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FFT
woni
Member
08-25-1999 11:18 AM
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I've come across quite an awful problem, programming of FFT. After picking
the data from the oscilloscope, I zero-padded on the data, which is not
inevitabel though. The data which is from the oscilloscope usually show a
sine wave. Therefore, apart from the area where the sine wave is, the
remaining area is almost flat, which means that the remaining area should
not affect the result of FFT. However, in what I made, it obviously changes,
whenever I move cursors which enclose the area I would like to transfer.
Please help me..
woni
the data from the oscilloscope, I zero-padded on the data, which is not
inevitabel though. The data which is from the oscilloscope usually show a
sine wave. Therefore, apart from the area where the sine wave is, the
remaining area is almost flat, which means that the remaining area should
not affect the result of FFT. However, in what I made, it obviously changes,
whenever I move cursors which enclose the area I would like to transfer.
Please help me..
woni
Christian Altenbach
Member
08-30-1999 01:17 PM
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Are you actually zero padding your sine waves??? This will cause tremendous
distortions and the remaining area will GREATLY influence the FFT
result..You also have to rescale the frequencies according to the relative
pad size.
Typically zero padding is done in the frequency domain where you just add
higher frequencies that are all zero, thus increasing the "resolution" of
your time-domain signal after backtransform.
I assume you are padding in order to get the number of points right for FFT
(vs DFT) to speed up your transforms. You probably should do it differently,
e.g use "interpolate array" to get the numbers up. You might still need a
windowing function to reduce spectral leakage.
Alternatively, you might also just add one of the windowing functions to
smoothly drop your waves to zero before "filling". This would get rid of
some "distortions" but it just does not feel right.
Please let me know if I was not clear enough.
Cheers
Christian
woni wrote in message
news:7q14qf$ie9$1@wisteria.csv.warwick.ac.uk...
> I've come across quite an awful problem, programming of FFT. After picking
> the data from the oscilloscope, I zero-padded on the data, which is not
> inevitabel though. The data which is from the oscilloscope usually show a
> sine wave. Therefore, apart from the area where the sine wave is, the
> remaining area is almost flat, which means that the remaining area should
> not affect the result of FFT. However, in what I made, it obviously
changes,
> whenever I move cursors which enclose the area I would like to transfer.
> Please help me..
>
> woni
>
>
distortions and the remaining area will GREATLY influence the FFT
result..You also have to rescale the frequencies according to the relative
pad size.
Typically zero padding is done in the frequency domain where you just add
higher frequencies that are all zero, thus increasing the "resolution" of
your time-domain signal after backtransform.
I assume you are padding in order to get the number of points right for FFT
(vs DFT) to speed up your transforms. You probably should do it differently,
e.g use "interpolate array" to get the numbers up. You might still need a
windowing function to reduce spectral leakage.
Alternatively, you might also just add one of the windowing functions to
smoothly drop your waves to zero before "filling". This would get rid of
some "distortions" but it just does not feel right.
Please let me know if I was not clear enough.
Cheers
Christian
woni
news:7q14qf$ie9$1@wisteria.csv.warwick.ac.uk...
> I've come across quite an awful problem, programming of FFT. After picking
> the data from the oscilloscope, I zero-padded on the data, which is not
> inevitabel though. The data which is from the oscilloscope usually show a
> sine wave. Therefore, apart from the area where the sine wave is, the
> remaining area is almost flat, which means that the remaining area should
> not affect the result of FFT. However, in what I made, it obviously
changes,
> whenever I move cursors which enclose the area I would like to transfer.
> Please help me..
>
> woni
>
>
