# Carrier generation at receiver in DSB-SC modulation?

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. ?

• But in costa's receiver quiescent frequency of the oscillator is same as carrier in modulated signal y(t). Let's say y(t) can have any frequency between ( 90 -107 MHz ). Is it required to change quiescent frequency of the oscillator whenever $w_c$ changes. – Virange Nov 17 '15 at 11:10