Standard workflow
In a first step a complex cable harness is divided into a finite number of straight segments. For each segment the program checks for any metallic shapes surrounding the cables. All cables in a segment, in combination with additional metallic shapes, define its cross-section. The whole process is called Meshing.
In a second step the primary transmission line parameter per unit length (R', L', C', G') will be calculated from each segment by a static 2D field solver. After that each segment will be transformed into an equivalent circuit and finally all circuits will be connected together to one single electrical model representing the whole cable. This process is called Modeling.
The second step implies that only TEM propagation modes can be considered and this fact causes the following constraints which are described below:
TEM propagation mode means that there are at least two separate conductors necessary to enable one single propagation mode (to enable forward and return current). In general N conductors are necessary to enable N-1 propagation modes. One single wire inside one space without any reference (typical antenna structure) won't be modeled correctly for a frequency higher than DC.
The generated equivalent circuits are only valid inside a frequency range from DC to fmax. This is due to the fact that the primary transmission line parameters are static parameters and only valid if the geometric dimensions behind the calculation have been significant smaller than the shortest wavelength of the propagating wave.
Additional effects caused by discontinuities like bends, deviations will not be considered.
In a third step the electrical model of the cable can be further processed in the circuit simulation. For this, the model will be automatically transferred to the in-built circuit simulator where the user is able to define several loadings (passive/active, linear/non-linear) and to calculate the transmission behavior of the cable in time and frequency domain.
Additional workflow for hybrid cable-to-field coupling
Many industrial applications deal with cables inside an additional metallic environment (e.g. ground planes in laboratory set-ups, car chassis). In presence of such reference conductors one propagation mode is of special interest. This mode is called common mode and consists of the current sum of all wires inside the cable bundle and the corresponding return current back through the reference conductor. Significant common mode is often the reason for considerable EMI/EMS problems.
If a reference conductor is part of the configuration the method used by CST CABLE STUDIO is able to calculate the common mode by summing up all currents in the cable bundle during a circuit simulation (possible in both frequency- or time domain). The common mode current along the cable path can automatically be passed to a 3D field solver where it is used as an impressed field source. This method is called the uni-directional cable-to-field coupling.
In case of the uni-directional field coupling, the cable itself is not physically present during the 3D field calculation and because of this, the reaction of the generated field (generated by the impressed current) back to the cable will be neglected. This approach limits the range of applications to configurations where most of the radiated energy will not be scattered back to the cable. This is true for may configurations with cables along open metallic chassis. But the assumption is not true incase of a resonant cable inside a nearly closed metallic enclosure. Therefore, when using the "current substitution method" the user has to check if the application fulfills the necessary assumption.
Note: In case of a uni-directional field coupling, if there is no reference conductor CST CABLE STUDIO will always predict a common mode current of zero. Any oscillation antenna modes probably existing in the higher frequency range (when dimension of lambda equals length of cable) won't be considered because the basic method is only able to simulate TEM-modes.
The uni-directional field coupling can also be used for considering the common mode impact of an external electromagnetic field onto a cable. Here, the 3D field solver will calculate the tangential electric field along the cable path (while the cable itself is not physically present). The 3D field solver will automatically convert the tangential electric fields to voltages and pass them to the built-in circuit simulator. In the circuit simulation the voltages are used to calculate the induced currents on the cable. The limitation of this "field substitution method" is equal to the "current substitution method".
If the described uni-directional coupling methods are not sufficient, CST CABLE STUDIO also offers the most general, namely the bi-directional coupling between cable-and field-solver in time-domain. In this case, currents and voltages are exchanged in every time step between the 3D field- and the circuit-simulator, representing cables and loadings. This method can (and has to) be applied in case of resonating structures, where radiation and irradiation effects act simultaneously.