FAKRA Coaxial Cable in Vehicle Signal Transmission

FAKRA coaxial cable is a high-speed and high-frequency cable that transmits radio frequency signals or high-definition camera signals. Its typical structure is shown in Figure 1, which mainly includes: FAKRA connector, FAKRA Inline connector, coaxial cable, and PCB board-end connector. With the growing demand for automobile intelligent driving systems and in-car entertainment systems, the signal transmission rate in the car has increased, resulting in a gradual increase in the transmission frequency of the corresponding connection cables.

In parts matching and automotive applications of FAKRA coaxial cables, the two important electrical properties of return loss and insertion loss will be significantly affected, resulting in a decrease in the quality of the transmitted signal, affecting user perception, and even leading to functional failure. This article mainly analyzes the impact of FAKRA connectors, FAKRA Inline connectors, coaxial cables, and PCB board-end connectors on the signal quality of the entire transmission link, and proposes engineering methods to reduce the impact.
In parts matching and automotive applications of FAKRA coaxial cables, the two important electrical properties of return loss and insertion loss will be significantly affected, resulting in a decrease in the quality of the transmitted signal, affecting user perception, and even leading to functional failure. This article mainly analyzes the impact of FAKRA connectors, FAKRA Inline connectors, coaxial cables, and PCB board-end connectors on the signal quality of the entire transmission link, and proposes engineering methods to reduce the impact.
Before analyzing the impact of FAKRA connectors on signal quality, it is necessary to first understand the design standards of the connector, and then analyze the potential influencing factors around the standards. The main reference interface size standards of FAKRA connectors are ISO20860-1 and USCAR-18, and the main test standards are ISO20860-2, USCAR-17 and USCAR-2. The interface size standard defines the main dimensions of FAKRA connectors in the axial and radial directions, including male connectors and female connectors, as shown in Figure 2 (extracted from ISO20860-1).

FAKRA coaxial cable connection structure diagram

Figure 2 Wire-end male connector interface size standard

In the axial direction, certain dimensions are within a dimensional tolerance range. After the male and female connectors are plugged together, there is a gap at the connection interface. The gap after insertion caused by the interface size tolerance affects the electrical performance, and the size of the 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 materials have different phenomena, which will also affect the impedance matching effect. Therefore, even if the interface structure size meets the standards, for connectors with different insulation materials and interface structures, matching tests need to be conducted to verify whether the relevant electrical performance indicators are within the specified value range.

Interface dimensions of male connector

2. The impact of FAKRA Inline connector on the performance of the entire transmission link
During the vehicle wiring harness connection and assembly process, Inline connectors are inevitably used for docking. For example, regarding 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 is necessary to connect the three-segment wiring harness through the instrument wiring harness, body wiring harness and ceiling wiring harness, which will produce an Inline connector.
As shown in Figure 3, a connection comparison was made: one cable is a complete 400mm sample 1, and both ends are FAKRA connectors. The other one is 4 sections of 100mm sample 2 of equal length, connected in series through 3 pairs of FAKRA Inline.

FAKRA coaxial cable and sample 2 coaxial cable

Figure 3 Sample 1 coaxial cable and Sample 2 coaxial cable
Through the insertion loss comparison in Table 1, it can be found that at the same frequency, Sample 2 has a greater insertion loss than Sample 1. It is due to the insertion loss of the three pairs of Inline connectors in the middle exerting an influence on the transmission link.

The impact of FAKRA Inline connectors on transmission links

Table 1, Comparison of Insertion Loss of Sample 1 Coaxial Cable and Sample 2 Coaxial Cable

As can be seen from Table 1, adding an Inline connector can introduce insertion loss. It is known from this that the more connectors are inserted, the greater the insertion loss. At the same time, for the selection application of Inline connection, the operating frequency needs to be considered. At different frequencies, the insertion loss is different. In practical applications, the quality stability and consistency of Inline connectors also need to be verified.
In addition, especially when it comes to Inline connections from different manufacturers, the overall performance after switching needs to be evaluated and tested. When inline connectors are matched, if impedance mismatch occurs, it will cause return loss, resulting in reduced signal output power and increased insertion loss. The conductor loss, dielectric loss and radiated energy of the Inline connector will cause the insertion loss to increase, resulting in a reduction in the signal output power.

