The coherence varies in the interval 0 ≤ ≤ γ γ x y 2 ( f ) ≤ ≤ 1 . If γ γ x y 2 ( f ) = 1 it means that the signals are perfectly correlated or linearly related and if γ γ x y 2 ( f ) = 0 they are totally uncorrelated. If a linear system is being measured, x ( t ) being the input and y ( t ) the output, the coherence function will be unitary all over the spectrum. However, if non-linearities are present in the system the coherence will vary in the limit given above.