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GATE EC 2025 Set 1 — Question 28

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MSQ+1 / -0MediumAM Modulation & DemodulationAnalog CommunicationsCommunicationsDSB-SC & SSB

Communications → Analog Communications → AM Modulation & Demodulation

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Question

Consider a message signal m(t)m(t) which is bandlimited to [−W,W][-W, W], where WW is in Hz. Consider the following two modulation schemes for the message signal:
  • Double sideband-suppressed carrier (DSB-SC): fDSB(t)=Acm(t)cos⁡(2πfct)f_{DSB}(t) = A_c m(t) \cos(2\pi f_c t)
  • Amplitude modulation (AM): fAM(t)=Ac(1+μm(t))cos⁡(2πfct)f_{AM}(t) = A_c (1 + \mu m(t)) \cos(2\pi f_c t)
Here, AcA_c and fcf_c are the amplitude and frequency (in Hz) of the carrier, respectively. In the case of AM, μ\mu denotes the modulation index.
Consider the following statements:
(i) An envelope detector can be used for demodulation in the DSB-SC scheme if m(t)>0m(t) > 0 for all tt.
(ii) An envelope detector can be used for demodulation in the AM scheme only if m(t)>0m(t) > 0 for all tt.
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