# FFT, IFFT and phase shifting

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RH ZA on 18 Apr 2019
Commented: RH ZA on 18 Apr 2019
Hi everyone, please i need some help !
i have a rectangular pulse i did the FFT it gaves me a sinc forme and i try to make a phase shifting of this pulse, but when i applicate the ifft on my code i get an horrible forme !!! here is my code :
%XXXXXXXXXXXXXXXXXXXXXXXXXX FFT with bilt in function XXXXXXXXXXXXXXXXXXXX
fs=1e+10;
ts=1/fs;
t=[0:ts:(1.6384e-6)-ts];
A=2;
y=A*trapmf(t,[100e-9 105e-9 205e-9 210e-9]);
subplot(2,1,1)
plot(t,y);
grid;
N=length(t); %which is equal to 16384 so pow of 2
%XXXXXXXXXXXXX FFT XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
Xf=fftshift(fft(y));
f=fs.*(-N/2:N/2-1)/N;
subplot(2,1,2)
stem(f,abs(Xf)/length(y));
title('FFT');
%XXXXXXXXXXXXXXXXXXXXXXXXX PHASE SHIFT XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
phi= -100;
ind = 1:numel(Xf)/2; %Postive frequencys
ind = numel(Xf)/2+1:numel(Xf); %// upper half (negative frequencys)
Xf(ind) =abs(Xf(ind)).*exp(1i*angle(Xf(ind))+1i*phi); %// subtract 1i*phi
%XXXXXXXXXXXXXXXXXXXXXXXXXX IFFT XXXXXXXXXXXXXXXXXXX
y1=ifft(ifftshift(Xf));
figure(2)
plot(t,real(y1));
grid on;
title('IFFT');

Catalytic on 18 Apr 2019
Is this what you want
Xf=fftshift(fft(y));
f=fs.*((0:N-1) -ceil((N-1)/2))/N;
subplot(2,1,2)
stem(f,abs(Xf)/length(y));
title('FFT');
%XXXXXXXXXXXXXXXXXXXXXXXXX PHASE SHIFT XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
phi= -100;
Xf=Xf.*exp(+1i*phi*2*pi*f);
%XXXXXXXXXXXXXXXXXXXXXXXXXX IFFT XXXXXXXXXXXXXXXXXXX
y1=ifft(ifftshift(Xf));
figure(2)
plot(t,real(y1));
grid on;
title('IFFT'); RH ZA on 18 Apr 2019
No, just shiftting it, i tried that with your method but no results, should i give all the frequencys to the equations or just the half ? because with all the frequencys the equation gives complex numbers