Standardizing RF Connections in Vehicles
As modern automobiles integrated GPS navigation, cellular connectivity (4G/5G LTE), satellite radio (SDARS), and keyless entry systems, automotive OEMs required a standardized, high-volume radio frequency (RF) connector system. Standard industrial SMB or SMA coaxial connectors were inadequate—they were slow to screw together on fast-moving assembly lines and lacked secondary locking mechanisms to survive engine vibration.
FAKRA (an acronym for Fachnormenausschuss Kraftfahrzeugindustrie, the German Automobile Standard Committee) connectors solved this problem by combining an industry-standard SubMiniature version B (SMB) coaxial contact insert inside a rugged, color-coded plastic mechanical housing standardized under DIN 72594-1 and USCAR-18.
Mechanical Architecture and Color-Coded Keying
The fundamental innovation of the FAKRA connector system is its dual-stage mechanical keying and standardized visual color system.
1. SMB Coaxial Sub-Assembly
At the core of every FAKRA connector sits a modified 50Ω SMB coaxial contact. The center pin carries high-frequency RF signals, while the outer metallic crimp sleeve provides 360-degree ground shielding against external electromagnetic interference. FAKRA interfaces typically operate reliably from DC up to 6 GHz, with a characteristic impedance of 50Ω.
2. Plastic Housing and Mechanical Keying
The metallic SMB insert is snapped inside a durable plastic housing featuring a primary locking clip and a secondary retaining sleeve. To prevent factory operators from plugging a GPS antenna cable into a Bluetooth transceiver port, FAKRA defines 14 distinct mechanical keying profiles, designated by letters A through N (and Z for universal keying).
Each keying code corresponds to a specific physical notch pattern and a bright, standardized plastic color:
- Code A (Deep Black): AM/FM radio antennas.
- Code C (Signal Blue): Global Positioning System (GPS) / GNSS navigation.
- Code D (Bordeaux Violet): Cellular telephony and wireless connectivity.
- Code E (Leaf Green): Television / multimedia streaming.
- Code I (Beige): Keyless entry and remote start systems.
- Code Z (Water Blue): Universal mechanical keying, designed to mate with all key codes. Used primarily in prototyping, test setups, and multi-purpose diagnostic equipment.
Design Rules for Integrating FAKRA on PCBs
To maximize RF performance when launching FAKRA connectors onto a circuit board:
- Design Coplanar Waveguide (CPW) PCB Launches: The transition from the SMT or thru-hole FAKRA center pin to the PCB trace is a primary source of RF reflection. Implement a Coplanar Waveguide with Ground (CPWG) trace geometry on the top PCB layer, calculating trace width and ground clearance gap to maintain an exact 50Ω characteristic impedance.
- Place Ground Vias Adjacent to Retaining Legs: FAKRA connectors are available in straight and right-angle PCB mount configurations. Place multiple low-inductance ground vias immediately adjacent to the connector's mechanical solder legs to tie the outer coaxial ground shield directly to the PCB internal ground plane.
- Account for Cable Attenuation at 6 GHz: Standard RG-174 coaxial cable exhibits significant signal attenuation at frequencies above 2 GHz. For high-frequency cellular 5G or high-band GNSS runs exceeding 2 meters, specify low-loss automotive coaxial cables such as RTK-031 or DAC-031 to prevent unacceptable link budget loss.