Spectrum using FFT

[cschneider]

Hi,

i am trying to convert a time-signal to a frequency signal using FFT Spectrum (Mag-Phase).vi. As the frequency expected is in the Terahertz range (10^15 Hz) and the frequency range is very smallband, I need a good resolution in the frequency range, but as the Labview output starts at 0 Hz, it's not very well resolved. So I need a way to increase the resolution (i. e. decrease the df of my output signal), but I don't know how to do this...can somebody help me?

thanks in advance,

christian

[Ton Plomp]

The resolution of an FFT is the inverse of the acquisition time.

So 2 seconds of data will make a df of 0.5 Hz.

Ton

[EricLarsen]

Yep, the way to increase resolution is to take more data. Here's an article explaining that:

http://zone.ni.com/devzone/cda/tut/p/id/4278

The tough part for you is the frequency of interest. If you're really in the terahertz range, which is actually 10^12, to satisfy Nyquist you'll need to be sampling at least twice your frequency. For a df of 0.5 Hz, that 2 seconds of sampling is going to be terabytes worth of data. That's a lot of numbers. You'll have to make some good decisions about exactly what df you need and if you have the hardware to handle that much.

[Tim_S]

i am trying to convert a time-signal to a frequency signal using FFT Spectrum (Mag-Phase).vi. As the frequency expected is in the Terahertz range (10^15 Hz) and the frequency range is very smallband, I need a good resolution in the frequency range, but as the Labview output starts at 0 Hz, it's not very well resolved. So I need a way to increase the resolution (i. e. decrease the df of my output signal), but I don't know how to do this...can somebody help me?

Extend your time sample if you are just looking to increase the df (df ~ 1/T where T is the time length). You may want to look at a Zoom FFT in the Sound & Vibraion Measurement Suite if you are only looking at a small portion of the frequency range.

Tim