Chip-scale atomic clocks can help with UAV SWaP design challenges

2A portable atomic clock is just the ticket for many UAVs, and the more SWaP-optimized the better. The Chip-Scale Atomic Clock (CSAC) fits the bill with the low power draw and accurate performance inherent in its design.

Unmanned Aerial Vehicles (UAVs) began as tools for military surveillance. As their capabilities expanded, they found usage in civilian applications such as border patrols and drug interdiction, while on the military side the expanded capabilities led to missions using armed UAVs.

Throughout their use, accurate clocks have been required for UAVs to carry out their missions. A principal need has been navigation; UAVs typically use a clock that has been synchronized to Global Positioning System (GPS) for very accurate timing. However, when the GPS signal is lost, the clock is used to provide a “holdover” function that integrates with a backup navigation system, usually some form of an Inertial Navigation System (INS). The clock’s holdover performance is important because, in military applications, GPS signal loss is sometimes due to intentional jamming, which can persist for long periods of time.

Accurate clocks are also needed in UAV communications. As UAV sensor payloads have advanced from still photos to video, to video integrated with infrared and other sensor data, high-density encrypted waveforms have been employed to transmit this data, as well as to receive vehicle control data. These waveforms can only stay synchronized with stable, accurate clocks.

Layered on top of these application requirements are the demands of Size, Weight, and Power (SWaP). Almost every component in the electronics of a UAV – whether part of the basic airframe or part of the specialized payload – is being pushed to reduce SWaP so that a given UAV can increase its mission duration (for more “persistent surveillance” in military terminology), or so that it can add more sensor capabilities without shortening mission duration. The choice of clock onboard can positively or negatively affect SWaP in UAV design.

The choices

A variety of clock choices have historically been available for UAV applications. Temperature-Compensated Crystal Oscillators (TCXOs) have been an appealing choice because of their low SWaP, and because their relative lack of accuracy and stability can be compensated by disciplining them to a GPS signal. However, the drawback to the TCXO solution occurs in GPS-denied scenarios, whether intentional (jamming) or unintentional. During a 4-hour GPS outage, the TCXO would exhibit 731 µs holdover. It is then up to UAV system engineers to determine whether or not this is a tolerable amount of drift for their intended mission. Too much drift can cause an unacceptable amount of uncertainty in the INS system, or can cause Inter-Symbol Interference (ISI) in the UAV’s communications systems.

Oven-Controlled Crystal Oscillators (OCXOs) offer comparatively better accuracy, stability, and temperature coefficient. Accordingly, the typical holdover time for a 4-hour GPS outage is reduced to 69 µs. However, OCXOs have drawbacks in SWaP – they are larger and consume considerably more power than TCXOs.

Atomic clocks have also been used in UAVs, though they have been confined to long-endurance missions where worst-case extended holdover performance may be required, and to larger UAV airframes that can afford the 10 W of power and extra space needed to operate a conventional gas-cell atomic clock.

Enter the Chip-Scale Atomic Clock (CSAC)

The Chip-Scale Atomic Clock (CSAC) provides a SWaP-optimized alternative to the traditional lineup, offering the advantages of the atomic clock without the SWaP disadvantages (Table 1). It is only one-eighth the volume of the conventional gas-cell clock and runs on only about 1 percent of the power. In fact, it needs only about 4 percent of the power required to operate the OCXO while being more accurate and taking up less space. It is larger than the TCXO alternative and consumes more power, however it compensates for this with a very large improvement in holdover performance.

Table 1: Several clock choices with varying features are available for UAV payloads.
(Click graphic to zoom by 1.9x)

Designed for low-power applications

The CSAC was deliberately designed with low-power applications in mind, and this was not achieved by simply shrinking the components used in existing architectures. Instead, a new architecture had to be invented, requiring innovations in semiconductor laser technology, MEMS processing, vacuum packaging, and firmware algorithms.

Take, for example, the glass resonance cell used in a conventional gas-cell atomic clock. The cesium (or rubidium) in the cell is heated to a vapor state and then the atoms in the vapor are illuminated to resonance by a laser or separate glass lamp cell. A photo-detector measures how much light actually makes it through the resonance cell (as opposed to being absorbed by the atoms inside the cell), thus producing a signal precisely tuned to the resonance frequency. That signal, in turn, drives a resonant element inside the clock that produces the clock’s actual timing signal output.

In the CSAC, the glass cell is replaced by a silicon MEMS device. A hole going all the way through the device is created by a Deep Reactive Ion Etch (DRIE) process, and the cesium is contained in the cell by two pyrex plates anodically bonded to both ends of the MEMS device. The cesium vapor is illuminated by a specially developed Vertical Cavity Surface Emitting Laser (VCSEL), and the light that gets through the cell is detected by a photodetector at the other end of the cell.

Figure 1 shows the key elements of the CSAC’s “physics package,” as it is called. In addition to the previously mentioned components, there is a cell spacer that ensures relatively uniform illumination from the laser light inside the resonance cell. There are also the upper and lower suspensions – also MEMS devices – made of polyimide spun onto silicon, with the silicon etched away as the last step in processing. Tension is deliberately put on the suspensions to hold the entire physics package in place. The heater traces, used to heat the cesium into a vapor state, are printed on both suspensions, as are all electrical traces needed to connect the physics package with the rest of the clock circuitry.

Figure 1: Elements of the Chip Scale Atomic Clock physics package.
(Click graphic to zoom)

A frame spacer, engineered to be slightly shorter than the rest of the parts that stack up to make the physics package, holds the whole assembly together and ensures tension in the suspensions. The assembly is then wire bonded to a leadless chip carrier that connects it to the rest of the clock circuitry, and then covered with a ceramic lid that is brazed to the LCC. However, the key to the low power consumption of the physics package is that the brazing step occurs in a high-quality vacuum. The vacuum surrounding the physics package gives it extremely high thermal isolation, so very little power is expended keeping the physics package at the correct temperature once it’s reached (which typically takes less than 2 minutes). The inside of the ceramic lid is coated with a getter material so that the quality of the vacuum can be maintained throughout the life of the CSAC.

The clock circuitry itself also contributes to the low power consumption of the CSAC. Each part is optimized for low power, and many clock functions that have traditionally been performed in hardware are instead done in the firmware of the CSAC.

Less SWaP tradeoff

In conclusion, though there are clock options available, the CSAC – which was designed for low power applications and has an architecture that improves on TCXOs and OCXOs – meets the timing challenges of today’s UAVs while improving upon the SWaP characteristics of alternative solutions. For example, Symmetricom’s Quantum SA.45s CSAC operates off a single +3.3 V supply and can be integrated onto a board as easily as any other component. Because it combines high stability and accuracy with low SWaP characteristics, it gives engineers one less headache when trying to meet the challenges of UAV payload design. As the UAV application space expands, so do the design challenges – both on the airframe itself and the electronics that fill it – and selecting a CSAC will help address these challenges.

Steve Fossi is a Director of New Business Development at Symmetricom. Steve’s technical background is in RF and microwave measurements, and semiconductor test. He brings a great deal of experience to the real-world applications of Symmetricom’s oscillators and atomic clocks.