Device manufacturers across a wide range of sectors – communications, radar, military vehicles, missiles, RF ablation and test/measurement – have long relied 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 aluminum 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 (see Figure 1) 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.
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 +/- 3 Ohms. While some manufacturers offer 50 +/- 2 Ohms, the best in the industry feature a standard spec of 50 +/- 1 Ohms. This tight impedance control provides a stable cable assembly with very good electrical performance that minimizes reflections.
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 center 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. PTFE (polytetrafluoroethylene) 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 minimize the amount of heat transfer to the cable so it can maintain its physical properties and optimize 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 center conductor and the lower dielectric constant. Care must be taken to minimize 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 optimized for both flexible coaxial cables and RF connectors. The completed assemblies minimize 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.
Source: http://www.edn.com/design/wireless-networking/4433298/How-to-select-flexible-cable-assemblies-
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