Problems with Signal Analysis

[Colt]

I have problems with Signal Analysis.The Vi which samples value from the acquisition card is in an while loop and samples each loop a value.the values were put in an array.I want to make different signal analysis(fast fourier transformation,powerspectrum,...) with these values.The problem is, that the graph shown in the diagramm is correct,but the value on the x-axis don't match with the analysis.What have I to do?Have I to create a waveform?If yes,how?

[MattBradley]

I'm not sure exactly what your data format is, but if you are doing any FFT's, you are going to need the time information or your X-Axis will be incorrect (Which sounds like what you have).

So to answer your question, yes, you will need to create a waveform. Waveform->Build Waveform.

[Colt]

Thank you for your help!Now i have build an waveform and it works so far. But I need frequency values on the x-axis.I don't know how to get the frequency values there.At the moment there are values which are calculated out of the number of sampled values and dt used by building a waveform.

[MattBradley]

Not sure I understand you. Are the x-axis values different from what you want or are they wrong?

[Colt]

When I make for example an FFT, i want to see the frequencies on the x-axis which are in the signal.But in my Labview Diagramm the values on the x-axis have nothing to do with the frequencies in my signal.The FFT seems to be correct,but the x-axis doesn't show what I want to see.I don't know what I have to change. In some NI examples it works,but I can't see why.

[Naïs et Jérôme LANGE]

If you want to have the 'good frequencies', you must set your dt.
if you use LV6 and waveform, the dt is the inverse of your sampling
frequency.
If you use older version, you can set your dt buy bundling dt value and the
array of point. you also can set the multiplier value in the
attribute/propertie node.
dt is always the inverse of the sampling frequency.


--
Jerome LANGE
Strasbourg
andromed@club-internet.fr

"Colt" a écrit dans le message news:
506500000005000000EB7B0000-1021771306000@exchange.ni.com...
> When I make for example an FFT, i want to see the frequencies on the
> x-axis which are in the signal.But in my Labview Diagramm the values
> on the x-axis have nothing to do with the frequencies in
my signal.The
> FFT seems to be correct,but the x-axis doesn't show what I want to
> see.I don't know what I have to change. In some NI examples it
> works,but I can't see why.

[Randy_H]

Colt,

Hoave you looked at the shipping examples that come with LabVIEW? There are many examples for doing Signal Analysis (Processing). This is the best place to start looking for help. There is even a Power Spectrum example. All you will need to do is to replace the generated waveform with one from a DAQ card.

Randy Hoskin
Applications Engineer
National Instruments
http://www.ni.com/ask

[Randy_H]

Colt,

Hoave you looked at the shipping examples that come with LabVIEW? There are many examples for doing Signal Analysis (Processing). This is the best place to start looking for help. There is even a Power Spectrum example. All you will need to do is to replace the generated waveform with one from a DAQ card.

Randy Hoskin
Applications Engineer
National Instruments
http://www.ni.com/ask