Material, plating, and other developments are helping cables and connectors meet tough requirements for communications, military, and aerospace applications.
Microwave and optical cables and connectors currently serve a variety of industries. Yet they always have the common objective of delivering a transmission path for both highspeed digital and high-frequency analog signals. Their diverse capabilities and performance are derived from their design as well as other factors, such as materials. As it does in every industry, design follows demand. In other words, the evolution of such factors is driven by the "hot" applications of both today and tomorrow. In the microwave space, research and development for these products continues to get a boost from new communications standards and technologies. Yet military and aerospace also are evolving into stronger and stronger drivers, thanks to satellite and military-backbone development efforts.
Although cutting-edge wireless communications technologies like WiMAX continue to drive cable and connector developments, fiber optics remains an active area as well. Some years ago, fiber-optics technology was considered obsolete due to the advent of wireless. The cost and effort to lay the cable seemed likely to make this delivery method extinct. Yet fiber optics has remained strong in certain niches and terrains and is even at the heart of some major undertakings. For example, a global consortium recently came together to construct a high-bandwidth, subsea, fiber-optic cable system that will link the US and Japan. The 10,000-km linear cable system, dubbed Unity, will address broadband demand by providing the capacity needed to sustain the growth in data and Internet traffic between Asia and the US. Initially, it is expected to increase Trans-Pacific lit cable capacity by about 20 percent with the potential to add up to 7.68 Tb/s of bandwidth across the Pacific.
The Unity consortium is a joint effort by Bharti Airtel, Global Transit, Google, KDDI Corp., Pacnet, and SingTel. The Trans-Pacific cable will provide connectivity between Chikura, which is located off the coast near Tokyo, to Los Angeles and other West Coast network points of presence. At Chikura, Unity will be seamlessly connected to other cable systems to further enhance connectivity into Asia. The Unity consortium selected NEC Corp. (www.nec.com) and Tyco Telecommunications (www.tyco.com) to construct and install the system during a signing ceremony held in February. Initial capacity is targeted to be available in the first quarter of 2010. The new five-fiber-pair cable system can be expanded to up to eight fiber pairs with each fiber pair capable of carrying up to 960 Gb/s. The construction of this Trans-Pacific infrastructure will cost an estimated $300 million.
Clearly, this project is awesome in its breadth and purpose. Although most cables and connectors will not be involved in such lofty projects, however, their achievements should still be deemed impressive. As frequencies increasingly reach up to millimeter wavelengths, for example, interconnect components can have even greater impact on a system's signal loss and noise figure. In microwave cables, the cable's diameter also determines the frequency of propagation. Although cables with a larger diameter may support higher power, they will offer lower frequency than smaller-diameter cables. As a result, cables and cable assemblies are often offered in various diameters.
A new micro-sized foam dielectric coaxial cable from Radio Frequency Systems or RFS (www.rfsworld.com), for instance, boasts a 1/6-in. outer diameter. Known as CELLFLEX MicroFlex, it is similar dimensionally to braided and "conformable" cable. Unlike most coaxial cables of these small dimensions, however, it does not use a braided outer with a solid dielectric. Instead, MicroFlex comprises a corrugated outer coupled with a polyethylene (PE) foam dielectric (Fig. 1). That foam dielectric promises to reduce attenuation, thereby improving system gain and noise performance. The corrugated outer provides additional shielding and structural benefits. The device offers characteristic impedance of 50 1 O. Maximum operating frequency is 18 GHz. The RF peak voltage rating for the cable is 740 kV.
When reliability is a requirement for coaxial cables, it is essential that selection is done very carefully. According to distributor RF Depot (www.rfdepot.com), coaxial cables are a lot like fuel gauges. Not much appears wrong until something goes wrong. Also, escaping RF energy makes no noise, no sound, and leaves no puddles. In the absence of easy, reliable verification and replacement or repair, a reliable cable also becomes important to continued system performance.
