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Different plot shape for fft depending on time sample spacing

Different plot shape for fft depending on time sample spacing

Different plot shape for fft depending on time sample spacing

(OP)


I am having the following issue. I am trying to analyse the input and output signal of my filter using fft. However, I am not sure if my fft is correct. My input signal is the product of a sawtooth and sine waveforms both with frequencies 6kHz and 32kHz, respectively. Both individual signals have an amplitude of 1 before being multiplied.
Depending on my time values I get different result for my fft plots. If I use values of time spaced out by the period of my sampling frequency, I get a "nice" looking graph with clear peaks as you would expect in a Fourier Transform (see Fig.1). However, if I use values of time spaced out by the period of the signal being sampled, the graph has more frequency components (see Fig. 2). I really don't know which graph is correctly displays my FFT. Also I have the problem that I used an online tutorial to achieve my FFT but I do not understand why the guy did what he did in his code which I do not understand. However, if I plot the absolute values of FFT, I get graphs with similar shapes but the amplitudes jump up to 30 or 40 more (see Fig. 3 and 4). Also for Fig.2 why is there a small wiggle at the start of the filtered signal (time domain) vs time ? Please your help in performing FFT accurately would be of great help. Thanks in advance.

CODE --> Matlab

%Time values using sampling frequency period.
fsampling = 80000;
tsampling = 0:1/fsampling:5000000*Tsampling;


%Time values using sawtooth frequency period.
fsampling = 80000;
fsawtooth = 6000;
Tsawtooth = (1/fsawtooth);
tsawtooth = 0:1/fssampling:20*Tsawtooth;


 %Piece of code to use FFT that I don't understand.
 L0 = length(product); 
 NFFT0 = 2^nextpow2(L0);
 Y0 = fft(product,NFFT0)/L0;
 FreqDom0 = fs/2*linspace(0,1,NFFT0/2+1);


%Performing FFT using just abs.
 IPFFT = abs(fft(product));
 plot(t, IPFFT);
 plot(FreqDom0, 2*abs(Y0(1:NFFT0/2+1))); 

RE: Different plot shape for fft depending on time sample spacing

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