that require atomic clock performance.
FIGURE 1 shows exterior and interior views of the MAC, while FIGURE 2 is a block diagram of the clock. The vertical-cavity surface-emitting laser (VCSEL) with thermoelectric cooler (TEC) generates the light source at the appropriate wavelength. The laser light is directed into the resonance cell to stimulate the Rb atoms. Use of a VCSEL, as opposed to the traditional lamp design, results in a relatively low-power, small-form-factor package while eliminating frequency jumps and preserving short-term stability. The new TEC enables fast temperature response, increased temperature set-point resolution, and a larger temperature range.
FIGURE 1 Top view (left), inside view (center) and bottom view (right) of MAC. (Photo: Microchip)FIGURE 2. Block Diagram of MAC. (Diagram: Microchip)
The temperature-compensated crystal oscillator (TCXO) drives an FPGA-based direct digital synthesizer (DDS) for higher accuracy with minimal board space intrusion, differential signaling and additional power isolation. Linear microwave control, which has direct impact on frequency stability as measured by the Allan deviation (ADEV), lock times and temperature compensation, is a key improvement.
The resonance cell subassembly contains the Rb gas mixture. It is surrounded by an oven with C-field (static magnetic field) coil necessary for controlling the temperature and magnetic field, respectively, of the Rb atoms. Dual magnetic shields mitigate the effects of external magnetic fields. The photodiode printed-circuit-board assembly detects CPT resonance of the clock. The resonator is fundamentally unchanged and therefore not expected to impact the quality factor, Q, of the oscillator.
The signal-to-noise ratio (SNR) of the CPT signal, on the other hand, has improved thanks to the updated control electronics design, faster servo-loop algorithms and use of lower noise electronics. This is evident in the less noisy clock transition for the MAC-SA5X (orange trace in FIGURE 3) versus the predecessor (black trace). Because the 1-second ADEV is proportional to 1/(Q×SNR), the short-term stability is improved in the new design.
FIGURE 3. CPT resonance of MAC. (Image: Microchip)
PERFORMANCE
This next generation of the rubidium atomic clock leverages substantial improvements in both hardware and software. These improvements, coupled with more than a decade of experience in practical CPT technology, have allowed for significant insight into physics behavior and interrogation techniques. This has resulted in improvements to key performance parameters such as temperature range, stability, retrace and lock times. These metrics will be reviewed in the following sections by comparing data from a sample of pre-production engineering units.
ADEV. Short-term frequency stability of the oscillators is represented in FIGURE 4 as an ADEV measurement. The MAC-SA5X has two performance classifications: The SA53 is the base-performance (red dots) and the SA55 is the high-performance (red squares). The MAC-SA55 has a 1-second integration period, tau (τ) = 1 second, ADEV requirement of less than 3 × 10-11, that follows a 1/√τ behavior to τ = 1000 seconds. ADEV rises at 105 seconds to accommodate the mid-/long-term frequency drift of the oscillator, with a generous margin. The base-performance version MAC-SA53 has a looser ADEV specification of less than 5 × 10-11 at 1 second that follows a 1/√τ behavior to 100 seconds.
On average (dashed line), the sample units had a 1-second ADEV of about 1.2 × 10-11. A narrow grey line represents the average values of the data set plus two standard deviations, and the orange line represents a sample unit that closely mirrored the average performance (limited sample size of five for long-term testing).
Two notes on Figure 4 are worth mentioning: The standard deviation line has a larger spread from average as the observation interval increases and a small (~2 × 10-13) bump exists in the measurement at 400 seconds. The former is due to increased measurement noise as there are simply fewer data points for longer τ. The latter is believed to be a result of the heating, ventilation and air conditioning (HVAC) system in the laboratory as it cycled. All MACs are compensated to reduce temperature effects, as will be discussed later. However, these units were not compensated at the time of testing and were more susceptible to HVAC temperature effects compared to full-production units.
FIGURE 4. Frequency Stability vs. Observation Interval (τ) of MAC Sample Units. (Image: Microchip)
Aging. Long-term frequency drift (monthly aging rate) of the MAC has a requirement of 1 × 10-10 per month and 5 × 10-11 per month for the SA53 and SA55 variants, respectively. It is important to note that the majority of sample units fall well within the tighter 5 × 10-11 per month requirement and accordingly affect the average mid-/long-term stability in the ADEV plot. Future production units that only meet the baseline SA53 performance could have inferior stability beyond τ = 100 seconds, compared to our sample data.
TDEV. The time stability of the phase is represented in FIGURE 5 as a time deviation (TDEV) measurement. This type of test is important to compare oscillators, since it gives an estimation of time error accumulation due to only the free-running oscillator itself by removing time or frequency errors at the beginning of the test. The graph uses the same color scheme as the ADEV plot to indicate average data (dashed line), average plus two standard deviation data (thin line) and a sample unit as an orange trace.