The standard version of RFS' Micro- Flex is deployed in applications that accept a lower power rating. In basestation equipment cabinets and communications- equipment rooms, it can be used as a jumper cable for connections between equipment backplanes, subcomponents, and as interconnect jumpers. Thanks to the company's perfluoroalkoxy (PFA) foam dielectric, MicroFlex also is available in a high-power version. The enhanced power rating makes the cable well suited for internal connections to power splitters and dipoles within base-station antennas. To ensure correct antenna performance, these internal cables vow to be consistent while exhibiting superior uniformity.
Communications also is the target of a series of ruggedized mast cables from M/A-COM (www.macom.com). Specifically, the FE series of ruggedized mast cables is intended for use in ground mobile-communication applications (Fig. 2). To prevent damage from harsh military environments, such as being deployed to an external antenna from a mobile vehicle, the series was designed with an internal wire-wound layer and rugged external jacket. The series promises to withstand harsher physical environments than standard cable, such as crushing, abrasion, or bending, while maintaining acceptable VSWR and insertion- loss performance. The FE39WW, for example, offers insertion loss of 5.881 dB/100 ft. at 1 GHz. Cables are available in outer diameters ranging from 0.25 to 1.4 inches with a variety of connectors. Because its goal is to provide maximum durability and strength, the construction of the FE ruggedized mast cable family uses an integral helically wrapped, stainless- steel spring. In addition, an extruded polyurethane outer jacket protects it from abrasions and UV damage.
To better satisfy the rigorous requirements of military and airborne applications, M/A-COM also extended its family of lightweight cables this past October. Compared to traditional cable constructions, the FA20RXLW, FA29RXLW, FA46RXLW, FN49RXLW, and FN52RLL promise to deliver 25 to 31 percent savings in weight. The company specifies performance to 8 GHz. The five new cable types cover a variety of outer diameter sizes and constructions. All assemblies feature reduced weight and volume connectors, which retain conventional ETNC, SMA, and N interfaces while incorporating new materials. The assemblies claim to withstand the same harsh environments as the company's traditional assemblies including altitudes from ground level to more than 70,000 feet and temperature variations from -55 to +200C.
A fiber-optic approach to ruggedness is taken by the LITEflight Enhanced Performance (EP) cable family from Tensolite (www.tensolite.com), which targets aerospace. Compared to its predecessors, this series provides all of the features necessary to function in harsh environments but with lower loss, tighter bend radius, improved thermal stability, and better handling during termination and installation. LITEflight EP is available in multiple sizes, configurations, and temperature ratings to 260C. Tensolite also recently released a line of coaxial cable up to 40 GHz. The line comprises the TLL40-111A (125 type) and TLL40- 1130A (150 type). The model TLL40- 1111A 40-GHz, low-loss flexible coax offers 50-O impedance. Nominal insertion loss is 91.6 dB/100 ft. at 40 GHz.
FORGING A CONNECTION
Material developments also drive the evolution of connectorsas does plating and other factors. At Rosenberger (www.rosenberger.de), for example, AuroDur is the new standard gold surface for all of the company's connector series. The AuroDur surface consists of a thin gold layer on a non-magnetic, chemically deposited layer of nickel that is 2-3 m Ni and 0.15 m Au. The gold plating promises to satisfy the high mechanical and electrical demands of RF connectors. Before load, outer-conductor contact resistance on typical coaxial connectors measured 0.20 to 0.33 mO. After load, that value changed to only 0.31 to 0.45 mO. The AuroDur surface also boasts superior intermodulation properties. Maximum magnetic field emission was measured at 10 to 100 nT, which easily surpasses the target of 432 nT. The new Rosenberger RF cable assemblies are among the plethora of products now stocked by distributor RFMW Ltd. (www.rfmw.com).
For harsh environments, ITT (www.itt.com) just released a fiber-optic connector. Previously, the LC configuration that is commonly used to connect fiber-optic devices failed to function reliably in high-temperature or high-vibration environments. Yet the Canon PHD SuperLC connector vows to meet or exceed Lucent 640-252-053 and Telcordia GR-326 requirements. It can withstand high temperatures and thermal cycling as well as elevated levels of shock and vibration. The connector's durability can at least partially be attributed to a thermal plastic housing and a locking feature that prevents mechanical release. In addition, a tunable optical cartridge functions as a stand-alone optical element separate from the connector housing. It accepts multi-mode or single-mode ferrules. At high temperatures, the locking mechanism also prevents thermal stress to the latch.
