Selecting cable for high-speed apps

What should you choose as fiber and copper still fight for the right to carry our data

BY RALPH RAIOLA
Editor
Electronic Products Magazine

In somewhat ironic fashion, as the world heads toward more and more wireless applications, the use of wire and cable in high-speed environments becomes more of a necessity. “Wireless is the competitor, but physiologically it may be hard for users to make the transition,” said Carroll Wontrop, a senior system engineer at Danaher Motion (Wood Dale, IL).

“There is concern about wireless communication being easily interfered with from other wireless communication.” Therefore, more than ever, the pressure is on design engineers and system designers to make the right cabling choices.

So, what exactly should EEs know and consider when specifying cable for high-speed data-transfer applications? I posed this exact question to some industry professionals from a variety of backgrounds. Hopefully, this primer will serve to make the choice a bit easier.

Fiber vs. copper

The gauntlet was thrown down ages ago. While fiber-optic cable has the decided edge in terms of performance—and common thought is that it will replace copper fully at some point—copper has hung in there, mostly due to its reliability, quality, and to the fact that much of the existing infrastructure is already copper friendly. So, many designers may be unsure as to which way to go.

“Both solutions are equally reliable if deployed correctly,” said Emad Soubh, high-speed cable technology manager at Samtec (New Albany, IN). “Otherwise both have issues. But copper has far less issues, and is more widely understood and backward compatible.”

That being said, recent developments by Corning—which I report on in this month's Outlook section—may have the ability to change the game in favor of optical solutions. But professionals seem divided on whether fiber solutions will eventually send copper packing for good.

“In my opinion they will always coexist,” Soubh said. “As far as one winning in the long run, I just simply don’t see it happening.”

But Wontrop believes fiber has definite advantages that will enable it to dominate in the market. “I think copper connections will continue to exist, but as connector issues get worked out with fiber optics—because of its inherent noise immunity—it has the potential to dominate,” Wontrop said. He notes that “switching to fiber-optic cable requires that users change the connector receptacle, and for higher speeds they will need to change or reprogram on-board FPGA chips.”

On the datasheet

Datasheet specs for high-speed cables abound, but those I spoke with agreed on some of the more important ones. Pat Wastal, a senior vice-president of IP&E at Avnet (Phoenix, AZ) pointed to attenuation, impedance, propagation delay, skew, shielding effectiveness, and capacitance as the most important specs an EE can retrieve from a datasheet. Soubh recommends designers should begin their selection by looking at insertion loss, cable length, and flex life specifications.

Cable types

While copper and fiber constructions are the two basic types available for high-speed cable, there are many different kinds with a variety of options to choose from in each category. “Some of the more common [copper] solutions include miniature coax, controlled impedance ribbon, twin-ax, and shielded twisted pair,” said Wastal. Other types of copper-based cables include, parallel pairs, quad-ax, and RF cable (RG family, microwave coax).

There are three main types of fiber-optic cable: single-mode, multimode, and plastic-optical fiber (POF). Single-mode—which features a single strand of glass fiber—is more expensive than mulitmode, but provides higher transmission rates and up to 50 times more distance. Lower-cost POF is a lower-cost option that compete with glass performance, but only on shorter runs.

Some high-speed cables are now being designed to handle signal, power, and data transfer. But there are inherent problems with packing everything under one jacket. “This will be great, but the problems of high-power noise (poor common mode rejection) on signal and data transfer signals will need to be worked out,” Wontrop said.

Impending 10GbE push

More and more products coming out are being specifically designed for 10-GbE applications, which will rely very heavily on the wire and cable being used to carry data signals. Soubh believes this push is putting a lot of stress on cable manufacturers, who must now take much more into account about the overall system.

“Cable manufacturers have to pay really close attention to every component and have to tightly specify every component,” he said. “Every step in the process of building the cable needs to be qualified and constantly measured to make sure nothing is deviating or getting out of the tight processing window.”

Wontrop was more optimistic about the transition. “Most industrial applications do not need it now,” he said. “But when it becomes available, users will figure out ways to use the added bandwidth to their advantage.”

Issues and concerns

Regardless of the type of cable being used, the designer must be prepared to deal with the inevitable issues that typically crop up in high-speed applications and environments. Wontrop cites a number of concerns designers have when designing with high-speed cable, including power loss that can occur over long runs, eliminating the effects of nearby radiated and conducted electrical noise, the cable jacket’s ability to withstand the intended environment, and compatibility with termination connectors.

Shielding and materials

Many types of shielding are available, depending on the application and cost targets. These included standard braid, spiral, Al/Mylar, foil, flat wire, Al/Mylar braid or spiral combination, foil and braid or spiral combination, double braid, double spiral, and flat wire and braid or spiral combination.

For copper cables, jacketing materials range from PVC to Teflon (FEP), TPE, TPR, and Halogen-free polymers. Center conductors are typically constructed from silver-plated copper or tin-copper material.

Applications

Certainly many applications use—or need—high-speed cabling. Industries such as packaging, converting, semiconductor, and electronic all rely on high-speed connections, said Wontrop. “High-speed connections with communication rates greater than 10 MHz are finding their way into all sorts of industrial applications.”

Testing

Cable manufacturers test their products for many different variables and conditions. Soubh recommends EEs pay particular attention to testing for impedance profile, multiple-line insertion/return loss, time delay, and skew. In-process measurements, should be used to ensure that the process can produce stable/consistent product. ■