3. The impact of FAKRA coaxial cable on the performance of the entire transmission link
Coaxial cable is a basic unit composed of two coaxial and mutually insulated cylindrical metal conductors. The cable diagram is shown in Figure 4.

Coaxial cable structure

Figure 4, coaxial cable structure

When considering FAKRA coaxial cables, it is generally accepted that the insertion loss of a coaxial cable is equal to the sum of the connector and cable losses. This article needs to emphasize the impact of the concentricity and contact status of pins and jacks on coaxial cable insertion loss. “Poor contact” will lead to an increase in coaxial cable insertion loss, and may also manifest as signal instability or direct open circuit. To determine the quality of the contact state, in addition to testing the contact resistance of the male and female connectors after they are mated, the plug retention force specified in the standard can also be tested to measure and determine the contact state. The test methods for inner and outer conductors are clearly defined in the ISO20860-2 test project.
Another factor that affects the insertion loss of coaxial cables is the operating frequency range of the cable. Figure 5 shows the test curve of insertion loss of a certain type of cable. It can be seen that the insertion loss test curve changes linearly within the “linear band” of the cable’s operating bandwidth, but outside the bandwidth. The test results change non-linearly and produce mutations at certain frequency points, which brings non-negligible impact and consequences to the signal transmission of the entire link.

Insertion loss performance beyond the operating bandwidth of the cable

Figure 5. Sudden changes in insertion loss performance outside the cable’s operating bandwidth.

4. The impact of FAKRA board-end connector on the performance of the entire transmission link
Compared with line-end connectors, FAKRA board-side connectors differ in structure except for the interface. The more common difference is that there are two designs for the end face of the public outer conductor: insulator and air. There will be performance differences between these two interfaces after they are plugged into the female connector.
Another important factor that affects the performance of the male end connector is the design of the connection between the connector tail and the PCB board. Figure 6 shows a board-end connector with the inner conductor flatly attached to the signal line of the circuit board. Figure 7 is a schematic structural diagram of the board-end connector after cutting off part of the outer conductor.

Board end connector appearance

Figure 6, board-end connector

Cut off part of the outer conductor of the board-end connector

Figure 7, part of the outer conductor of the board-end connector is cut off

In order to achieve impedance matching, these factors will affect signal transmission:
The distance between the root of the conductor in the board-end connector (the part close to the insulating 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 operating frequency exceeds a certain threshold, both the board connector, the PCB board, and the corresponding welding parameters must be fully demonstrated to avoid sudden mutations in high-frequency signals in certain frequency bands.

5 Conclusion
It can be seen from the above that when selecting FAKRA coaxial cables, you need to pay attention to the working bandwidth of the cable and connector. Throughout the link, the operating bandwidth of each device will have an impact on signal transmission. For connectors of the same category from different manufacturers, it is necessary to refer to the product technical specifications to confirm the consistency of selection and matching.
In the actual application of vehicle cables, for coaxial cable applications in bending areas, considering transmission loss and bending durability, special bending-resistant cables with lower losses and larger diameters are usually used. For electrical appliances arranged in wet areas, special waterproof and dustproof FAKRA connectors are usually used. For placement in vibration areas, FAKRA connectors that meet the vibration requirements also need to be considered.
In short, FAKRA connectors, FAKRA Inline connectors, coaxial cables, and PCB board-side connectors will all affect the signal transmission of the entire link. By analyzing and mastering its main influencing factors, we can formulate corresponding design and testing measures to reduce and eliminate these effects, which is beneficial to the reliability and stability of high-frequency signal transmission.