# aliasing in frequency domain problem

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fima v on 1 Apr 2020
Commented: fima v on 2 Apr 2020
Hello ,i am trying to show frequency domain transform of 3 sinuos signal.I samples it twice the frequency of the highest frequency in the signal.
Althogh i get 3 harmonics but i dont get them at the 10,30,70 Hz and their amplitude is not 10(As shown in the code bello)
Where did i go wrong?
Thanks.
t=0:0.001:1
f1=10;
f2=30;
f3=70;
y1=10*sin(2*pi*f1*t);
y2=10*sin(2*pi*f2*t);
y3=10*sin(2*pi*f3*t);
y4=y1+y2+y3;
% twice the sampling rate
Fs=2*70;
Ts=1/Fs;
Tn=0:Ts:1;
fft_L=length(Tn);
y4_samples=10*sin(2*pi*f1*Tn)+10*sin(2*pi*f2*Tn)+10*sin(2*pi*f3*Tn);
%stem(Tn_new,y4_samples);
ff=fft(y4,fft_L);
plot(abs(ff));  Ameer Hamza on 1 Apr 2020
Edited: Ameer Hamza on 1 Apr 2020
First, you are taking fft of y4 which is sampled at 1000Hz. Check the following code to see how it can be dome properly
f1=10;
f2=30;
f3=70;
% twice the sampling rate
Fs=2.05*70; % sampling frequency is a bit above 2 times to get all the peaks.
Ts=1/Fs;
Tn=0:Ts:1;
fft_L=length(Tn);
y4_samples=10*sin(2*pi*f1*Tn)+10*sin(2*pi*f2*Tn)+10*sin(2*pi*f3*Tn);
%stem(Tn_new,y4_samples);
ff=fft(y4_samples);
ff1 = abs(ff/fft_L);
fft2 = ff1(1:floor(fft_L/2)+1);
f = Fs*(0:fft_L/2)/fft_L;
plot(f, fft2);

fima v on 1 Apr 2020
Hello Ameer,in aliasing i was looking for a copy of our signal in a different location
our signal+our signal shifted, this just a shift.
coud you please show a shifted copy?as shown in the figure bellow.
Thanks. Thanks.
Ameer Hamza on 1 Apr 2020
Sorry, I don't have much knowledge about this stuff. Since this question is specifically related to signal to process, I think you will have a better chance of getting an answer on an appropriate forum: https://dsp.stackexchange.com/
fima v on 2 Apr 2020
I am struggling to understand how to contol the amplitude of the harmonics in our transform.
Thanks.