Okay so on the trace in the paper you can see that there are 3 peaks telling you that in 10ms (that is 1x10^-2 s) there are 3 wavelengths past a set point.
Frequency is wavelengths past a set point per second, and you know that 10ms goes into 1s 100 times. Therefore you know the approx frequency to be 3 (in 0.01s) x 100 (to get to 1s) giving you a frequency of 300Hz.
Hope that helps.