fft of higher data

[altenbach]

Well, you did not include any data with your VI as requested, how am I supposed to test?

You clearly have a DC offset because all your data is above zero. Look at it!!!

(All AC components average out to zero).

[Dost]

here is the attached data

[altenbach]

I'd suggest to set the display for the spectrum to a logarithmic Y axis. Since most spectral components are below 1e-6 you won't see much otherwise.

The rest seems to work fine. The frequency range goes to 1000, so you probably only want to display up to 500Hz.

Message Edited by altenbach on 03-29-2007 08:08 AM

[Dost]

Thank u very much for the reply,however i could not get how did u change the frequecny from 1000 to 500 ( sorry for it) and which  is this new VI that u have used in ur progame that has vrm2 written inits icon.where can i get this vi.Thanks

[Dost]

I think u have used the  phase and amplitude spectrum.vi that calcualtes the signle side spectrum that y the frequency is become 500 ( am i right ) Btw i have found FFT spectrum vis  is found in two places 1,analyze->wave measurmenet->power,psd,fft

2 analize->signal processing->frequency domain

Could u please suggest which one shoule be use for the FFT analysis.I really appreciate for your kind helpfull suggestions.

Regards

[Dost]

Hi its me again sorry to bother u too much but i  really stucked in to it.I have  done as per ur VI however i could not find the VI that u have used for vrms.  I have used from analize ->signal processing->frequency domain. but it has different icon than yours.if it is the same as u have used then why y-axis resolution is not like u.I have used lograthmic y axis.I trie d to use your scale on y axis but did nt succeed.Any help.

Regards

[altenbach]

It's the same thing as you are using. Only the icon has changed (I am using it in LabVIEW 8.20)

[Dost]

I have one more question to ask,I have sampled the data at 1000 hz but i saved the data in my data file at every milli second for about 50 seconds. this mean i have got about 50000 points but even than as u can see my FFT  has DC off set.Is there any way that i can extrapolate my data so that i can have data points at  every second fot two minutes giving me 120 data points for every second.