You can use the freq_meter component from ahdlLib - this will output the instantaneous frequency. Also, the calculator has a means of plotting instantaneous frequency these days (but you're better off using freq_meter or something like it, because it will solve for the crossing point and hence get the frequency more accurate than if it is interpolated by the calculator). You might want to look at sourcelink solution 11015938 - probably needs to be updated (by me, since I wrote it) to reflect the fact that the calculator has a built in instantaneous frequency measurement function (think it's just called "freq" - can't remember off the top of my head).

Andrew.