请问用FFT算法如何求出基波幅值

模块********************************************************
'FFT0 数组下标以0开始 FFT1 数组下标以1开始
'AR() 数据实部 AI() 数据虚部
'N 数据点数，为2的整数次幂
'NI 变换方向 1为正变换，-1为反变换
'***************************************************************

Public Const Pi = 3.1415926

Public Function FFT0(AR() As Double, AI() As Double, N As Integer, ni As Integer)
Dim i As Integer, j As Integer, k As Integer, L As Integer, M As Integer
Dim IP As Integer, LE As Integer
Dim L1 As Integer, N1 As Integer, N2 As Integer
Dim SN As Double, TR As Double, TI As Double, WR As Double, WI As Double
Dim UR As Double, UI As Double, US As Double
M = NTOM(N)
N2 = N / 2
N1 = N - 1
SN = ni
j = 1
For i = 1 To N1
If i < j Then
TR = AR(j - 1)
AR(j - 1) = AR(i - 1)
AR(i - 1) = TR
TI = AI(j - 1)
AI(j - 1) = AI(i - 1)
AI(i - 1) = TI
End If
k = N2
Whil