EMC (electromagnetic compatibility) is the ability of electronic devices and systems to function reliably in their intended electromagnetic environment without causing or experiencing undue electromagnetic interference. Electromagnetic susceptibility, or EMS, is an essential aspect of electromagnetic compatibility. It refers to the extent to which a device, system, or material can be influenced or affected by external electromagnetic fields. Electromagnetic susceptibility therefore indicates how sensitive a particular object is to electromagnetic interference (EMI). Electromagnetic interference can come from various sources, such as radio frequency signals, electronic devices, power lines, and other sources of electromagnetic radiation.
EMS is a critical aspect in the design and testing of electronic systems, especially in applications where reliable operation is essential. Engineers use various techniques and strategies to improve the EMC of devices and ensure that they can operate effectively in environments with electromagnetic interference.
Testing for EMS involves exposing the device or system that controls the electromagnetic fields to evaluate its performance under such conditions. Standards and regulations may dictate the levels of electromagnetic susceptibility that devices must withstand to ensure proper functionality and safety.
This article looks at the electromagnetic susceptibility of cables routed in a car from the battery to an electrical device. The source of interference is electromagnetic radiation at the car's antenna. In addition, solutions for improving the electromagnetic sensitivity by means of a 3D electromagnetic simulation of the cables laid in the car are shown.
Figure 1 shows a Jeep with an antenna as a radiation source in the FM broadcast band.
Figure 1: The model of a jeep with an antenna on the roof
The chassis of the car is modeled as a perfect conductor, the window material is made of glass (Pyrex) and the tires are made of rubber. All materials were imported from the CST Studio Suite material library. There is an antenna on the roof of the Jeep that radiates in the FM broadcast band. In Europe, Africa, Australia and New Zealand, the FM frequency ranges from 87.5 to 108 MHz. The electromagnetic field of the antenna is a source of electromagnetic interference for the cable bundle. It runs from the battery in the engine compartment to an electronic device, such as the windshield wiper motor, in the rear of the car near the rear windows. The cable bundle is shown in blue in Figure 2.
Figure 2: The Jeep model with the cable routing
To analyze the electromagnetic sensitivity, three wires with varying jackets and connection types are arranged together in the bundle. As the cross-section of the bundle in Figure 3 shows, the first wire is an unshielded wire. It consists only of a red inner conductor and a green dielectric as insulation for the wire. The second wire also has a blue copper shield. The shielded wire likewise features green insulating jacket . The shield of this wire is connected to the chassis of the car via a 30Ω resistor, which is why the shielding is not optimally completed. The third wire is geometrically exactly the same as the second wire. The only difference lies in the shielding connection. Its shield is directly connected to the chassis and there is no resistance between the shield and the system's earth.
Figure 3: The cross-section of the cable bundle with three wires laid from the battery to the device.
An interesting and powerful feature of CST Studio Suite is the Schematic View, which offers the possibility of circuit analysis in conjunction with 3D simulation. As the schematic view in Figure 4 shows, the antenna is connected to an external connector that excites the antenna with a Gaussian signal. This signal covers the entire desired frequency band of the simulation. Here the simulation ranges from 50 MHz to 150 MHz. As previously mentioned, the shield of one of the wires is terminated with a 30Ω resistor, and the other is connected directly to the chassis. Three probes, P_Unshielded, P_ShieldedwithResist and P_ShieldedId, can be seen in this figure, representing unshielded, not optimally shielded and ideally shielded wires.
Figure 4: The schematic model with the antenna connection and the wire terminations
First, a 3D simulation is carried out. The S-parameters of the antenna connection and electromagnetic fields are calculated. The S-parameters shown in Figure 5 demonstrate the radiation of the antenna at approximately 102.5 MHz.
Figure 5: S-parameters of the antenna
Figures 6 and 7 show the surface current on the Jeep chassis at 102.5 MHz and 70 MHz. Since the antenna radiates at approximately 102 MHz, 70 MHz is an arbitrary frequency at which no resonance occurs and is not one of the harmonic antennas. From these two images of the surface current, it can be clearly seen that the antenna radiation is stronger at 102.5 MHz.
Figure 6: Surface current on the vehicle chassis at 102.5 MHz
Figure 7: Surface current on the vehicle chassis at 70 MHz
Following the 3D simulation, a circuit analysis can be carried out using the schematic user interface. The results are shown in Figure 8. This shows the voltages in the wires over the frequency range.
Figure 8: Induced voltage on three different wires in the cable bundle
The curves in Figure 8 show that the induced voltage is reduced by using a copper shield. However, there is a higher potential for electromagnetic protection if the shield is ideally terminated with an optimal connection to the vehicle chassis.
Electromagnetic compatibility focuses on the protection of electronic systems using various methods to ensure the safe operation of electronic devices. Developing a system that has all the desired features and meets the required EMC standards requires considerable effort. The development process involves numerous rounds of laboratory measurements and the setup of various measurement setups, which takes considerable time and incurs significant financial costs. CST Studio Suite offers an excellent opportunity to optimize both time and cost in the development of a viable system.
In this article, simulations were carried out using the CST Studio Suite. The electromagnetic field and surface currents on the surface of the Jeep were calculated. This was done through a 3D simulation in conjunction with a schematic view that performed the simulation of the circuit system. The results of the simulations show that the electromagnetic radiation from the car antenna can affect the function of devices connected to cables in the car. However, designed shielding with optimal grounding can reduce EMC problems.