1. I do remember the lesson on the 20 sample points for one period, Since if I give 15000 data points, the time step will be 25/15000(s), the reciprocal value of it will be the sampling frequency which is 600Hz, very thankful for that insightful advice. Besides before trying it with 15000 data points, if I give it 8000 data points, it will work well without any error, just the result looks very terrible. Do you know the reason why the case of 8000 data points work but 15000 data points does not work?
2. Does the number of elements for the mesh and the time step influence the convergence? Since from my point of view, If I increase the time steps, there will be less possible that the sharp change of the input will occur, therefore the convergence will be easier to be completed.