Cables and connectors ensure that signals pass through RF/microwave systems with minimal distortion.
Cables and connectors are often thought of as “patches” to get a high-frequency signal from one location to another. While their roles in a system may be that simple, they can also have a major influence on the performance of that system. For that reason, cables and connectors should always be chosen wisely for RF/microwave applications, with performance levels that exceed the requirements of those applications.
Coaxial connectors are designed to be electrically “invisible” as much as possible, providing interfaces for component packages, circuit boards, and coaxial connectors with the lowest possible insertion loss and VSWR. In recent years, coaxial connectors have been extended to higher frequencies, accommodating broadband components and test equipment operating into millimeter-wave frequency ranges.
A connector’s frequency range is a function of its inner pin diameter. The smallest-dimension RF/microwave connectors, with 1-mm pin diameter, provide frequency coverage as wide as DC to 110 GHz, increasingly replacing more narrowband millimeter-wave waveguide flanges as the interconnection of choice. As millimeter-wave applications, such as 60-GHz point-to-point radios, expand, the demand for higher-frequency connectors operating through 40 GHz and higher also grows.
In addition, specifiers are generally seeking coaxial connectors in a variety of different mechanical configurations, from traditional end-launch connectors to blind-mate and vertical printed-circuit-board (PCB) launch connectors for fabricating compact multilayer circuit boards
For example, Molex, Inc. recently introduced a high-speed vertical-launch 2.4-mm connector capable of operation to 50 GHz (Fig. 1). Designed for mounting anywhere on a PCB to allow increased circuit and interconnection density, these connectors feature a compression-mount design to simplify mounting high-frequency connectors to a PCB. The connector receptacle attaches to a PCB by means of two 0-80 UNF screws. The connectors accommodate a wide range of board thicknesses including 0.57 to 2.79 mm with a continuous ground connection between the connector and the PCB. The connectors, with a stainless-steel body built to handle over 500 mating cycles, exhibit low VSWR of 1.20:1 through 50 GHz.
1. This line of vertical-launch 2.4-mm coaxial connectors is capable of supporting a continuous bandwidth of DC to 50 GHz. (Photo courtesy of Molex, Inc.)
For those interested in the long evolution of RF/microwave coaxial connectors, an application note from Maury Microwave—“Microwave Coaxial Connector Technology: A Continuing Evolution” (Application Note 5A-021), written by Mario Maury, Jr.—is available for free download from the firm’s website. The 21-page application note details some of the long evolution of RF/microwave coaxial connectors to their current configurations. As the note explains, coaxial connectors are available in both sexless and sexed forms. Sexless connectors join two identical halves while sexed connectors have male and female connector halves, usually with both conductors employing male and female configurations. The note also describes three different grades of connectors commonly used in the high-frequency industry: production, instrument, and metrology. The precision and performance accuracy/repeatability improves with each connector grade, as does the cost, with metrology-grade connectors providing traceability to national standards.
Coaxial cables are also evolving, with a growing number of suppliers for specialized cable assemblies [including those with hand-formable cables, low passive-intermodulation (PIM) performance, and phase-stable cables]. The choice of cable assemblies was once between those based on semirigid cables and those with flexible cables. Semirigid coaxial cables provide superior electrical performance but cannot be bent to shape without special tools. Flexible coaxial sacrifice some of the electrical performance of semirigid cables, but provide ease of installation through easy bending and shaping.
A third type of cable assembly, the hand-formable cable, is becoming more popular since it features electrical performance that is between the levels offered by semirigid and flexible cables, but can be readily shaped to fit an application. It is well suited for many system and test-equipment applications that require high performance, but may also need last-minute or on-site shaping of the cable assembly to make proper connections.
Hand-formable cable assemblies are designed for a limited number of flexures— typically about 500—and are typically supplies in cable diameters that match to the sizes of semirigid cables, such as 0.047 and 0.141 in. The similar cable diameters allow the use of similar connectors with hand-formable cables, and for ease of replacement of semirigid cables as needed.
