Problems with obtaining actual results using the FFT VI.

[black mamba]

I'm using the FFT express VI and graphing the results. My experiment involves decreasing the amplitude of a 100Hz acoustic pressure wave, ideally to zero, by phasing it with another signal of the same frequency and amplitude. According to other analysis I believe that the amplitude from the FFT of the signal is increasing and then decreasing. However, I am not able to visually see this in real-time on the graphical display.

The change in amplitude is happening quickly, every few ms. Is this timing to fast for the process to maintain proper resolution? Is there a way to make the output from the FFT more sensitive?

Thanks for the help.

[rpursley8]

Are you looking at the output in dB or Linear? If you set it to linear output, you will see the changes you expect. The variations are harder to see in dB mode.

[johnsold]

The period of a 100 Hz signal is 10 ms. If your amplitude is changing every few ms, an FFT is not going to reflect this accurately. The FFT is basically defined for constant amplitude during the segment of signal being analyzed. The FFT also typically requires several cycles to give useful results. If it changes during this time, other techniques must be used to detect that. There is a large body of literature on time-frequency analysis dealing with this topic.

Lynn