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FFT of Voltage Waveform... (Check my work)
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Glenn0010:
Hi All,

In the file attached I have some scope data I captured of an IGBT switching.

I've tried running an FFT on excel however I'm unsure the results I've achieved. The first peak I can understand since there is a large DC component, however the scond peak I am not sure of, it's aslo happening at the sampling frequency of the scope.

I am really rusty with the fourier stuff so if anyone can poin any glaring messes I made I'd appreciate it.

I've followed the steps in this link to achieve the FFT
http://www.stem2.org/je/Excel_FFT_Instructions.pdf

Cheers
Dr. Frank:
Hello,
the formatting of your FFT output is strange for me.
As you input time domain data on the order of 100ns, I would expect FFT output  frequencies in the MHz range, but not in the 10-9. I also can't imagine at the moment, why there is a peak at the end frequency of the FFT.

Anyhow, if you set the X-axis formatting to a maximum of 1E-11, and the y-axis to e.g. 600 maximum, you'll see the typical odd harmonics (1, 3, 5, 7..) of a step response or of a square wave. Maybe a logarithmic Y-axis output would also help.

Frank
Mechatrommer:
you need many cycles of the signal, not the only one of its transient respond.
Etesla:
How 'accurate' the output of an FFT is depends largely on the number of samples, the time between samples, and the number of cycles of the waveform that are represented in the input data.

For example, taking the FFT of a single cycle of a sin wave will yield something that does not at all represent the 'expected' spectrum of a sin wave, which would be a single 'impulse' at the frequency of the sinusoid.

For this reason, I am not surprised that the results of your FFT didn't match your expectations, since you only have the rising edge of your waveform as the input. To get a good idea of the frequencies involved in your waveform, I think it would be better to get a scope shot of maybe 20 or 50 cycles, and take the FFT of that.

For your FFT, its obvious that something is wrong with your x axis which I'm guessing you wanted to represent frequency. Your interesting data should be around 1 Mhz. I plotted the FFT of your data. I also plotted the FFT of a single cycle sin wave vs 10 cycles of sin wave.

gf:

--- Quote from: Etesla on April 27, 2020, 05:09:14 pm ---For example, taking the FFT of a single cycle of a sin wave will yield something that does not at all represent the 'expected' spectrum of a sin wave,

--- End quote ---

Not true for a single cycle (granted that it is exactly a single cycle).

To understand this we need to keep in mind that the DFT does not calculate the "spectrum of the given samples", but it calculates the (discrete) spectrum of a hypothetical infinite periodic signal which is formed by repeating the given samples again and again, an infinite number of times.

So if the samples cover exaxtly a single cycle of a sine wave, then repeating the samples again and again forms an infinite sine wave, and the DFT result is fine. But if the begin and end of the samples don't stitch the waveform seamlessly, then the result of repeating the samples is no longer a sine wave and we'll face an effect called "spectral leakage".


--- Quote ---To get a good idea of the frequencies involved in your waveform, I think it would be better to get a scope shot of maybe 20 or 50 cycles, and take the FFT of that.

--- End quote ---

... and apply a window function to mitigate spectral leakage.

gf
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