Demodulation of DSBSC Signals

1

• Demodulation of DSB-SC Signals
• What is Demodulation?
• Re-translating the received spectrum to its
  original position

2

• The receiver uses a local carrier, same as the
  sending carrier, followed by a low-pass filter to
  recover m(t)
• This is called synchronous detection or coherent
  detection

3

• 2 Amplitude Modulation
• For the Suppressed Carrier (SC) scheme, the
  receiver must generate an carrier signal,
  identical to the carrier used by the sender
• It is difficult when the distance is large
• How about letting the transmitter to send the
  carrier signal also?

• Not economical for the 1-to-1 communication
• Suitable for the broadcast (1-to-M) communication

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• However, we need

• In textbook,

5

• Example 4.19 (Fig. 4.35)

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• Angle Modulation (Sec. 4.8, textbook)
• For the amplitude modulation, we have obtained

• The information content appears as the amplitude
  part of the carrier
• We also can put the information content into the
  angle part of the carrier
• The angle involves frequency and phase
• There are two ways

7

• Phase Modulation (PM)

• Frequency Modulation (FM)

• General Form

• The Instantaneous Frequency (IF)

8

• Phase Modulation (PM)

• Frequency Modulation (FM)

• For some basic m(t), we can use the indirect
  method (IM) to evaluate the IF
• For example, if m(t) is a periodic triangle
  wave, the IM can be used for both FM and PM (Ex.
  4.21)
• However, if m(t) is a square wave, the IM can
  only be used for FM (Ex. 4.22)

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• Example 4.21(Fig. 4.43)

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• Example 4.22 m(t) is digital

• Note that we are sending digital signals
• Frequency Shift Keying (FSK) (Fig. 4.43)

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• Need to use the Direct Method (DM)

• Phase Shift Keying (PSK) (Fig. 4.44)