Pulse amplitude modulation (PAM)


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Gives brief idea about pulse amplitude modulation.

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Pulse amplitude modulation (PAM)

  1. 1. Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. Example: A two bit modulator (PAM-4) will take two bits at a time and will map the signal amplitude to one of four possible levels, for example −3 volts, −1 volt, 1 volt, and 3 volts. Demodulation is performed by detecting the amplitude level of the carrier at every symbol period. Pulse-amplitude modulation is widely used in baseband transmission of digital data, with non-baseband applications having been largely superseded by pulsecode modulation, and, more recently, by pulse-position modulation.
  2. 2. Analog pulse modulation PAM PWM PPM Digital pulse modulation PCM DM ADM
  3. 3.  In the pulse amplitude modulation of pulsed carrier is changed in proportion with the instantaneous amplitude of the modulating signal x(t). So the information is continued in the amplitude variation of PAM signal.  the carrier is in the form of train of narrow pulses .  If you compare the PAM system with the sampling process ,you will find these two processes are identical.  The PAM signal is then sent by either wire or cable or it is used to modulate a carrier.
  4. 4.  Natural PAM (Chopper PAM)  Flat top PAM
  5. 5.  Generation of natural PAM Continuous modulating signal x(t) Low pass filter Multiplier Pulse train generator PAM signal
  6. 6. MOSFET acting as switch x(t) c(t) Natural RL PAM Signal
  7. 7. PAM signal Low pass filter Demodulated PAM signal
  8. 8.  The PAM signal can be detected (demodulated) by passing it through a low pass filter.  The low pass filter cutoff frequency is adjusted to fm so that all the high frequency ripple is removed and the original modulating signal is recovered back.
  9. 9. Waveform of Flat-top PAM
  10. 10. Due to the aperture effect an amplitude distortion introduces. This distortion can be corrected by equalizer
  11. 11. Advantages:  Better SNR due to increased signal power.  Generation is easy. Disadvantage:  Aperture effect introduces distortion.
  12. 12. Parameters Natural PAM Flat top PAM Nature of sampling Train of finite duration pulses Train of finite duration pulses Circuit arrangement Uses a chopper Uses a sample n hold circuit Signal power Increases with increase in the pulse width Increases with increase in the pulse width Bandwidth requirement Increases with the reduction in pulse width Increases with reduction in the pulse width Effect of noise Moderate Moderate Waveforms
  13. 13.  Since the information is contained in the amplitude variations, the noise will contaminate the information
  14. 14.  Q. Explain generation and demodulation of PAM signal with the help of suitable diagrams. (10 marks-May 2008)