问题
I have a current signal which I obtained from cadence simulation over 30ns time but my current actually varies (which I call active region) over certain period. I extracted the data of current signal over 1ps fixed step and plotted the current. Then to analyze frequency spectrum and powers spectral density I used FFT and PSD in MATLAB. But I am confused about what this analyse mean and how can realize it.
The operation frequency of my circuit is 2GHz. So my goal is to see the frequency spectrum and power at this operation frequency. Now so far I understood I need to normalize the signal while doing FFT in MATLAB. I did it. If my current signal is x, I did FFT using the equation FFT_x = abs(fft(x)/sqrt(N)) where I N = length(x), e.g., length of my current signal. But I do not get any peak at my operation frequency (2 GHz) and not even in any other frequency also. The FFT is like below (NB: I shifted the plot to one sided for my analysis of my operation frequency).
But if I don't normalize I get peak at my operation frequency (2 GHz) where I used FFT_x = fftshift(fft(x)); The plot is given below:
Now the later plot is not normalized but I have something to realize my goal. But I cannot say it as my FFT as I read in various forums that I need to normalize the signal. While normalizing it the spectrum is almost flat. How can I use it for my analysis.
Also my PSD is similar like the plot of normalizing FFT except the amplitude value is different if I do it over normalizing FFT as psd = abs(fft(x).^2);
Do you have any idea how do I do the analysis here. Did I do any mistake?
Also it would be nice if I could get some good reference for FFT and PSD for my case.
After solving the above problem. I have two concern. In many papers or references I have read that windowing is needed to do FFT. Is it needed? What is the importance of this? if so, How can I do that?
Another one is - how to show the Perseval's theorem? I mean the power before FFT and the power after FFT is same as per Perseval's theorem. How can I code it in MATLAB?
来源：https://stackoverflow.com/questions/44664216/fft-and-psd-of-current-signal-and-analysis