As shown in fig., In this scheme input signal is square and passed through a narrow(high Q) bandpass filter tuned to \$ w_c\$.Finally the ouput comes to be a sinusoid of frequency \$ w_c\$.
But to implement this scheme you need prior information of \$ w_c\$ because you need to tuned your bandpass filter at \$ w_c\$.
My question is that if you know \$ w_c\$ then why not just use a crystal oscillator to generate carrier of same. why use all PLL, filters etc. ?