Lecture 5 Basic Modulation Techniques (I)

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Lecture 5Basic Modulation Techniques (I)

• Principles of Communications
• Fall 2008
• NCTU EE Tzu-Hsien Sang

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Outlines

• Linear Modulation
• Angle Modulation
• Interference
• Feedback Demodulators
• Analog Pulse Modulation
• Delta Modulation and PCM
• Multiplexing

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To modulate or not to modulate, that is the
question

• Recall the simple digital modulation for
  transmitting 0s and 1s with fiber optics.
• In some aspects, digital modulation is much
  simpler than analog modulation.
• Modulator alters a carriers shape so that the
  alteration the intended message (information).
• Modulation, historically, is done on the RF
  carrier wave. Thus the conversion from message
  signals to RF signals is also called modulation.

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Linear Modulatoin

• General form
• The envelope A(t) has a one-to-one correspondence
  with the message m(t).
• Example I DSB (Double SideBand) Suppressed
  Carrier (SC)

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• Coherent (Synchronous) Demodulator (Detector)
  The receiver knows exactly the phase and
  frequency of the received signal.

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• What if the receiver reference is not coherent?
  ---- A phase error case (?(t), unknown, random,
  time-varying, )

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Lets synch up!

• Carrier recovery Regenerate the carrier at the
  receiver ( fc and ?(t) )
• Example Square circuit

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• What are we expecting? It will become much
  clearer if we look in the frequency domain.
• FT of xr2(t)

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Side Show The importance of synchronization

• In communications, many things heavily depend on
  the synchronization between the Rx and the Tx.
  There are many usages of the word
  synchronization. So be careful

• Example The French composer Pierre Schaeffer
  (1910-1995) had done probably the first
  tele-synchronized act of art in 1944 to celebrate
  the liberation of Paris.

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• Example Global Positioning System Use the time
  differences of received signals from satellites
  to determine the location.
• Q Can you do a coarse estimation of what it
  takes to have a 10m precision?
• The speed of light
• The modulation scheme

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• Example II Amplitude Modulation (AM), its
  simply DSB with carrier

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Demodulation

• Coherent detection precise but requires carrier
  recovery circuit.
• Incoherent detection envelope detection Simple
  receiver (LPF) but requires sufficient carrier
  power (a lt 1) and fc gtgt W. ( In theory, fc gt W is
  ok, but a good LPF is needed.)
• Think about the advantages and disadvantages of
  analog signal processing (against DSP).

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This is the way to figure out the power
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How much power is for the message?
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Single Sideband (SSB) modulation A good example
of signal manipulation

• Method one of generation of SSB signals Sideband
  filtering, easy to understand, but difficult to
  implement. Why?

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• Method 2 Phase-shift modulation