FFT output resolution

[scottcolston]

I am currently trying to conduct an FFT on a signal in LabVIEW.

The signal has been sampled at 10kHz.

The output from the FFT VIs are integer values e.g.. 1,2,3,4,5..... I am looking for 2 particular frequencies at 14.3Hz and 15.1 Hz. Just now I get a peak at 14 and 15 but I want to be more specific. Is this possible?

Thanks

Scott

[srikrishnaNF]

hi scottcolston,

  FFT is a discrete fourier transform of a signal.i.e if you sample any signal at fs samples/second,you will get the frequency components at multiples of fs with maximum at fs/2...

Thanks and regards,

srikrishnaNF

[scottcolston]

Not too sure what you mean by this.

 If my sample rate is 10kHz what does a peak at 15 on the output represent in Hz?

Thanks

[srikrishnaNF]

hi scottcolston,

  what i mean,say if you sample your sgnal at 10khz and if you calculate your FFT,maximum it will plot upto 5khz and after that it will be repeating .And the distance between each frequency components in FFT will be your 10khz...So FFT cant be continuos one and you cant find the values for intermediate values.

Thanks and regards,

srikrishnaNF

[johnsold · Accepted solution]

Read the detailed help for the FFT.  It says that the resolution df is fs/N where fs is the sampling frequency and N is the number of samples.

So if you are getting df = 1 and fs = 10000, then N must also be 10000.  If you want resolution to 0.1 Hz, you need 100000 samples.

Lynn

[scottcolston]

Thanks Lynn,

This is where I was going wrong.

Scott

[TCPlomp]

In short the resolution is the inverse of the sample time.

Ton