Hi,
I am tasked with taking the FFT of a 1D array of 2310 points. The dt value between each individual point is .142 seconds. So i believe the sampling frequency is around 7.0422 Hz. I have been trying to format the frequency axis correctly, so that my graph will correctly display at which frequency my peak is at. I have taken the FFT in three different ways, each displayed in the VI attached.
The first way, the graph is labeled "Resitance FFT using dt". After taking the FFT of the data, i wire the output to a build waveform vi with the dt value set to .142. The results of this graph display that i have a peak somewhere between 0 and .15 Hz, which is what i am trying to achieve, but i believe this is incorrect because the x-axis has the same scale as the original data(time) and not frequency.
The second way, the graph is labeled "using df". The df value is .003048. I believe this method is the correct way to scale my frequency axis, but the results i receive give me a peak at a frequency from 0 to 0.0025hz. To check these results i decided to wire the output of my FFT to a filter with a low cut-off frequency of 0.01 hz and a high cut-off frequency of .5 hz, and then take the inv FFT. The results of this display a sinusoidal wave on the graph labeled "FFT->filter->inv FFT" which is exactly what the whole goal of this program was to achieve. What i cannot seem to wrap my head around is that my FFT plot labeled "using df" which I scaled using df has a peak at a frequency much lower than the low cut-off frequency of my filter, so after i take the inv FFT of that data, nothing should appear since it should be filtered out. To me this suggests that the way i scaled the first FFT plot labeled "Resistance FFT using dt" is correct. Is there an explanation as to why i am obtaining these results?
Finally, the last way i displayed my FFT of the data was by simply wiring the output of the FFT vi to a waveform graph. This graph is labeled "no scaling", and i believe this FFT is incorrect because when you look at the x-axis the final point is at 2310, exactly the number of data points, so the x-axis is not scaled correctly at all.
So, what is the correct way to scale my frequency axis? Or does anyone know why i am obtaining these odd results and how to fix it? if i left something out of this post/ you need more info to help solve the probem, please let me know.
Any suggestions/corrections are much appreciated. Thanks for your time.
