FFT Problem

[Dost]

Hi

I am  a new in signal analysis.I  have used FFT VI in Labview. I have about 40,000 data points that is sampled at a rate of fs=1000 hz (T=40 sec). I am interested in analyzing my data to see information within100 hz range. Plz suggest how can i do this? I think if i do downsampling i would lose the number of my data points as I have taken 40,000 for just 40 seconds only.

Anyone with DSP experinece Plz suggest and advice ............. looking forward for a reply

desperate user

[Nann]

What are you concerned about?  Are you concerned about the resolution or computation time?

Since you have taken 40 seconds of data, the frequency resolution is not higher than 1/40 Hz if you compute the FFT power spectrum.  If you downsample the data, the frequency resolution does not change, which is still 1/40 Hz. 

If the frequency components of your signal are simple, say several tones within 100Hz range, you might be able to use AR power spectrum to improve the resolution.  However, the AR power spectrum need more computation time.  If your signal is stational, say the first second of signal contain all the information already, you can just use a segment (the first second, for eample) to compute the AR power spectrum. 

[Dost]

thanks for the reply.However what do u mean by AR power spectrum.how I can use this in Labview.what is computation time is it mean time during which i have stored the data i.e 40 second in my case.my problem is that I have small duration of data I want to increase this lenght up to 2 minutes is there any solution.......

[Nann]

AR Power Spectrum is a VI in the Advanced Signal Processing Toolkit.  The VI has higher frequency resolution but it is time consuming.  If you increase the measurment to 2 minutes, I think you have to use FFT.  Zoom FFT might be also useful to speed up since you are only concerned about frequencies within 100Hz.  Zoom FFT is available in the Sound and Vibration Toolkit.