Advances and trends in RF connection
When considering the exponential growth of transferred data, most headlines are captured by wireless innovation, but advances continue to be made in the wired domain, not least the technologies that connect the one to the other. For it remains the case that the most important data travels along cables, prior to wireless transmission, and at the end of every cable is a connector to transfer data to the next PCB or system.
Radio frequency (RF) connectors seek to transfer data across multi-megahertz radio frequencies with minimal data loss from reflection and dispersion, in a cost-effective manner. With the majority destined to work with coaxial cable, the aim is to maintain the shielding that a coaxial design offers and, in that sense, the performance of the cable or connector is only as good as the sum of its parts.
When choosing a connector for any application, factors to consider include selecting acceptable levels of resistance, with 50 or 75ohm representing characteristic impedances; reflection, with 1.0 VSWR representing no reflection; and RF leakage tolerance, with a totally shielded connector achieving none whatsoever. Affecting these performance characteristics is a gamut of additional electrical, mechanical, environmental, material and economic considerations, because the reality is that producing an ideal RF connector simply isn’t possible. Saying that, new connector designs aim to get as close to ‘ ideal’ as possible.
Connector manufacturers continue to improve reliability and device quality, by turning to new materials and manufacturing techniques. Recent years have seen the emergence of specialised connectors, such as those able to withstand extreme temperatures. With hundreds of RF connectors to choose from, here’s a list of the main trends in RF connection as we see them:
The factors driving the miniaturisation trend are well known; the dominance of handheld/mobile devices (including wearables) drive requirements for smaller, lower-profile components capable of higher speeds, and the increase in complexity and integration of devices is matched by the components they comprise. The trend towards sub-miniature and micro-miniature connectors can be seen in whatever type of RF connector you choose to look at, whether it is MMCX taking over from MCX, or SMPM taking over from SMP, and lest we forget, the UMCC (take a look at Hirose’s U.FL range - right - for a truly tiny connection!). When small size and high performance are important, Samtec’s RF047 flexible microwave cable assembly is ideal for high speed, high cycle test applications where the size of a U.FL connector is required but a more rugged and reliable interface is needed and offers 500 cycles and 10 GHz performance with a U.FL cable plug.
Higher frequency and high power
Examples within our line card include Amphenol’s SMA RF connectors (right), which have a VSWR performance up to 26.5 GHz and cater to multiple PCB thicknesses. Molex also caters to high-frequency applications, with its RF solutions range serving DC to 65 GHz and Hirose offers high-speed coaxial connectors supporting up to 50 GHz that meet the MIL-STD348B standard. Notably, two of our suppliers, Molex and TE Connectivity recently signed a Dual Source Alliance to collaborate on new connector and cable assembly products for high-speed applications i.e. 56 GHz and beyond. When choosing connectors for high frequency applications, don’t forget that the accepted limits are generally 90% of their theoretical cut-off frequency.
Characteristics that influence the choice of materials used for RF connectors include contact force and surface smoothness, electrical and thermal conductivity, machinability, and environmental conditions, like temperature range. Brass and stainless steel remain the materials of choice for RF connector housings, but since 2008, bronze copper beryllium has replaced copper beryllium for the spring contacts. White Bronze (a copper, tin and zinc alloy) plating for RF connectors has been a popular choice over nickel or silver for some time, thanks to its improved electrical and intermodulation performance, better wear characteristics and low insertion loss.
Driving more recent developments are mission-critical applications such as automotive, military and medical. For example, connectors for the medical market (e.g. scopes and probes) must be non-magnetic, so here a traditional nickel-plating for the contacts is replaced with electro less nickel phosphorous. As an example, Molex offers non-magnetic solutions with virtually any RF connector and cable to minimise distortion in medical and aerospace imaging applications while meeting high-frequency and -density requirements.
Molex's FAKRA II SMB connector system
Another avenue of interest for RF connectors is plastic. In recent decades, various electrically conductive plastic products have been developed. With low density and good electrical and dielectric properties, connectors using electrically conductive plastic are of growing interest and, as a result, plastic is becoming a regular feature of the materials mix. An illustration of this can be seen in FAKRA type RF connectors (such as the FAKRA II SMB connector system from Molex). With colour coded plastic housings that simplify identification, plus unique keying to prevent mis-mating, they perform up to 3 GHz and meet the mechanical and environmental needs of the automotive industry.
Avnet Abacus stocks RF connectivity solutions from some of the world's leading connector manufacturers, and our pan-European team of technical specialists is on hand to help you find the right one for your design. Get in touch by clicking the Ask an Expert button to the right of this post.