Fig. 11. The measured response data fitted with a transfer function, derived from a hypothesis that the expression was a product of a linear part and a non-linear exponential part. The exponential part affects the phase lags only (see solid and broken lines). We obtained the simplest transfer function: B(s)=[9/(s+3)]²e^-0.02s. It is observed that B(s) fits well for both the gain and phase data.