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The analytic signal formed from a real signal has a spectrum that

  • A.Is symmetric about zero frequency
  • B.Contains only negative frequencies
  • C.Is identical to the original spectrum
  • D.Contains only positive frequencies

Answer: D

Adding \( j \) times the Hilbert transform to a real signal cancels the negative-frequency half, so the analytic signal is one-sided in frequency. It is the natural representation for a modulated signal, since its magnitude is the envelope and its phase the instantaneous phase, both of which are exactly what a demodulator recovers. Symmetry about zero is the property of the original real signal. A negative-only spectrum would follow from subtracting rather than adding. And the whole point is that the spectrum changes.

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