CST2013: Time Domain Solver Overview

A time domain solver calculates the development of fields through time at discrete locations and at discrete time samples. It calculates the transmission of energy between various ports or other excitation sources and/or open space of the investigated structure. Consequently a time domain solver is remarkably efficient for most high frequency applications such as connectors, transmission lines, filters, antennas etc. and can obtain the entire broadband frequency behavior of the simulated device from a single calculation run.

In CST MICROWAVE STUDIO two time domain solvers are available. One is based on the Finite Integration Technique (FIT), just called Transient solver, the second one is based on the Transmission Line Method (TLM) and is referred to as TLM solver. Both solvers work on hexahedral grids, however, the mesh setup is slightly different and classified as Hexahedral and Hexahedral TLM mesh type, respectively.

Transient Solver

The Transient solver based on the Finite Integration Technique (FIT) applies some highly advanced numerical techniques like the Perfect Boundary Approximation (PBA)? in combination with the Thin Sheet Technique™ (TST) to allow accurate modeling of small and curved structures without the need for an extreme refinement of the mesh at these locations. This allows a very memory efficient computation together with a robust hexahedral meshing to successfully simulate extremely complex structures.

The fields are calculated step by step through time by the ”Leap Frog” updating scheme. It is proven that this method remains stable if the step width for the integration does not overcome a known limit. This value of the maximum usable time step is directly related to the minimum mesh step width used in the discretization of the structure. Therefore, the denser the chosen grid, the smaller the usable time step width.

This means that a high mesh resolution of a small detail inside a given structure possibly determine the global timestep and therefore the total simulation time. In these cases it might be suitable to activate the Multilevel Subgridding Scheme (MSS) to combine the profit of a detailed resolution with an adequate solver time. See the Advanced Mesh Strategies Overview page for further details.

In addition features like AR-Filtering or S-Parameter symmetries and reciprocity help to increase the performance of the solver. Furthermore, the simulation becomes even more efficient when applying hardware acceleration like GPU or MPI computing.

TLM Solver

A TLM solver computation is a time domain analysis also based on a hexahedral mesh. The TLM solver has many of the features of the Transient solver and basically shares a similar application range, where it is especially well suited to EMC/EMI/E3 applications. It offers a very efficient octree-based meshing algorithm which efficiently reduces the overall cell count.

The TLM solver is able to model special material types and compact models like Shielded cables, Thin panel and Coated metal, Slots  and Seams or Vents.

Areas of application

• scattering parameter matrices (S-Parameter)

• electromagnetic field distributions at various frequencies (see monitors)

• antenna radiation patterns and relevant antenna parameters (see monitors)

• signal analysis such as rise times, cross talks etc. including a spice network extraction

• structure design by using the optimizer or the parameter sweep

• time domain reflectometry

• radar cross section calculations using farfield/RCS monitors (see farfield overview and monitors)

• simulation of dispersive materials (see material overview)

How to start the solver

Before you start the solver, you should make all necessary settings. See the Time Domain Solver Settings Overview for details. Both time domain solvers can be started from the Time Domain Solver Parameters dialog box, where the mesh type can be selected either as Hexahedral or as Hehahedral TLM to activate the Transient or TLM solver, respectively.

A solver run can only be started if at least one port or some other excitation source as plane wave or a far- or near-field source is defined.

 If there are multiple ports defined, these ports may be stimulated in four different ways if you start the solver: You can start the solver

• with all ports stimulated sequentially,

• with only one single port stimulated,

• with some selected ports stimulated sequentially,

• or with some selected (or all) ports stimulated simultaneously.

To start the solver, all results of other solvers (frequency domain solver, eigenmode solver, etc.) will initially be deleted. Old results of the same time domain solver will not be deleted as long as they are not effected by a change of the time domain solver settings, the time domain solver parameters, the source definitions or any change in the modeled structure.

Solver logfile

After the solver has finished you can view the logfile by selecting Post Processing: Manage Results Logfile . The logfile contains information about solver settings, mesh summary, solver results and solver statistics.