The spectrum of the electromagnetic pulses concentrates mainly in the very low frequency bands with frequencies < 250 kHz, and therefore the dimensions of the structure is very small compared to the wavelength. Solving this kind of problems using the Finite Difference Time Domain (FDTD) method requires huge number of time steps (millions of time steps for some problems) due to the Courant stability condition. Because the cell sizes of the FDTD computational domain are very fine compared to the wavelength, the solution will include high frequency errors. Consequently, as the time steps increase the numerical errors accumulate and the solution becomes unstable.
In order to tackle this problem, we eliminate the high frequency errors at each time step by using FIR filter. This filtering process is applied at each time step to filter the high frequency noise. This method provides stable results over millions of time steps, and it is able to simulate the electromagnetic lightning effect on complicated structures and electronic equipment.
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