What is the difference between time-domain and frequency-domain solvers in CST?

A time domain solver calculates the development of fields through time at discrete locations and at discrete time samples. It obtains the entire broadband frequency behavior of the simulated device from a single calculation.

A frequency domain solver assumes a time-harmonic dependence of the fields and the excitation. Each frequency sample requires a new equation system to be set up and solved; thus, the relationship between calculation time and frequency samples is linear. Therefore, the frequency domain solver usually is fastest when only a small number of frequency samples need to be calculated.

The performance of the time domain solver degrades for strongly resonant structures or if the device operates at very low frequencies. In such cases, the frequency domain solver may be faster, especially if just a few frequency samples are sufficient to characterize the structure’s behavior. On the other hand, the simulation time of the frequency domain solver increases more rapidly with an increase in the number of mesh cells than the simulation time of the time domain solver, which makes the time domain solver more suitable for electrically larger problems.