1. A method comprising: selecting different Nyquist frequencies to be sampled; for each Nyquist frequency, sampling the extrapolation value of the S-parameter matrix; extrapolating the real part in the out-of-band region of the S-parameter matrix, performing a Hilbert transform on the extrapolated real part of the S-parameter matrix to compute the imaginary part of the S-parameter matrix, comparing the computed imaginary part to the imaginary part of a causal impulse response to obtain a difference signal, and selecting the Nyquist frequency and the extrapolation value as an optimal extrapolation curve when the difference signal is within a desired tolerance; and performing an inverse Fast Fourier Transform on the optimal extrapolation curve.