Help with removing the transient part in FFT

I want to remove the transient psrt from my fft of TE to clearly see the frequency components. I have tried removing the transient part say first 0.05 sec, but it doesn't work

My code is here: https://uk.mathworks.com/matlabcentral/answers/1937134-help-with-computing-fft?s_tid=prof_contriblnk

Answer 1

[Bhanu Prakash]

Hi Siddharth,

As per my understanding, you want to remove the transient part in FFT.

To remove the transient part of FFT, you must apply a window function to the input signal before computing the FFT. Consider the code shown below:

N=128; 
window_start=1; 
window_stop=20; 
x=rand(N,1); 
x(window_start:window_stop)=x(window_start:window_stop).*hann(window_stop-window_start+1); 
X=fft(x); 
plot(abs(X));

where, "N" is the FFT size and "x" is a 128x1 matrix containing random values ranging in the range (0,1). To remove transient in a particular interval, "window_start" & "window_stop" are used.

The window function "hann" is applied to the values of "x" in the interval (window_start, window_stop), to remove the transient in that interval. Then the FFT is computed with the help of "fft" function.

For more information on the above-mentioned functions, you can refer to the following documentation:

For "rand" function:

https://www.mathworks.com/help/releases/R2023a/matlab/ref/rand.html?s_tid=doc_srchtitle

For "hann" function:

https://www.mathworks.com/help/releases/R2023a/signal/ref/hann.html?s_tid=doc_srchtitle

For "fft" function:

https://www.mathworks.com/help/releases/R2023a/matlab/ref/fft.html?s_tid=doc_srchtitle