frequency FFT

[pascal2007]

hello,
How to measure the frequency of a given signal?
I attached my example to help me.
In the first program,  I generate a sinusoidal function for 2^N points. The function is Y=20*cos(2*pi*25*X*(pi/180)); I save the file with a given name.
In the second program, I read the file and I pass it through the FFT Power Spectrum.vi to get the frequency?
When I use 2^11 points, the position of the frequency-peak (in the wave form graph) is at 0.002.
When I use 2^22 points, the position of the frequency-peak is at 10^-6 time. It changes for each number N.
The amplitude of the peak is the same and equal to about 175.

How I can do to get the same frequency of my signal, independant of N??
are my examples correct?
Thank you

[falkpl]

You should make your signal a waveform so the FFT has some time information.  Otherwise there will be no scale (Hz) on the X axis of the FFT. 

Paul

[pascal2007]

Hi Paul,

Can you be clearer please, I don’t understand what do you mean, “I make my signal a waveform”? I am new in Labview. Do you have an example on labviw 6.1 or 7.1?

Thank you a lot and Sorry for my disturb.

Ls. Pascal

[falkpl]

See the waveform -> build waveform wire the array into Y and the dt gets the sampling delta time of the signal.  It this case it is period of the sign wave divided #pts/sin wave simulated.  usually it is derived from the acquisition rate.