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Choosing flexible cable assemblies

16 Mar 2015  | Roger Kauffman

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Device manufacturers across a broad range of sectors—communications, radar, military vehicles, missiles, RF ablation and test/measurement—have long depended on semi-rigid cable assemblies. When designing modules within a device, these assemblies are a natural choice for their consistency and stability from an electrical performance perspective.

But because semi-rigid assemblies come with an outer-shield made of solid copper or aluminium tube, system designers must create three-dimensional models to determine routing within the system. The cables require particularly precise designs, so that during assembly, each end locates exactly where the connection needs to occur. Design cycles for point-to-point cable assemblies with this type of cable tend to be longer.

The manufacturer then needs to procure pre-bent cables that fit properly into the intended configuration within the module. The pre-bending adds to the complexity and the cost of the cables while also increasing the difficulty for assemblers when routing the cables within devices containing multiple assemblies.

Simplified installation with cables that fit into smaller places
To resolve this challenge, some system manufacturers in recent years have sought microwave cable assemblies with comparable characteristics – particularly shielding effectiveness – but with more flexibility to allow assemblers to do all the bending while using standard cable configurations. This greatly simplifies installation and gives designers more options since they can fit the cables into smaller spaces.

By relying on flexible cables (figure) instead of the semi-rigid type, assemblers can simply connect one end, snake the cable through or around any devices, and then connect the other end. They can thus draw two-dimensional assemblies and configure them in a straight length – knowing that when the cable is installed, it can be easily bent to fit correctly.

Figure: Flexible microwave cable assemblies, such as these from Molex, can deliver high performance and simplify the design process.

The bendability characteristic can also lead to longer-lasting cables. They do not suffer as much performance degradation as semi-rigid cables, for which the bending by hand places additional stress on the cable and the solder joints. This can compromise long-term reliability and forces assemblers to be painstakingly careful, which increases the overall time to assemble each device.

Solid core vs. air core flexible cables
System designers choosing to work with flexible semi-rigid replacement cables can choose from two types:

 • Low-loss solid core
 • Ultra-low-loss air core

When purchasing semi-rigid replacement cables from leading manufacturers, it is important to specify helically-wrapped foil covered by a braided shield that produces a shielding effectiveness greater than 100 decibels. This protects signals from internal and external interference, and is comparable to semi rigid-cables while offering flexibility for ease-of-use.

Both types of flexible cables also feature tight impedance control. The typical impedance rating for RF and microwave cables is 50 ± 3Ω. While some manufacturers offer 50 ± 2Ω, the best in the industry feature a standard spec of 50 ± 1Ω. This tight impedance control provides a stable cable assembly with very good electrical performance that minimises reflections.

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