Why can't I see very low frequencies in the Spectrum FFT window in LabChart?

Which frequencies can be resolved in the Fast Fourier Transform (FFT) window of the Spectrum function is sensitive to the FFT window width (FFT Size in the Settings dialog) and which ‘bins’ (or data ranges) the frequencies are collected. The Spectrum picture can change significantly at different FFT sizes.

The FFT works on a certain number of data points over the selection to be analyzed. The larger that number, the greater the frequency resolution (the number of frequency 'bins' into which the signal is resolved). Small FFT sizes give a higher amplitude accuracy but a lower frequency resolution.

The FFT Size determines the bin width. For example, at the default setting of 1K (1024) and a sampling rate of 200/s, the ‘bin’ width is 0.2 Hz (Bin width = Sampling Rate / FFT Size). Therefore, frequencies below 0.2Hz cannot be seen.

To view possible peaks at 0.01 Hz at a sampling rate of 200/s, you need an FFT size of 16 K or larger. At this fine resolution, low frequencies dominate the spectrum.

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