# is there any problem in my FFT?

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SUNHAK KIM on 28 Oct 2021
Commented: David Goodmanson on 28 Oct 2021
AB = C (time domain data)
A and C : known
B : unkown
length of A,B and C is 512
N1 = 512;
t = t1(1:N1-1);
A = A1(1:N1-1);
C =C1(1:N1-1);
T1 = tpp*N1;
fpp = 1/T1;
a1 = 0.3906;
nr = N1*a1;
fs = 1/tpp;
df1 = 0:fs/N1:(N1-1)*fs/N1;
dfp1 = df1(1:nr);
fft1 = fft(A);
fft_Mag_A(1) = (1/N1)*abs(fft1(1));
fft_Mag_A(2:nr) = (2/N1)*abs(fft1(2:nr));
fft_Phase_A = angle(fft1(1:nr));
fft2 = fft(C);
fft_Mag_C(1) = (1/N1)*abs(fft1(1));
fft_Mag_C(2:nr) = (2/N1)*abs(fft1(2:nr));
fft_Phase_C = angle(fft1(1:nr));
B = fft_Mag_C/fft_Mag_A*cos(w*t+fft_Phase_C - fft_Phase_A);
fft3 = fft(B);
fft_Mag_B(1) = (1/N1)*abs(fft3(1));
fft_Mag_B(2:nr) = (2/N1)*abs(fft3(2:nr));
fft_Phase_B(1:nr) = angle(fft3(1:nr));
C1 = fft_Mag_A*fft_Mag_B*cos(w*t+fft_Phase_A+fft_Phase_B)
here is a problem.
why C1 and C(data) doesn't match perfectly each other when i draw graph?
There's a slight error.
David Goodmanson on 28 Oct 2021
Hi SK,
could you explain the purpose of doubling the magnitudes between 2 and nr, and also why the magnitude and phase of C are based on fft1 and not fft2?