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Analog Angle Blog

Sophisticated Cables: Not Just for RF

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 10 GHz and are approaching 30, 50 and even 100 GHz, 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 1 mm and less. At those dimensions, developing and installing reliable connectors without impedance bumps is an amazing electrical and mechanical challenges.

However, it's not just RF cabling that is seeing advances. Despite the fact that DC and signaling interconnections may seem to be old news, mature, and fully developed, two cable developments for radically different applications – one in production, one the R&D phase – recently caught my attention. They demonstrate that thinking “it's just some quality copper and insulation” is a gross oversimplification here, as it is for all passives ad 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).

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/sec2 , travel speed up to 5 m/sec, and travel lengths up to 100 m. 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.

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 10×. (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).

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?

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1 comment on “Sophisticated Cables: Not Just for RF

  1. jimfordbroadcom
    March 28, 2016

    For those who have not worked with motion control and don't have a feel for what 5 m/s velocity means, it is really fast!  I once worked for a wirebonder manufacturer, and we had bondheads moving at up to 1 m/s, and that was plenty fast enough to cause serious injury.  Multiply that by a factor of 5, and I'd expect limbs to be severed and people coming in contact with such machinery to be killed.  Of course, mass of the moving object comes into play, but I'm assuming if you have to worry about the cables' reliability, the mass is substantial.

    At that bonder manufacturer, we had some issues with reliability of connectors as well.  I remember that company A's connectors mated with company A's connectors were fine, but company B's connectors mated with either company A's connectors or company B's connectors had issues.  The connectors in this particular series were supposed to be compatible among manufacturers, but the ones from company B were inferior.  Connectors from other series were fine across the board.  Not a pleasant situation with hundreds, if not thousands, of mated pairs with every combination of connectors from both manufacturers on boards and cables out in the field!

    Moral of the story; neglect cables and connectors at your own peril!

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