The PFD has two inputs: the external input signal, Fin, and the output of the Feedback divider, Ffb. It is critical that these two inputs, Fin and Ffb have the same frequency, although they will not have the same phases. The PFD outputs a DC voltage from the phase difference of the two input signals. An ideal PFD generates a DC voltage that is linearly proportional to the phase difference between the two input signals, however in practice PFDs are never perfect, hence the output is never perfectly linear.