Influence Analysis and Countermeasures of Fakra Coaxial Cable in On-Board Signal Transmission

With the increasing demand of intelligent driving system and in-vehicle entertainment system, the transmission rate of in-vehicle signals increases, resulting in the transmission frequency of the corresponding connecting cables gradually increasing. FAKRA coaxial cable harness is a high-speed and high-frequency Wire Harness for transmitting radio frequency signals or high-definition camera signals. It mainly includes FAKRA Connector, FAKRA inline connector, coaxial cable and PCB board connector.

When FAKRA coaxial cable is used in parts matching and automobile applications, two important electrical properties, namely, return loss and insertion loss, will be obviously affected, resulting in the degradation of the quality of the transmitted signal, the user's perception and even the functional failure. This paper mainly analyzes the influence of FAKRA connector, FAKRA inline connector, coaxial cable and PCB connector on the signal quality of the whole transmission link, and puts forward the engineering methods to reduce the influence.

Before analyzing the influence of FAKRA connector on signal quality, it is necessary to know the connector design standard, and then analyze the potential influencing factors around the standard. The interface dimension standards of FAKRA connectors mainly refer to ISO20860-1 and USCAR-18, and the test standards mainly include ISO20860-2, USCAR-17 and USCAR-2. Interface size standard, which defines the main axial and radial dimensions of FAKRA connector, including male connector and female connector.

In the axial direction, some dimensions are within a dimensional tolerance range, and there is a gap at the connection interface after the male and female connectors are plugged into each other. The gap after insertion caused by interface dimension tolerance affects the electrical performance, and the size of air gap affects the degree of impedance matching. In addition, starting from the standard, the insulating dielectric material at the end face of the outer conductor only defines the design requirement of 50Ω. In the products of different companies, the insulating dielectric material is different, which will also affect the impedance matching effect. Therefore, even if the interface structure size meets the standard, it is necessary to carry out matching tests for connectors with different insulation materials and interface structures to verify whether the relevant electrical performance indexes are within the specified numerical range.

In the process of Wiring Harness connection and assembly of the whole vehicle, it is inevitable to use inline connector for docking. For example, for the connection between the entertainment host and the external antenna, the entertainment host is arranged in the instrument panel area, while the external antenna is at the rear of the roof. Generally, it needs to be connected by three sections of wiring harnesses: the instrument wire harness, the body wiring loom and the ceiling cable harness, which will lead to inline connector.

It can be found that the insertion loss of sample is larger than that of another sample at the same frequency, because the insertion loss of the middle three pairs of inline connectors exerts an influence on the transmission link.

It can be seen that the insertion loss can be introduced by increasing the inline connector, which shows that the more connectors are inserted, the greater the insertion loss will be. At the same time, for the selection and application of inline connection, it is necessary to consider the working frequency. Under different frequencies, the insertion loss is different. In practical application, it is necessary to verify the quality stability and consistency of inline connector.

In addition, especially when it comes to inline connections from different manufacturers, it is necessary to evaluate and test the overall performance after switching. When inline connectors are mated, if impedance mismatch occurs, it will cause return loss, which will lead to signal output power reduction and insertion loss increase. The conductor loss, dielectric loss and radiation energy of inline connector will increase the insertion loss, resulting in the decrease of signal output power.

When considering FAKRA coaxial cable, it is generally considered that the insertion loss of coaxial cable is equal to the sum of the losses of connector and cable. It should be emphasized in this paper that the concentricity and contact state of the pin and jack have an impact on the insertion loss of coaxial cable. "Bad contact" will lead to the increase of the insertion loss of coaxial cable, and it may also be manifested as unstable signal or direct open circuit. To judge the contact state, we can not only test the contact resistance of male and female connectors after mating, but also test the mating retention force specified in the standard, so as to measure and judge the contact state. Test methods of inner and outer conductors are clearly defined in ISO20860-2 test items.

Another factor that affects the insertion loss of coaxial cable is the operating frequency range of the cable. It can be seen that the test curve of the insertion loss changes linearly within the "linear frequency band" of the working bandwidth of the cable, but the test results change nonlinearly outside the bandwidth, and there is a sudden change in some frequency points, which brings about noticeable influence and consequences to the signal transmission of the whole link.

Compared with line-end connectors, FAKRA board-end connectors are different in structure at the interface, and the common difference is that there are two designs of the end face of the male outer conductor: insulator and air. After the two interfaces are plugged into the female connector, there will be differences in performance.

Another important factor that affects the performance of the male connector is the design of the connection between the connector tail and PCB.

In order to achieve impedance matching, all these factors will affect the signal transmission: the distance between the root of the inner conductor of the board-end connector (near the insulation medium of the connector) and the PCB board, the width of the signal line on the board, the working bandwidth of the PCB board, and the size of the openings on both sides of the signal line on the board.

When the working frequency exceeds a certain threshold, the board connector, PCB board and the corresponding welding parameters should be fully demonstrated to avoid the abrupt change of high-frequency signals in some frequency bands.

5 Conclusion

It can be seen from the above that when choosing FAKRA coaxial cable, attention should be paid to the working bandwidth of cable and connector, and the working bandwidth of each device will affect the signal transmission in the whole link. For connectors of the same category and different manufacturers, it is necessary to refer to product technical specifications to confirm the consistency of selection and matching.

In the practical application of the whole vehicle cable, for the coaxial cable application in the bending area, considering the transmission loss and bending durability, a special bending resistant cable with lower loss and larger diameter is usually used. For electrical appliances arranged in wet areas, special FAKRA connectors which are waterproof and dustproof are usually used. The FAKRA connector that meets the vibration requirements should also be considered for the layout in the vibration area.

In a word, FAKRA connector, FAKRA inline connector, coaxial cable and PCB board connector will all affect the signal transmission of the whole link. By analyzing and mastering the main influencing factors, the corresponding design and test measures are formulated to reduce and eliminate these influences, which is conducive to the reliability and stability of high-frequency signal transmission.