A DSB-SC modulated signal is demodulated by applying it to a

A DSB-SC modulated signal is demodulated by applying it to a coherent detector.
(a) Evaluate the effect of a frequency error ?A? in the local carrier frequency of the detector, measured with respect to the carrier frequency of the incoming DSB-SC signal.
(b) For the case of a sinusoidal modulating wave show that because of this frequency error, the demodulated signal exhibits beat at the error frequency. Illustrate your answer with a sketch of this demodulated signal.