Shielding

What does “shielding” mean?

Shielding refers to the measures taken inside a cable connection to place a protective barrier made of conductive material around the cables in order to reduce electromagnetic interference and ensure interference-free transmission. The purpose of this shielding is two-fold: it prevents external signals from penetrating inside the cable, and it prevents the cable’s own signals from radiating out and influencing other electronic systems in the vehicle.

Why is shielding necessary?

Due to the many driver assistance systems that exist in modern vehicles, an increasing number of wired transmission paths with high bandwidths are used. These systems require interference-free data transmission, particularly in areas relevant to safety such as the chassis, brakes, steering and drive. Two-level NRZ binary coding is no longer used to transmit high data rates via copper cables. Instead, multi-level line coding is used, such as that specified in 10GBASE-T1 (IEEE 802.3ch-2020), which uses PAM4, a 4-level pulse amplitude modulation technique, to support data transmission at a rate of 10 Gbit/s via twisted pair cabling. This makes it possible to double the transmission rate. If the number of levels increases while the maximum voltage remains the same, the voltage difference between the individual levels decreases. However, this distance is important to be able to differentiate between the states in the receiver. Due to the smaller distance between the levels, interference on the transmission path can more easily disrupt level differentiation in the receiver, and the bit rate increases. As the electric cables of many different systems in vehicles are often laid in bundles, the probability of interference further increases. Effective shielding with correct electrical connections is an important measure for preventing system function failures.

Reducing the effect and radiation of electric fields through the use of appropriate shielding

Interference fields can be categorized according to their frequency. Electrostatic and quasi-static or low-frequency electric fields can be reduced by a braid made from well-conductive material around the signal-carrying wires. Here, the braid acts according to the Faraday cage principle. An externally applied electric field causes the charge to be distributed in the metal braiding, and this creates an opposing field inside the braiding which cancels out the external field by superimposing the two opposing fields. As the frequency of the electric field increases, the shielding effectiveness increases through the “skin effect”. The “skin effect” refers to the displacement of current from the center to the surface of an electric cable as the frequency increases. In addition to the frequency, the penetration depth of the fields also depends on the conductivity. Due to the skin effect, high-frequency electric and electromagnetic fields can also be shielded by highly conductive, thin metal foil or metal-coated film. The shielding is effective both against undesirable radiation of signals (emitted interference, Electro Magnetic Interference (EMI)), and against the effect of signal cable faults (interference immunity, Electro Magnetic Compatibility (EMC)). This applies equally to electric fields, magnetic fields and electromagnetic fields. 

Reducing the effect and radiation of magnetic fields with the use of appropriate shielding

Low-frequency magnetic fields of up to several hundred kilohertz can penetrate metal shielding and induce interference currents in signal cables. Highly permeable materials are used as shielding to provide protection against these fields. The effect of this shielding is that the magnetic fields are bundled in shielding material so that the magnetic field cannot penetrate through to the signal cable. However, as these highly permeable materials are very expensive and can restrict the flexibility of a cable, low-frequency magnetic fields are avoided by differential signal transmission and twisting the two signal conductors. Differential signal transmission uses two wires with half rated voltage but different signals, and the differential voltage between the two wires is analyzed in the receiver. By twisting these two wires, the direction of the induced interference current changes in each loop and is compensated by symmetrical twisting in the center. This process is applied, for example, in unshielded twisted pair (UTP) cables. With high-frequency, magnetic and electromagnetic fields, the skin effect of the electrically conductive material has a damping effect. A distinction should be made here between braided shields and foil-type shields. When using wire mesh as the shielding, the effectiveness of the shielding decreases towards higher frequencies, which is due to the incomplete covering of the surface. The magnetic part of the field penetrates through the small holes in the braid at high frequencies. To increase the effectiveness of the shielding, an electrically conductive foil is placed under the braid of high-quality cables and provides protection against the remaining high-frequency fields.

What role does the shielding connection play in terms of the system’s reference potential?

A cable that is shielded with a highly conductive braid and a metal foil or metal-coated film does not provide reliable protection if the ends of the shielding are not correctly connected to the system’s reference potential. Without a connection to the system’s reference potential, no compensating currents can flow, which means that the shield only has a low impact on the interference fields. If the shield is connected to the reference potential at one end of the cable, it only provides protection against electric fields. To provide protection against magnetic fields above the kilohertz range, the shield must be grounded on both sides in order to enable the conduction of current. In the contact area, the shield must be contacted on all sides (360°) by the shielded connector casing as otherwise openings develop, through which high-frequency interference fields can penetrate and signals can be radiated out.

How is shielding effectiveness measured?

Measured values are required to compare the shielding effectiveness of cables protected by different shield braids. A distinction is made between low-frequency and high-frequency measurements. The transfer impedance or coupling resistance is determined in the low-frequency range. With this measurement, a current is applied to the outside of the shield and the voltage is measured between the inner conductor and the shield. The shielding effectiveness is measured in the high-frequency range. The shielding effectiveness can be determined by triaxial measurement, for example. In this case, the shielding effectiveness is the ratio between the signal being fed into the shield and the signal being radiated out through the shield.