measure of the degree of linear, noise-free relationship between the measured system input and output signals at each frequency
NOTE 1 The coherence function is defined as
w is the frequency
Sff (w) is the power spectrum of the input signal
Sxx (w) is the power spectrum of the output signal
Sxf (w) is the inputoutput cross spectrum.
NOTE 2 g2 (w)=1 indicates a linear, noise-free relationship between input and output.
NOTE 3 g2 (w)=0 indicates a non causal relationship between input and output.