This video (by request) gives an overview of how to the FFT Math function on the Tektronix TDS2000 series oscilloscope. (same applies to the TDS1000). A brief review of FFT fundamentals is given, followed by a demonstration of using the FFT function and controls, ending with a description of the mathematical relationships between the scope settings and the FFT results. More modern scopes will have different (easier to use) controls for the FFT than this 12+ yr old scope, but it is instructive to understand the operation of this feature.
More detail regarding the basics of an FFT on a scope can be found in this video:
A copy of the note pages from this video can be found here: