What is being returned from FFT Spectrum.vi?

[ejosephw]

I'm using the FFT Spectrum (Mag-Phase).vi and inputing a single sine wave.  The resulting outputs, magnitude and phase, are arrays.  But how do I know what frequencies correspond to each element.  Basically I need to pull out the mag/phase at the specified input frequency, and I need to know which element in the array corresponds to this frequency.  I can manually look at the array and tell, but I have to input different sines and don't want to have to look each time it changes.

[Conseils]

Check out the following on FFT basics

"The use of the FFT for frequency analysis implies two important relationships.

The first relationship links the highest frequency that can be analyzed to the sampling frequency and is given by the following equation.

Fmax = fs/2

(A)

where F_max is the highest frequency that can be analyzed and f_s is the sampling frequency.

The second relationship links the frequency resolution to the total acquisition time, which is related to the sampling frequency and the block size of the FFT and is given by the following equation.

Δf = 1/T = fs/N

(B)

where Δf is the frequency resolution, T is the acquisition time, f_s is the sampling frequency, and N is the block size of the FFT."

http://zone.ni.com/reference/en-XX/help/371361A-01/lvanlsconcepts/fft_fundamentals/