FFT data
FFT data
(OP)
Dear all
This is semi-related to an earlier thread. I have performed a Fast Fourier Transform on a set of acceleration data within Excel using the Analysis Toolpak -> Data Analysis -> Fourier Analysis (thanks, Michael!). Excel has produced a set of FFT data which is a series of complex numbers. Since there is very little documentation on this function within Excel, could someone cast light on the following:
- My input - acceleration - data is sinusoidal in nature (also only a single frequency) the maximum amplitude being 0.5g. The complex numbers (FFT data) produced from Excel on this data I have converted using (x^2+y^2)^0.5 where x+yi is the format of the complex number; this apparantly yields the "magnitudes of the FFT". However, the magnitude of the dominant frequency in the FFT is at least 10 times higher than the input - I don't understand why this is, but there must be a good reason why.
Would appreciate a potted summary of the complex output for FFT in Excel and any general remarks about FFTs.
Many thanks,
-- drej --
This is semi-related to an earlier thread. I have performed a Fast Fourier Transform on a set of acceleration data within Excel using the Analysis Toolpak -> Data Analysis -> Fourier Analysis (thanks, Michael!). Excel has produced a set of FFT data which is a series of complex numbers. Since there is very little documentation on this function within Excel, could someone cast light on the following:
- My input - acceleration - data is sinusoidal in nature (also only a single frequency) the maximum amplitude being 0.5g. The complex numbers (FFT data) produced from Excel on this data I have converted using (x^2+y^2)^0.5 where x+yi is the format of the complex number; this apparantly yields the "magnitudes of the FFT". However, the magnitude of the dominant frequency in the FFT is at least 10 times higher than the input - I don't understand why this is, but there must be a good reason why.
Would appreciate a potted summary of the complex output for FFT in Excel and any general remarks about FFTs.
Many thanks,
-- drej --





RE: FFT data
RE: FFT data
I always analyse a known signal when using a new FFT algorithm just to check what calibration factors etc the author has used.
Cheers
Greg Locock
RE: FFT data
-- drej --
RE: FFT data
RE: FFT data
Scaling the frequency axis is easy. The first line is at 0 Hz, each line thereafter is 1/T Hz higher, if the frame is T seconds long.
If there were N samples in the frame then only the first N/2 bins are needed for amplitude work, the second half are used to define phase.
The amplitude scaling in a digital FT is a design choice made by the programmer, Excel appears to use a factor based on the number of samples.
Cheers
Greg Locock
RE: FFT data