FIGURE 5. Phase Stability vs. Observation Interval (τ) of MAC Sample Units. (Image: Microchip)
Based on the required stability performance of the SA55, the time error after three days for a free-running oscillator is predicted to be less than 650 nanoseconds. For the measured units, the MACs had a TDEV of about 230 nanoseconds at τ = three days, due to the long-term drift performance of our samples.
Phase Noise. Phase noise for the MAC has two classifications: base performance and high performance over the range 1 Hz to 10 kHz.
Average phase noise data is well below the requirements, for our samples.
Temperature Effects. As a small Rb oscillator, the MAC inherently has low sensitivity to environmental temperature perturbations compared to most commercial quartz oscillators. To further improve performance, each MAC is characterized and compensated with a high-order polynomial fit of temperature effects to reduce peak-to-peak frequency changes below 5 × 10-11 over a wide operating range. The SA53 has a two times relaxation for this requirement.
Retrace. Retrace specifications are provided to indicate the expected frequency change of an oscillator due to that oscillator being powered off and back on again. The MAC retrace test is defined as follows:
The MAC is powered on, and its frequency offset (from nominal) is measured after 24 hours.
Power is removed for 48 hours.
Power is turned back on, and its frequency offset is measured again after 12 hours.
The delta frequency between the two measurements is calculated to be within ±5 × 10-11.
A test verified the specification of ±5 × 10-11 after 12 hours.
For this test, however, we did not wait 12 hours to measure the retrace frequency change. Instead, we began measuring immediately after power was turned back on. The measured data from sample SN00011 is indicative of typical performance and shows how the MAC retrace frequency delta is well within ±1 × 10-11. This unit had a slightly positive delta and meets the retrace requirement in minutes — far sooner than the modest 12-hour specification.
The sample units as a whole performed similarly to the sample SN00011.
Warm-up Time. Defined as the time to reach atomic lock, warm-up time is the point at which atomic resonance is attained and the short-term stability performance of the oscillator will be achieved. Test average and standard deviation data is well within the requirement of 8 minutes at temperatures greater than –10°C. At colder temperatures, the requirement is 12 minutes.
Typical performance is about four minutes to achieve lock at a starting temperature of 25°C. This has been a major design focus; all MACs are designed and tested to quickly achieve lock at all temperatures.
Power Consumption. Average power consumption in a 25°C environment is about 6 W. Warmer environments reduce the power consumption, due to less required heating of the resonance cell to achieve the appropriate temperature.
1PPS Disciplining. A 1-Hz (1PPS) input and output signal are new features for the MAC. The 1PPS output is derived directly from the TCXO, and its stability performance is therefore tied to the RF output performance. The 1PPS input accepts a reference signal from a primary reference clock to calibrate the MAC’s 1PPS (and RF) output. The algorithm will simultaneously steer the phase and frequency to that of the external reference (1PPS input), ultimately achieving accuracies of less than 1 nanosecond and 1 × 10-13, respectively. This feature is quite useful for applications where absolute frequency or phase errors need to be minimized and is similar to the function available on the CSAC.
The MAC can quickly calibrate its RF output by turning on the 1PPS disciplining feature to correct a 1.4 × 10-8 frequency error in minutes. A user can adjust the disciplining time constant to accommodate for noisier 1PPS input signals, if necessary.
g-Sensitivity Testing. Vibration and g-sensitivity testing was conducted. Static acceleration effects, such as a “tipover” test, on atomic clocks are minimal, and they exhibit a sensitivity of several parts per trillion per g. The MAC significantly outperformed a commercial oven-controlled crystal oscillator or OCXO. This type of performance is important for applications where the equipment is placed on its side, for instance.
Unlike static acceleration, effects due to random vibration profiles are determined mostly by the TCXO and will adversely affect the performance. Preliminary testing of the MAC has shown an effective sensitivity of several parts per billion per g. TABLE 1 describes the profile used to test the MAC from “MIL-STD-810, Fig. 514.7E-1, Category 24.” The profile was applied to all three axes tested.
Table 1. Random Vibration Profile Expressed as Power Spectral Density (PSD). (Data: Microchip; Graphic: GPS World)
The g-sensitivity may be calculated from the dynamic phase-noise measurement. The total effective g-sensitivity was determined by taking the magnitude due to the random vibration profile applied in all three axes.
The total effective g-sensitivity due to the random vibration profile is about 2.4 × 10-9 per g. Results of the worst-case sensitivity are summarized in TABLE 2.
