Simulating Crosstalk and EMI in Cables

Cables not only transfer the power needed to run electrical equipment, but also the data signals needed to operate them. To prevent errors and device failures, the same attention must be paid to the choice and installation of the cabling as is paid to the rest of the system. Dealing with data means considering signal integrity (SI). Since data signals are modulated electrical pulses, anything that introduces noise into the cable can corrupt the information, causing equipment to lose performance, malfunction or simply fail altogether. Crosstalk and impedance mismatch are common sources of SI problems; cables generally consist of multiple wires travelling together. The fields generated in one wire can, without proper shielding, couple to others and induce currents in them, while signals can be reflected at the interfaces between cables if the impedances do not match. Analyzing these issues uses RF and microwave techniques and expertise. It is not just interference from other data signals that one needs to worry about, however. Electromagnetic interference (EMI) can come from a range of sources within the system and the wider environment. Switched-mode power supplies generate noise, while lightning strikes and electrostatic discharge (ESD) introduce transients that often cause damaging current surges in devices. Even the interaction between the equipment and its casing can be enough to interfere with data signals. As well as being immune to external radiation, the cables themselves should not radiate either. Electromagnetic compatibility (EMC) is a legal requirement and this means that they must pose little interference risk to other devices. Every advance in technology pushes cable design requirements further. High-speed devices demand cables capable of handling everhigher bit-rates. Automotive and aeronautical systems, increasingly reliant on electronic control and communications systems, need cables that are lightweight yet also measure up to stringent safety regulations. Consumer electronics meanwhile are pushing toward standardized multipurpose cables, where one lead might be used for anything from charging a mobile phone to controlling a printer or transferring data to a hard drive. In light of these developments, designers have turned to cable harnesses, where multiple cables – sometimes a hundred or more – are tied together and travel along the same conduit as well as hybrid cables, which contain both signal and power wires together. The most familiar example of a hybrid cable is probably USB, but custom cables of many configurations are used in industrial applications. For such complex cable designs, traditional design rules to calculate the cable’s properties become unwieldy. Simulation offers a way to develop and check an arbitrary cable design and optimize its layout and shielding for better performance.