fft use

[claudi]

hello everybody,

I need to obtain the frequency content of a signal, but the occuring frequencies are propably not integer multiple of each other. If I use the FFT the amplitude is not calculated correctly (see attached vi). I guess that I'm not allowed to use FFT.vi but unfortunatly I've no idea which *.vi I have to use instead.

Can anybody help me?

concerning to the attached vi try:
ampl.1=ampl.2=1
freq.1=1
freq.2=6

result after fft:
ampl.1=1
ampl.2=0.7
location with peak detection gives also wrong values.

And second question: Does the rms identify the right values or is it also confuesed?

Thanks Claudia

[rpursley8]

You can use the FFT but you need to increase the number of samples. The more samples you have the closer you will get to the correct amplitude.

[DSPGuy]

Claudia,

I would recommend using the Extract SIngle Tone Information.vi to perform the peak detection in the frequency domain. This VI is designed to do exactly what you are trying to do. I have modified your example to demonstrate.

[claudi]

Hello DSPGuy,

I can't open it because I've only version 7.0
Can you please save it in an older version?

Thank you Claudia

[claudi]

to rpursley8

if I increase the number of samples then it matches the right amplitude. But if I increase the number of samples to high then the second amplitude is to high.
In my case I don't know the occurent amplitudes and frequencies so I'm not sure that this will work out because it means to me that every frequency has a proper number of samples that I have to find. Mmh this way should be difficult, maybe there is an other one.

But thank you for you proposal
Claudia

[rpursley8]

Try making the number of samples and the number of samples per second the same. The simulations I ran show that the amplitudes seem OK.

[DSPGuy]

Hi Claudia,

Sorry I missed your post earlier. Here is the VI saved in LabVIEW 7.0.

[claudi]

@ DSPGuy

thank you very much for your reply. You were absolutly right. I use it and it works really fine 🙂

claudia