Sophisticated cables are not just for RF

By Peter Clarke

Cables and their connectors don’t get much attention, but that situation is changing dramatically in the RF arena. As frequencies of operation for mass-market products go past 10GHz and are approaching 30, 50 and even 100GHz, RF interconnects are no longer restricted to expensive, phase-matched, low-temperature coefficient components primarily used in the lab or specialized mil/aero situations. We’re now seeing standard cables with diameters of 1mm and less. At those dimensions, developing and installing reliable connectors without impedance bumps is an electrical and mechanical challenge.

However, it’s not just RF cabling that is seeing advances. Despite the fact that DC and signalling interconnections may seem to be mature, and fully developed, two cable developments for radically different applications – one in production, one in the R&D phase – caught my attention recently. They demonstrate that thinking “It’s just some quality copper and insulation” is a gross oversimplification, as it is for all passives and interconnects. If you have time, check out the performance parameters for the latest Power over Ethernet standard, as well; they are tough ones!

The first item I saw is the OLflex Servo series from the Lapp Group, an extremely flexible power/signal cable for long-travel applications, Figure 1, such as motors on tracks, robotic arms, and industrial machinery. This complex cable assembly uses a combination of fine strands of bare copper, polypropylene insulation, and copper-braid shielding (for EMI/RFI protection); it also has a polyurethane outer jacket for resistance to physical wear, abrasion, UV, and oil.

Figure 1. The OLflex Servo series of power and signal cables from the Lapp Group are designed for long-reach repetitive-motion applications, with significant acceleration, speed, and travel lengths (photo Lapp Group).

This cable even has motion-related specifications, parameters you don’t often associate with “conventional” cabling, including acceleration up to 50 m/s2, travel speed up to 5m/s, and travel lengths up to 100m. There’s also a minimum bend-radius parameter for continuous flexing of between 7.5 and 10× the cable diameter depending on the specific version. The outer diameter ranges between 10 and 40 mm, which is a function of the specific internal conductor sets the customer orders, there are about 30 offered sets of matchups and gauges for power and signal, see here. Anyone who says “what’s the big deal….it’s just some cable” is either naïve or simplistic. An installation’s long-term reliability, often in situations where failure has serious safety implications, is just as dependent on the cable’s integrity as it is on anything else. While this cable is not a fundamental technical breakthrough, it requires mastery of materials properties and selection as well as sophisticated manufacturing techniques and equipment.

Next: Stretching to new lengths

In very different part of the interconnect world, there’s some fascinating work looking to leverage carbon nanotubes to build conducting fibers that can stretch by significant amounts. These may find use in applications ranging from exoskeletons to pacemaker leads (their most-common source of failure is their leads and attachment). Researchers at the University of Texas (Dallas) wrapped sheets of these tiny nanotubes as a sheath around a long rubber core, Figure 2, see “Scientists Stretch Electrically Conducting Fibers to New Lengths.” Unlike conventional copper cables, where stretching the wire reduces the cross section and thus increases their resistance, there is little change in the resistance of these cables when stretched even by a factor of ten. (The 24 July 2015 citation and abstract of the academic paper in Science is ” Stretch, wrap, and relax to smartness,” but the full paper is behind their subscription wall.)

Figure 2. Researchers at the University of Texas have developed a very stretchable, electrically conducting fiber made of layers of carbon nanotubes and rubber that can bring performance benefits as a flexible, interconnect for pacemakers, and perhaps be the core of small-scale artificial muscles (University of Texas).

The applications go beyond the obvious. The researchers say these highly stretchable conducting fibers can be the basis (with added sheathing) for building strain sensors, as well as artificial muscles where the buckled nanotube shields act as electrodes, or even miniature torsional muscles if the fibers are twisted.

Of course, there’s often a huge chasm to cross between a lab development and a practical product or technology for even niche, specialized applications. It’s worth reminding ourselves that the apparently mundane role of a conducting cable actually plays a large part in successful, reliable designs. The advances of both researchers and manufacturing from DC to RF in materials, implementations, and configurations will have an impact that we may not appreciate, except in hindsight.

Have you ever had cables and interconnects be the gating item or your projects, or needed their technical innovation to complete the job?

Bill Schweber, is an engineer, author and editor and this article first appeared on EE Times’ Planet Analog website.

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Bandwidth demand drives connector design

Selecting heavy duty connectors for industrial applications

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