I am taking the FFT of some data and have two questions: how do I know I am correct and what are the units of my y-axis?
Let us start with question two. Here is my original time domain signal:
From what I have read the y-axis units of your FFT should be the same as your units on your time-domain signal which in my case is millimeters. Now when I take the FFT my results are as follows:
When I look at this data I am puzzled as nothing that I am doing is every 35mm. I know that my frequencies are very small, but that is expected with my appliaciotn. Here is the code that I am using to make this signal:
s = data - mean(data); %Removes DC component
len2 = 2^nextpow2(len); %Changes lentgh of signal
s = fft(s,len2);s2 = s(1:len2/2);xfft = Fs*(0:len2/2-1)/len2; %Converts x-axis to Hz
plot(xfft,abs(s2));
I wanted to add that even though I removed the DC compoent I still have a large spize at very close to 0Hz. Is that normal? Now back to my first question, how do I know this is correct? Any insight would be helpful.
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