FAKRA, USCAR, and the evolution of connectivity
In Rolling Connectivity: Automotive RF communications, I talked about the evolution of various RF technologies being adopted by carmakers to support entertainment, communications, safety, and other value-added applications. Here, I take a closer look at some of the unique connectivity solutions within the vehicle for RF systems and how they are evolving.
Antennas and amplifiers on the exterior of the vehicle connect via coaxial cable using FAKRA (Fachkreis Automobil, originally a German standard, literally "Automobile Expert Group") compatible connectors. These connectors, based on the SMB connector type, are used to connect coaxial cable to receivers/transceivers located in the vehicle for GPS, satellite radio, cellular communications, and some FM signals.
Due to assembly considerations in the vehicle plants, most roof-mount antennas have either short pigtails terminated with FAKRA connectors, or bulkhead FAKRA connectors on the bottom facing inside the vehicle. A cable run is installed in the vehicle to route the signals, and the cables are terminated on both ends with FAKRA connectors. In a vehicle with GPS, satellite radio, cellular, and FM, there could be four pigtails and four cables for a total of 16 connectors.
To simplify assembly, different colors are used for each signal type, and the colors correspond to a mechanical keying so that signals can't accidentally be misconnected. Figure 1 shows a color chart and corresponding mechanical keying.
Figure 1 Color codes and mechanical keying for FAKRA connectors (Source: Amphenol).
Many auto makers also require conformance to USCAR (United States Council for Automotive Research LLC, founded in 1992) specifications. Under USCAR, there is the EWCAP (Electrical Wiring Component Applications Partnership), which includes standards for RF connectors. In addition, nearly every automaker requires components to pass PPAP (Production Part Approval Process), a lengthy process of documentation, validation, pilot runs, and demonstration of production capability. Needless to say, supplying into the automotive industry isn’t for everyone.
When I began working in the automotive industry, there were only a handful of approved FAKRA connector suppliers. Rosenberger, Amphenol, and Tyco/AMP (now TE Connectivity) were all early suppliers and continue to be key players in this market. However, there are now dozens of suppliers all over the world who make connectors that are compatible with the standard. Many of these are, however, feeding into the aftermarket, and may not meet OEM specs. Figure 2 shows some typical examples of FAKRA connectors for the PC board and cable sides of the connection.
Figure 2 Some examples of FAKRA automotive RF connectors from Rosenberger. Top—PC board connectors; Bottom—cable connectors.
Amphenol specifies its FAKRA connectors for RG-174 and RG-316 series coaxial cables for frequencies up to 3GHz, which covers all applications today. Interestingly, TE and Rosenberger specify their FAKRA lines up to 6GHz; this may become important in the future if DSRC is widely adopted at 5.9GHz.
Critical considerations with these connectors and cables include assembly cost, complexity, and reliability. One complaint with the FAKRA design is that the connectors are somewhat difficult to assemble, especially the tiny center pins. In addition, a relatively high part count adds cost per connection. TE Connectivity has introduced an alternative system, somewhat confusingly called the Stripline RF connector system (Figure 3).
Figure 3 TE Connectivity Stripline RF connectors.
In this case, stripline doesn’t refer to a PCB transmission line, but rather a design wherein the coaxial cable is transitioned to an all-metal/air dielectric stripline geometry in the connector. Using advanced simulation, the structure was designed to minimize impedance changes through the connector. The result is a connector with lower cost, easier assembly, and better loss characteristics (above 3GHz) than standard coaxial connectors.
Figure 4 shows a snapshot from a 3D EM (electromagnetic) simulation, which was used to design the connectors.
Figure 4 Simulation of transitions from coaxial geometry to stripline geometry (from a presentation by TE Connectivity (then Tyco).
Figure 5 shows some details of the assembly. Instead of flaring the shield braid and pushing it over the connector body, then crimping on a ferrule as is done with a coaxial connector, the outer stripline conductor is pressed directly through the cable jacket to contact the shield.
Figure 5 The left view shows the outer conductor assembly to the cable by pressing directly into and through the cable jacket, eliminating manipulation of the shield braid necessary for a coaxial connector. The right views show the center stripline contact for one half and the assembly to the other half (from TE Connectivity presentation).
As more and more electronics are built into cars, including more sensors and RF devices, the connectivity solutions are bound to keep evolving.