fft of a finite duration real signal

Hello

I have signal which is finite in duration sampled at a certain frequency and has a lot of noise buried in the signal. I am trying to plot the spectrum using fft to find out what frequency the signal is in. I don't know if I am doing the fft right. The plot of time vs amplitude and spectrum is in the link below.

http://www.yorkphoto.com/york/slideshow/AlbumID=3615814002/PictureID=207156219002/a=100160453_100160453/

Can anyone please help? Thanks

Answer 1

[Walter Roberson]

What was the sampling frequency? It is difficult to make out on the graph but it appears you might have had 4 seconds of data at 30 kHz ?

Your frequency data is packed pretty tightly; you might want to zoom in on it. The interesting parts might be below 1 kHz or so.

[vsee]

There is a sample every 400ns which converts to a sampling frequency of 2.5MHz. Here is the code I used.

sig = csvread('tek0001CH1.csv',15);

init_ampl = sig(:, 2);

ampl = init_ampl';

fs = 2.5*10^6;

T = 1/fs;

L = 10000000;

t = [0:L-1]*T;

NFFT = 2^nextpow2(L);

Y = fft(ampl,NFFT/4+1)/L;

f = fs/2*linspace(0,1,NFFT/4+1);

subplot(2,1,1),plot(t, ampl);xlabel('time');ylabel('amplitude');

subplot(2,1,2),plot(f,20*log10(abs(Y)));xlabel('frequency');ylabel('20*log10|Y|');

There are a total of 10 million samples.

You are right about the frequency data being packed tight. I just want to know where the signal is so I can filter out the noise.