FFT scaling frequency axis

[loudoe77]

Hi, any help would be much appreciated, so my program reads in data values from an excel file, builds a waveform, takes the FFT of the waveform data, filters the FFT data, then takes the inverse FFT. My question is if the scaling of my graph for my frequency axis after taking the FFT is correct? I am expecting a high frequency around 0 Hz, then a slight spike around/ near 0.1 Hz, which seems to be about what i am getting.  I am unsure if i am using the correct method to scale my axis of the FFT plot  though because i am just using the build waveform VI. My dt value is .142, as i got that from the hardware i am using to obtain these values. Can someone verify if i am doing it correctly or if i am way off and need to do something to correctly scale it? I am newer to labview and have tried everything i can think of.

Also, I am using a bandpass filter with a low cut-off frequency of 0.01 Hz and a high cut-off frequency at 0.5 Hz. After applying the filter and taking the inverse FFT i get a sinusoidal wave as displayed on the graph labeled "Resistance FIR Filtered Inv FFT" which is exactly the end results that i want. Because of this result, it makes me think that possibly my FFT graph is scaled correctly since the peaks i am getting on the FFT graph happen between 0.01 and 0.5 Hz. 

[mterz]

Hi Lou,

To clarify, do you mean the df value for the FFT graph, or do you mean the actual scale on the axis on the front panel?

[mcduff · Accepted solution]

Did you see the VI I posted in your previous discussion?

(You should not post the same question in different threads)

FFT resolution is 1/Acquisition time

Your dt value is 0.142 which means your Sampling Frequency is 7.04Hz, the maximum frequency you can resolve is 3.52Hz. You have 2310 points which means your acquisition time is 328.02 seconds. So your df is 0.003 Hz. You are wiring your dt to your plot indicator (Resistance FFT) you need to change that to get a correct plot, change it to 0.003Hz.

mcduff

[loudoe77]

Hi Madison,

I mean the df value for the FFT graph. Sorry for the confusion

[loudoe77]

Hi mcduff,

Yes, i did see you reply in the other thread. I posted this thread a day before i posted the other one, and this thread was not getting any replies. So i posted a new one with a different description, hoping that the newer one would make more sense. I realized now i should have just edited this post.

Louis