Fourier Analysis
Fourier Analysis is used in modern test equipment to test for NEXT and FEXT. The earlier but still used testing technique was to generate a wide range of analog frequencies and then see how much energy was induced into adjacent pairs (crosstalk). This technique is referred to as a swept frequency response or test. When we say "swept" we mean to change the frequency gradually say from 1KHz to 100MHz. Clearly to do this your need a frequency generator and also a wide bandwidth receiver to detect all the frequencies generated. Needless to say this is an expensive piece of electronics to meet the strict international standards required for this test.

Modern test instruments use some pretty fancy mathematical models to do the same thing. But instead of generating numerous analog sine waves as unique frequencies, modern test equipment uses a square wave pulse. To appreciate why this is done please review the video below. The video explains that a square wave is an infinite number of specific sine waves added together. It was mathematically proven by Joseph Fourier a renowned mathematician. His math have been expanded on and improved to a point where we can use his knowledge of square waves to mathematically test cables with a variety of square waves, each different square wave covers a specific range of frequencies.

Please use this link to raise an Acrobat PDF document which is a paper written by Fluke that broadly explains the process of using square wave pulses to perform modern cable tests such as NEXT.

If you do not have Adobe Acrobat installed on your computer click here first.

To review this explanation of Fluke Testing Document click here.

I would definitely review my video on Fourier analysis before reviewing the Fluke Testing Document above. The video will give you a good introduction to the concepts which will help your understanding of the PDF document by Fluke Engineers.