question_answer

Draw a block diagram of a simple modulator for obtaining an AM signal. Briefly explain how amplitude modulation is achieved. Or Draw a block diagram of a simple modulator for obtaining an amplitude modulated (AM) signal explaining the function of each of its components.

Answer:

                                       The block diagram of a simple modulator for obtaining AM signal is shown below: Here the modulating signal is added to the carrier signal to produce the input signal, \[x(t)={{A}_{m}}\sin {{\omega }_{m}}t+{{A}_{c}}\sin {{\omega }_{c}}t\] This signal is passed through a square law device which gives the output, \[y(t)=Bx(t)+C{{x}^{2}}(t)\] Here Band Care constants. When this signal is passed through a band pass filter centred at \[{{\omega }_{c}}\] , the filter rejects the sinusoidal signals of frequencies \[{{\omega }_{m}},2{{\omega }_{m}}\] and \[2{{\omega }_{c}}\] and retains the frequencies \[{{\omega }_{c}},{{\omega }_{c}}-{{\omega }_{m}}\] and\[{{\omega }_{c}}+{{\omega }_{m}}\]. The output of the bandpass filter is thus an AM wave. Role of bandpass filter in amplitude modulation. The bandpass filter rejects d. c. signal, low frequencies \[({{\omega }_{m}},2{{\omega }_{m}})\], high frequency (2\[2{{\omega }_{c}}\]) and retains the band of frequencies centred around \[{{\omega }_{c}}({{\omega }_{c}},{{\omega }_{c}}\pm {{\omega }_{m}}).\]