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Choosing flexible cable assemblies

16 Mar 2015  | Roger Kauffman

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Other common characteristics of the leading flexible coaxial cables include silver-plated conductors, fluoropolymer (FEP) dielectrics, double shields, and an FEP jacket. The low-loss solid core cables feature an FEP dielectric with 70% velocity of propagation (VOP), while the ultra-low-loss air cables feature an air-enhanced FEP dielectric with up to 87% VOP.

The ultra-low-loss air core cable sets itself apart from the low-loss solid core cable by introducing air into the dielectric material. This creates a larger centre conductor in relation to the given outer conductor size and provides lower attenuation per given cable length. For system designers requiring low attenuation performance, ultra-low-loss is the ideal choice.

Additional considerations when choosing flexible cables
An additional consideration when choosing which flexible cable to use is the type of insulation. Extruded FEP insulation can be held to tight tolerances within the extrusion, which enables greater impedence control. polytetrafluoroethylene (PTFE) insulation, a different type of flouropolymer, tends to withstand higher temperatures, but is more difficult to extrude with the same tolerances.

For lead-free RoHS compliance, it is important to check if the cable assembly manufacturer uses high-temperature lead-free solder. The challenge is to minimise the amount of heat transfer to the cable so it can maintain its physical properties and optimise electrical performance. Most coaxial cables experience some dielectric growth when exposed to heat. If this is not controlled or accounted for in the design, the cable assembly will show degradation of VSWR, particularly at higher frequencies.

To help designers accustomed to semi-rigid cables adapt more easily to flex cables, another key aspect is to specify flexible-cable form factors that match the common sizes of semi-rigid cables used by the industry. The three most common form factors are 0.047" (diameter over the shield), along with 0.086 and 0.141".

Also, low-loss solid-core cables can be terminated with industry-standard semi-rigid connectors, while ultra-low-loss air core cables require specifically-designed connectors. This is due to the larger size of the centre conductor and the lower dielectric constant. Care must be taken to minimise heat transfer, and some adjustment to the cable preparation dimensions may be needed to accommodate dielectric growth. Cable assembly designers typically take this into consideration when developing connectors for cables; otherwise, the cable assemblies will have difficulty in achieving expected performance.

To assure optimal performance, cable manufacturers should offer completed cable assemblies with termination techniques optimised for both flexible coaxial cables and RF connectors. The completed assemblies minimise the voltage standing wave ratio and insertion loss at high frequencies, and they can be installed easily into any size device.

The industry is also offering low-loss cables using different dielectric materials that withstand higher temperatures. Examples include air-enhanced PTFE and PFA. Such cables tend to be more costly and are rarely sold as bulk cable. But the options of low-loss and ultra-low-loss cables, made with FEP dielectrics, offer a more cost-effective alternative for a complete, fully-tested cable assembly.

Device manufacturers can select from a wide range of interconnect solutions that are available in the market today and which offer flexible alternatives to semi-rigid cable assemblies.

About the author
Roger Kauffman is the Director of Product Management and Marketing for RF/Microwave Products at Molex. With more than 35 years in the electronics industry and over 25 years in RF/microwave interconnects and cable assemblies, Roger has been with Molex RF for more than 12 years. During his time in the industry, he has held a wide range of positions and responsibilities covering sales, applications engineering, product marketing, product management, and product development as well as a variety of sales and marketing-leadership roles.

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