Distortion analysis with FFT.
Anyone who has used Fast Fourier Transfer techniques to analyse the spectrum of a signal knows about the use of windows. Briefly, if your signal is not synchronous to the FFT process, you get spectral leakage, unless you use a window. The window does not completely eliminate spectral leakage but will minimize it. However, the window does modify the spectrum display and there are a myriad of window types, depending on what parameter of the spectrum you want to be as accurate as possible. Virtins Technology has published a lengthy document containing complete characterizations of many windows, including their transfer function and example application. Worth a read if you're in that sort of thing.
Extremely low level distortion measurement.
In the past, IC designer developed opamps and related parts that had such low distortion that it couldn't be measured even with state of the art equipment. Engineers had to invent new ways to characterize these parts. Michael Steffes has been involved in such adventures and has written it all up in 2019 for AudioXpress. His January 2020 column has a reference to work done by his collegue Xavier Ramus. That referenced article gives a very good insight into the challenges and solutions of very low distortion measurements. The original has been lost but Michael has resurrected it, and it can be downloaded here.