Table 2. Summary of g-Sensitivity. (Data: Microchip; Graphic: GPS World)
Table 1. Random Vibration Profile Expressed as Power Spectral Density (PSD). (Data: Microchip; Graphic: GPS World)
SUMMARY
Based on the CPT method of interrogation, a commercial miniaturized rubidium atomic clock has been developed with a wider operating temperature of –40 to +75°C and improved performance over its predecessor MAC-SA.3Xm. New features, such as the 1PPS input, allow users to connect a GNSS-derived signal to calibrate the clock and then maintain timing during GNSS-outages for longer durations thanks to improvements in stability performance. Retrace measurements of ±1 × 10-11, temperature stability of less than 5 × 10-11 and fast/consistent warm-up times along with the small size and power afforded by CPT technology enable a variety of mobile applications.
ACKNOWLEDGEMENT
This article is based on the paper “A Next-Generation, Miniaturized Rb Atomic Clock Reference for Mobile, GNSS-Denied Environments” presented at ION ITM 2020, the International Technical Meeting of The Institute of Navigation, held in San Diego, California, Jan. 21–24, 2020.
At Microchip Technology, WILLIAM KRZEWICK is the product line manager, JAMIE MITCHELL is the manager of engineering, JOHN BOLLETTIERO is an associate engineer, PETER CASH is the associate director of clock products, KEVIN WELLWOOD is the manager of software engineering, IGOR KOSVIN is the principal engineer of electrical engineering and LARRY ZANCA is the principal engineer of mechanical engineering.
Jackson Labs Technologies (JTL) has launched the PNT-6220 Assured Reference — a product combining low-Earth-orbit (LEO) signals, GNSS, terrestrial, wireline and atomic clock services in one small solution, specifically designed for critical infrastructure applications.
The PNT-6220 reference seamlessly combines concurrent L1, L2, L3 and L5 GNSS reception with a custom JLT-designed LEO-based Satellite Time and Location (STL) timing receiver. It also includes terrestrial receivers and PTP/IEEE-1588 edge grandmaster (EGM) and PTP/IEEE-1588-slave capability.
The PNT-6220 provides assured PNT for critical infrastructure applications such as those described in the directives of Presidential Executive Order 13905.
It can serve as a timing reference for 5G equipment, an ePRTC-capable reference, or a high-performance disciplined reference that supports PTP/IEEE-1588, STL, RF distribution and multi-frequency GNSS capability.
The PNT-6220 will be able to select the most optimal UTC reference input automatically and auto-switchover among its numerous reference inputs if one or more of them are jammed or spoofed, as well as average several references for additional stability and accuracy.
If all external references are jammed, the unit can provide UTC timing from its internal holdover oscillator with options that have less than 100-ns drift over 24 hours. The unit is also capable of outputting a GPS RF distribution signal driven by the internal flywheel oscillator, which allows glue-less retrofitting of any GPS-based legacy user equipment to the state-of-the-art reference sources the PNT-6220 can receive by simply plugging into the legacy equipment GPS antenna input.
Available Options
Numerous options are available for the half-width 19-inch-wide rack-mount box.
Orolia has introduced a low SWaP-C miniaturized rubidium oscillator, the Spectratime mRO-50, designed to meet the latest commercial, military and aerospace requirements where time stability and power consumption are critical. The oscillator is low SWaP-C — size, weight, power and cost.
The Spectratime mRO-50 provides a one-day holdover below 1 µs and a retrace below 1 x 10-10 in a form factor sized 50.8 x 50.8 x 19.5 millimeters. It takes up only 51 cc of volume — about one-third of volume compared to standard rubidiums — and consumes only 0.45 W of power.
he Spectratime mRO-50 miniaturized rubidium oscillator provides accurate frequency and precise time synchronization to mobile applications, such as military radio-pack systems in GNSS-denied environments. Its operating temperature of -10°C to 60°C (military version extends to -40°C to 75°C) is also suitable for UAVs and underwater applications.
Orolia is a leader in space-based atomic clocks and high-end crystal, rubidium, hydrogen maser and integrated GPS/GNSS clocks. The company also provides testing instruments for space missions that rely on high precision atomic clock technology.
Orolia’s Atomic Clocks team received the 2019 PTTI Distinguished Service Award in January for advancing the state of the art in high-stability atomic clocks and producing the only space-based passive H-maser in the world, operating on all Galileo satellites. Spectratime mRO-50 is the latest technology solution from this award-winning team.
“Through Orolia’s continuous commitment to innovation, we are proud to offer our customers more precise PNT data in a cutting-edge, lightweight form factor for mobile missions,” said Orolia’s Atomic Clocks Product Line Director, Jean-Charles Chen.
