You can also do the FFT calculation off line in python if you can capture a data plot from the scope. Most modern scopes let you transfer them to a PC to do this even if they can't do the FFT internally properly.
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Here's a you tube video with pointers to Python code that does exactly this.
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Dave's comments about the needed scope bandwidth still apply, however.
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I hope this helps without being too off topic...
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M
