激励函数的问题

求散射的时候平面波的激励信号可不可以指定啊?我想用正弦的和指数e(-at)类型的，这个怎么指定啊?是要在user define里面指定吗?要编程吗?希望高手指点一下啊！

(1)->Excitation Singals->New Excitation signal->
        User define(Edit)-->VB editor
(2)
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Function ExcitationFunction(dtime As Double) As Double
    'Assign the excitation signal value for the given time to the function name.
    ExcitationFunction =Exp(-a*(dtime))
End Function
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(3)save (singal name)

多谢小编解答～～～～～～～～～～～～