Evolving Long-Distance Data Transmission
To avoid performance bottlenecks, input/output (I/O) connections need to keep pace with processors so that data is moved around quickly and efficiently. By: Earle Olson, Former Business Development Manager, Aerospace
Even as high-data-rate copper-based connectivity is evolving, fiber-optic transmission is finding increased use. Creating location-independent architectures means that different subsystems must not be constrained by cabling distances. Optical fibers have the well-known advantages of long transmission distances independent of data rate, noise immunity, and small size/light weight. As UAVs move from platform-centric to network-centric models – ones that share sensor and payload information with ground-control stations, satellites, and remote terminals – computational-intensive processing is required both to separate the wheat from the chaff and to compress the data. VPX is a leading platform to meet these needs, as it gives users modularity, scalability, and support for board-level optical and RF connectivity.
Optimizing I/O and SWaP
To avoid performance bottlenecks, I/O connections need to keep pace with processors so that data is moved around quickly and efficiently. Systems are beginning to use gigabit and even 10 gigabit Ethernet to create onboard networks that connect sensors, processors, and communications. Saving space and weight are key design requirements for UAVs. In particular, weight savings are important to enabling UAVs to loiter on station for long durations and to carry heavier payloads. Savings measured in ounces at the component level yield pounds at the system level. The combination of higher speeds and demands for reduced size and weight means that traditional military connectors, such as the ubiquitous MIL-DTL-38999 circular connector, are often too large and not well-suited for high-speed systems. For these reasons, designers are looking for alternative solutions; whatever solution is chosen must be robust and rugged enough to withstand the shock and vibrations, temperature extremes, and other mechanical and environmental hazards that come with deployment in a UAV.
As I/O speeds increase, issues of signal integrity and power budgeting create new challenges. Simply put, high-speed signals are harder to manage than low-speed signals. The higher the interconnection speed, the more difficult it is to manage return loss, insertion loss, crosstalk, and similar factors that can degrade signals. While an ideal cabling system would have no intermediate connections between boxes, the real-world need for production breaks and modularity necessitates connectors in the path. A poorly designed connector will appear as a significant impedance discontinuity. The impact of the discontinuity is frequency-dependent – return loss and crosstalk increase with frequency – meaning that high-speed I/O connectors must be more carefully designed. Attenuation in the cable and insertion loss in the connector are also frequency-dependent, making power budgets more challenging at high speeds. Size, weight, and power (SWaP) issues remain most important in providing persistent surveillance, a better fuel-to-weight ratio, and the potential for smaller UAVs. While smaller, lighter connectors help meet SWaP goals, miniaturization cannot simply be accomplished at the expense of signal integrity or robust ruggedness. Nanominiature and microminiature connectors already exist, but these legacy connectors were not designed for high-speed signals.
Ready for 10 G
The gap in fast copper connectivity can be partially bridged with some connectors capable of 10 Gb/s performance. A connector that maintains shield continuity through the connector can be concatenated multiple times without degrading performance. These connectors offer field repairability, supporting a single 10 G Ethernet channel in a size 11 shell or four channels in a size 25 shell. A smaller eight-position connector in a size 8 shell uses a T-shaped contact pattern to provide noise cancellation and decoupling to minimize crosstalk and increase signal integrity. In this size 8 connector, the backshell is integrated into the plug body to sport a low profile, provide low weight strain relief, and furnish electromagnetic (EMI) protection. Nanoconnectors use the same T-shaped contact pattern as the size 8 connectors, but in a nanominiature size, as plugs are only 0.3 inch in diameter. (See Figure 1).
Light weight, high speeds
Even as high-data-rate copper-based connectivity is evolving, fiber-optic transmission is finding increased use. Creating location-independent architectures means that different subsystems must not be constrained by cabling distances. Optical fibers have the well-known advantages of long transmission distances independent of data rate, noise immunity, and small size/light weight. While there are many fiber optic connectors available, the two main categories of optical contacts are physical contact (PC) and expanded beam (EB). PC termini, typically using a ceramic single-fiber ferrule, achieve low loss by having the termini physically touching. EB connectors, on the other hand, rely on ball lenses to expand and then refocus the light across the interface. The noncontacting EB interface offers high-mating-cycle durability and easy cleaning, while the PC interface provides the lowest loss. The MT ferrule, with a capacity of 12 or 24 fibers, enables very-high-density fiber packaging. MT ferrules are available in both PC and EB versions.
For fiber connectivity, 38999-style connectors remain popular in UAV applications. TE recently introduced the MC801 connector, which combines ARINC 801 termini and a 38999-style shell (Figure 2). The genderless ARINC termini are considered easier to use, clean, and maintain than the pin and socket configuration of PC military-style contacts. ARINC 845, which covers expanded beam technologies, recently selected TE’s PRO BEAM EB16 termini as the ARINC 845 industry standard for rugged optic applications within commercial aviation applications.
What’s next for UAV connectivity?
With more sophisticated sensors, ever faster and more capable silicon, and more sophisticated computer architectures and software, one clear future for UAV systems is more bandwidth. Network backbones are already migrating from 1 Gb/s Ethernet to 10 Gb/s, with 40 and 100 Gb/s waiting in the wings. Streamlining designs with a goal of a common hardware set is also gaining ground. For example, designing interconnects to be compatible with a range of physical layer impedances – such as Fibre Channel, IEEE 1394, eSATA, and the like – can not only simplify system design but can also reduce the number and types of cables and connectors that must be stocked. Improved compatibility will enhance the idea of modularity and easy plug-and-play connectivity. While designers, in the end, will look for standardized high-performance systems and components, they still have choices when it comes to performance-based alternatives. Today’s performance-based system may well become tomorrow’s new standard.