We have a PC with digital-to-analog converter. With help of this converter we generate “pure” sine: y(t)=A*sin(2π*f*t+ ψ) with industrial frequency 50 Hz. This signal goes as input to electricity generating plant which has a goal to raise this signal and give to its own output. During this signal raise it gets distorted (there is some non-linear element in the plant scheme) and on the output of the plant we get not “pure” signal (using Furier expansion we have harmonics of higher than 50 Hz frequencies: 100 Hz, 150 Hz, 200 Hz and so on). We can see these higher frequencies when we analyze signal from the analog-to-digital converter (the same PC with digital-to-analog converter) that arrives from the output of the same(as in beginning of this post) electricity generating plant and the signal gets decreased to the same level as it was at input of electricity generating plant. THE PROBLEM IS: feed the electricity generating plant with signal that on the output it will be “pure” sine(there is a constraint: difference between main harmonic and any other from the first eleven not less than 52 decibel). Any ideas how to implement this?