Information Theory

1
Unit - III
2
Modulation

• Systematic alteration of a parameter of carrier
  wave (amplitude, phase and frequency) in
  accordance with instantaneous magnitude of the
  message signal.
• Generally, Frequency domain analysis provides
  more information about modulation operation.

3
Modulation
Based on the type of carrier wave
Continuous wave
Pulse
PAM PPM PWM PCM
Based on the type of modulating signal
Analog
Digital
ASK FSK PSK, etc
Based on the relationship between modulating and
modulated signal
Linear
Exponential
AM DSB-SC SSB VSB
FM PM
4
Advantages of modulation

• Frequency Assignment and multiplexing
• Practicability of antenna (minimum dimension of
  an antenna should be at least ?/10)
• Overcomes Hardware limitations (fractional BW,
  defined as absolute BW/center freq, should be
  within 1-10 for minimizing hardware cost and
  complexity)
• Reduction in noise and interference (FM, cellular
  N/W)

5
AM and DSB-SC modulation Techniques

• xc(t) Ac1µ.x(t)cos?ct
• x(t)lt1 (for normalization)
• 0ltµlt1 (to avoid envelope distortion)
• W ltlt fc (to form envelope)
• Frequency domain view
• DSB-SC signal can be generated by setting µ1 and
  suppressing the carrier. (synchronous detection)
• Power calculation

Over-modulation
6
Tone modulation and phasor analysis

• Let x(t) µAmcos?mt, xc(t) Accos?ct and µAm
  2/3. Draw the line spectra and phasor diagram for
  AM
• (i) undistorted transmission
• (ii) lower sideband is completely removed by the
  channels.

7
Modulators and transmitters

• Product modulator
• Square law modulator
• Balanced modulator
• Ring modulator (sampling)
• Switching modulator

8
Square law modulator

• A nonlinear device is used
• vout a1vin(t) a2vin2(t)
• vin x(t)cos?ct

9
Balanced modulator
Ring modulator
10
Envelope detection (a) Circuit (b) Waveforms

• Diode acts as half wave rectifier.
• In ve cycle C1 gets charged to peak.
• In ve cycle it will
• discharge through R1.

W ltlt 1/R1C1 ltlt fc

• v has a dc-offset and ripples. C2R2 acts as
  high pass filter thus smoothens o/p signals.
• DC offset can be separately extracted and used
  for AGC.

11
Synchronous (coherent) detection

• It is a product demodulator which can detect all
  types of linear modulation.
• The locally generated carrier must be exactly
  synchronized (pilot carrier)
• Drift in frequency (a pair of new tone in DSB)
• Drift in phase (Quadrature effect, fading)
• It is unattractive for AM (??) but very useful in
  SSB and digital CW demodulation.