Last fall, ITT debuted the TMDMP Micro-D filter connector for military and aerospace applications (Fig. 3). By utilizing an unusual internal filter design, this connector provides the density required to accommodate the ferrite tubes that are needed for Pi filtering. Completely intermateable with MILDTL- 83513, the TMDMP filter connector is available in five sizes: 9, 15, 21, 25, and 37 with contacts for both sockets (receptacle) and pins (plugs). The connector features rugged aluminum shells. Capacitance values include H, L, M, and T.
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Rugged environments also are home to the SMPM RF connectors from Tyco Electronics (www.tyco.com). These connectors claim to match the convenience and performance of SMP connectors in a form factor that is 30 percent smaller. They boast a low VSWR through 40 GHz. As a result, the SMPM connectors allow center-to-center spacing of 0.135 in. The 50-O connectors are available with either a smooth bore or fully detented interface to provide insertion forces of 2.5 or 4.5 lbs. The interface design complies with MIL-STD-348A. The connectors' small size makes them suitable for ground and airborne radar, antenna systems, and other communications systems with dense packaging. They are available in cablemount jacks, board-mount receptacles, and accessories like adapters and shrouds.
The SSPO from Spectrum Elektrotechnik GmbH (www.spectrum-et.org) is a subminiature, push-on, blindmate interconnect system. It was developed in response to industry demand for increased package density. The SSPO is approximately 70 percent of the size of SMP connectors. It offers impedance of 50 O over the DC-to-65.0 GHz frequency range. For the 1201 Precision series, VSWR is 1.15:1 typical to 50 GHz. Insulation resistance is a minimum of 5000 MO while centerconductor contact resistance is 6.0 mO maximum. The SSPO meets MIL-PRF-39012 for corrosion, vibration, mechanical shock, thermal shock, and moisture resistance.
In one article, it is virtually impossible to summarize the efforts of the seemingly endless number of companies involved in this space. One of the most interesting aspects of this segment is that so many of those companies have an extremely long heritage. For example, WL Gore (www.wlgore.com), which is known for developing the widely used synthetic material, polytetrafluoroethylene (PTFE), is celebrating its 50-year anniversary this year (see "Celebrating 50 Years Of PTFE At Gore," February 2008, p. 84). According to IW (www.iw-microwave.com), the former Insulated Wire, its heritage includes 100 years of combined transmission-line design experience and numerous patents relating to both cable and connector innovations. Pasternack Enterprises, Inc. (www.pasternack.com) is another well-known cable supplier. Although this company started in 1972 as a manufacturer of log amplifiers for the military and aerospace communities, it has evolved to offer extensive lines of RF coaxial and fiber-optic products. Other well-known "old timers" in this industry include Astrolab, Inc. (www.astrolab.com), Flexco Microwave (www.flexcomw.com), MegaPhase (www.megaphase.com), Micro-Coax (www.micro-coax.com), and Times Microwave Systems (www.timesmicrowave.com).
Many of these firms offer low-cost testgrade cables for use with a vector network analyzer (VNA). In addition, many wellknown component suppliers, including Mini-Circuits (www.minicircuits.com) and Synergy Microwave (www.synergymwave.com), offer phase-stable cables for a range of test applications.
Even Compel Electronics GmbH, which many industry experts think of as the "new kid on the block," just celebrated its 10-year anniversary. The firm's cableassembly line at Bebra, Germany, has manufactured more than 750,000 assemblies since 1999. In addition to power and low-frequency cable assemblies, Compel specializes in RF cable assemblies for flexible coaxial cables (up to 14 mm in diameter) and for semi-rigid and conformable semi-rigid coaxial cables up to UT 250. For a complete listing of cable and connector suppliers, please visit the online version of the Microwaves & RF Product Data Directory at www.mwrfpdd.com.