Some component suppliers, such as Micro-Coax, feature all three types of RF/microwave coaxial cables in their cable assemblies. The firm actually offers two different types of hand-formable cables, Alumiline cables with solid aluminum jackets and UTiFORM cables with tinned-braided outer jackets. The former delivers higher RF shielding and performance closer to semirigid cables, while the latter offers somewhat greater flexibility for ease of installation. Both hand-formable cable types are available in the same sizes as semirigid cables, such as with 0.047- and 0.141-in. diameters, allowing the use of the same connector sizes as for semirigid cables.
Mini-Circuits supplies its 141 Series of Hand-Flex cables as a replacement for 0.141-in. semirigid cables. These hand-formable cables, which operate from DC to 18 GHz, are designed for high power-handling capabilities, with ratings of over 500 W at 500 MHz and 90 W at 18 GHz. The 141 Series cables have a bend radius of 8 mm for forming tight shapes, with standard assemblies supplied with an FEP insulator jacket to minimize shorting during installation and use; versions are also available without the FEP jacket. SUCOFORM hand-formable cables from Huber + Suhner leverage the positive qualities of standard PTFE-insulator-based semirigid cables, but use a tin-soaked copper braid for the outer conductor for enhanced flexibility.
Hand-formable cable assemblies from WL Gore are also available in versions equivalent to 0.086- and 0.141-in. semirigid cables for applications from DC to 18 GHz. The smaller-diameter hand-formable cables have a minimum bend radius of 0.20 in. and maximum insertion loss of 0.98 dB/ft. at 18 GHz, while the larger-diameter cables offer a minimum bend radius of 0.25 in. with maximum insertion loss of 0.65 dB/ft. at 18 GHz.
Modern communications systems with their advanced modulation formats often require cable assemblies with exceptional phase stability and/or low PIM generation. A growing number of cable suppliers now offer cables and assemblies based on materials known to minimize PIM. Ferromagnetic materials (nickel, for example) can increase the level of PIM in a cable assembly or other high-frequency component. Poor-fitting connection junctions and rust on metal surfaces can also contribute to unwanted PIM distortion in a communications or test system.
In addition to supplying numerous low-PIM cables and cable assemblies, San-tron offers a free white paper to help specifiers better understand the causes and effects of PIM in cables and connectors: “Minimizing PIM Generation from RF Cables and Connectors.” The eight-page document, available as a free download, offers useful advice for specifiers hoping to curb the levels of PIM from the cables and connectors in their systems. The firm’s SRX™ line of low-PIM cable assemblies are available based on semirigid, hand-formable, and flexible cable types (Fig. 2).
2. The SRX line of coaxial cable assemblies is available with all three types of RF/microwave cables—semirigid, hand-formable, and flexible cables—with levels of PIM as low as -174 dBc depending on cable and connector types. (Photo courtesy of San-tron, Inc.)
What is considered acceptable PIM performance? That answer will depend on the requirements of a particular system. As an example, cables and connectors are among the most PIM-prone components in many systems. The LMR-SW™ cables from Times Microwave Systems have been developed for applications where PIM must be minimized. They feature a thin-wall aluminum outer conductor and can achieve better than -170 dBc PIM performance. A line of Type N connectors developed by Pasternack Enterprises was designed to terminate these cables with minimal additional PIM. The connector body is brass with white bronze plating, while contacts are gold plated and nickel free. The connectors, usable to about 11 GHz, are interchangeable with any Type-N connector meeting MIL-C-39012 specifications.
To ease the task of specifying coaxial cable assemblies, some suppliers, including Times Microwave Systems, provide online calculators that can be used to compute the attenuation (in dB/100 ft.) for a desired length of coaxial cable for a specific frequency (in MHz). The calculator, available here, can also predict the power-handling capability of a given type of cable assembly.