In the demodulation DSB-SC AM signals, there is a need for a phase-coherent or synchronous demodulator for recovering the message signal from the received signal. That is, the phase of the locally generated sinusoid should ideally be equal to the phase of the received carrier. What are the ways by which a sinusiod at the receiver can be generated phase-locked to the phase of incoming carrier signal ? Explain with the aid of block diagrams, and discuss their advantages and disadvantages.