This function defines a chirp signal in the form of an external function. A chirp signal is a sinusoid with a frequency that changes continuously over a certain band and over a certain time period:

The "instantaneous frequency" in this signal is obtained by differentiating the argument with respect to time t:

And we see that it increases from to . When applying the signal to a system it gives a good control over the exited frequency band, it is therefore often used for system identification.

The following c-function is used in the model:

double Chirp(double w1, double w2, double A, double M, double time)
          {
             double res;
             res=A*cos(w1*time+(w2-w1)*time*time/(2*M));
             return res;
          }
          

For a step by step tutorial see External Function—Chirp Signal.